STELLER SEA LION RESEARCH : NUTRITIONAL STRESS

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Nutritional Stress | Diet DeterminationNutrition of PreyEnergy NeedsAffect of Changes in Prey Quantity  | Affect of Changes in Prey QualityDetecting Nutritional Stress

NUTRITIONAL STRESS

Normal reference ranges, and age-related and diving exercise effects on hematology and serum chemistry of female Steller sea lions (Eumetopias jubatus).
Gerlinsky, C. D., M. Haulena, A. W. Trites and D. A. S. Rosen. (in press).
Journal of Zoo and Wildlife Medicine
abstract
Decreased health may have lowered the birth and survival rates of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska and Aleutian Islands over the past 30 yr. Reference ranges for clinical hematology and serum chemistry parameters needed to assess the health of wild sea lion populations are limited. Here, blood parameters were serially measured in 12 captive female Steller sea lions ranging in age from 3 wk to 16 yr to establish baseline values and investigate age-related changes. Whether diving activity affects hematology parameters in animals swimming in the ocean compared with animals in a traditional aquarium setting was also examined. Almost all blood parameters measured exhibited significant changes with age. Many of the age-related changes reflected developmental life history changes, including a change in diet during weaning, an improvement of diving capacity, and the maturity of the immune system. Mean corpuscular hemoglobin and mean corpuscular volume were also higher in the ocean diving group compared with the aquarium group, likely reflecting responses to increased exercise regimes. These data provide ranges of hematology and serum chemistry values needed to evaluate and compare the health and nutritional status of captive and wild Steller sea lions.

keywords     Diving, Eumetopias jubatus, hematology, marine mammal, serum chemistry, Steller sea lion
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Physiological constraints and energetic costs of diving behaviour in marine mammals: a review of studies using trained Steller sea lions diving in the open ocean.
Rosen, D.A.S., A.G. Hindle, C. Gerlinsky, E. Goundie, G.D. Hastie and A.W. Trites. 2017.
Journal of Comparative Physiology B 187:29-50.
abstract
Marine mammals are characterized as having physiological specializations that maximize the use of oxygen stores to prolong time spent under water. However, it has been difficult to undertake the requisite controlled studies to determine the physiological limitations and trade-offs that marine mammals face while diving in the wild under varying environmental and nutritional conditions. For the past decade, Steller sea lions (Eumetopias jubatus) trained to swim and dive in the open ocean away from the physical confines of pools participated in studies that investigated the interactions between diving behaviour, energetic costs, physiological constraints, and prey availability. Many of these studies measured the cost of diving to understand how it varies with behaviour and environmental and physiological conditions. Collectively, these studies show that the type of diving (dive bouts or single dives), the level of underwater activity, the depth and duration of dives, and the n utritional status and physical condition of the animal affect the cost of diving and foraging. They show that dive depth, dive and surface duration, and the type of dive result in physiological adjustments (heart rate, gas exchange) that may be independent of energy expenditure. They also demonstrate that changes in prey abundance and nutritional status cause sea lions to alter the balance between time spent at the surface acquiring oxygen (and offloading CO2 and other metabolic by-products) and time spent at depth acquiring prey. These new insights into the physiological basis of diving behaviour further our understanding of the potential scope for behavioural responses of marine mammals to environmental changes, the energetic significance of these adjustments, and the consequences of approaching physiological limits.
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Low prey abundance leads to less efficient foraging behaviour in Steller sea lions.
Goundie, E.T., D. A. S. Rosen and A.W. Trites. 2015.
Journal of Experimental Marine Biology and Ecology 470:70-77.
abstract
Breath-hold divers should adjust their dive behaviors to maximize the benefits and minimize the costs of foraging on prey patches of different densities at different depths. However, few studies have quantified how animals respond to changes in prey availability (depth and density), and how this affects their foraging efficiency. We tested the effects of changes in prey availability on the foraging behavior and efficiency of Steller sea lions (Eumetopias jubatus) by measuring diving metabolic rate, dive durations, and food intake of 4 trained sea lions diving in the open ocean on controlled prey patches of different densities at different depths. Sea lions completed bouts of 5 consecutive dives on high- or low-density prey patches at two depths (10m and 40m). We found that the rate of energy expenditure did not change under any of the imposed foraging conditions (meanąSD: 0.22ą0.02 kJ min−1 kg−1), but that the proportion of time spent consuming prey increased with prey patch density due to changes in diving patterns. At both depths, sea lions spent a greater proportion of the dive bout foraging on prey patches with high prey density, which led to high rates of energy gain (4.3 ą 0.96 kJ min−1 kg−1) and high foraging efficiency (cost:benefit was 1:20). In contrast, the sea lions spent a smaller proportion of their dive bout actively feeding on prey patches with low prey density, and consequently had a lower energetic gain (0.91 ą 0.29 kJ min−1 kg−1) and foraging efficiency (1:4). The 5-fold differences in foraging efficiency between the two types of prey patches were greater than the 3-fold differences that we expected based on differences in food availability. Our results suggest that sea lions faced with reduced prey availability forage less efficiently and therefore would have greater difficulty obtaining their daily energy requirements.

keywords     Dive behavior, Diving energetics, Foraging efficiency, Optimal foraging, Steller sea lion
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A nutrigenomic approach to detect nutritional stress from gene expression in blood samples drawn from Steller sea lions.
Spitz, J., V. Becquet, D.A.S. Rosen and A.W. Trites. 2015.
Comparative Biochemistry and Physiology: Part A 187:214-223.
abstract
Gene expression profiles are increasingly being used as biomarkers to detect the physiological responses of a number of species to disease, nutrition, and other stressors. However, little attention has been given to using gene expression to assess the stressors and physiological status of marine mammals. We sought to develop and validate a nutrigenomics approach to quantify nutritional stress in Steller sea lions (Eumetopias jubatus). We subjected 4 female Steller sea lions to 3 feeding regimes over 70-day trials (unrestricted food intake, acute nutritional stress, and chronic nutritional stress), and drew blood samples from each animal at the end of each feeding regime. We then extracted the RNA of white blood cells and measured the response of 8 genes known to react to diet restriction in terrestrial mammals. Overall, we found that the genomic response of Steller sea lions experiencing nutritional stress was consistent with diet restriction regulation in terrestrial mammals. Our nutritionally stressed sea lions down-regulated some cellular processes involved in immune response and oxidative stress, and up-regulated pro-inflammatory responses and metabolic processes. Nutrigenomics appears to be a promising means to monitor nutritional status and contribute to mitigation measures needed to assist in the recovery of Steller sea lions and other at-risk species of marine mammals.

keywords     Genomics, Expression profile, q-PCR, Diet, restriction, Biomarker, Monitoring
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Oxygen stores, carbon dioxide accumulation and nutritional status as determinants of diving ability of Steller sea lions (Eumetopias jubatus).
Gerlinsky, C.D. 2014.
M.Sc. Thesis, University of British Columbia, Vancouver, B.C. 105 pages
abstract
The diving ability of marine mammals is limited by body oxygen stores (TBO) and rates of oxygen depletion (diving metabolic rate; DMR), which can be expressed as the calculated aerobic dive limit (cADL). Diving ability must also be influenced by CO₂ production and control of ventilation. I investigated the factors that limit the diving ability of Steller sea lions (Eumetopias jubatus), including the effect of nutritional stress on the cADL. Specifically, I 1) determined the cADL of Steller sea lions by measuring TBO and DMR, 2) determined whether nutritional stress alters the cADL and 3) examined the post-dive elimination of CO₂, and the sensitivity of Steller sea lions to hypercapnia (high inspired CO₂). TBO was estimated from measured blood oxygen stores and body composition―and metabolic rate, breathing frequency and dive behaviour were recorded prior to and during a period of nutritional stress where animals lost ~10% of their mass. Animals breathed ambient, hypercapnic or hypoxic (low O₂) air to experimentally alter pCO₂ levels and decrease rates of CO₂ elimination and O₂ consumption. I found that the TBO (35.9 ml O₂ kg-¹) and cADL (3.0 minutes) in actively diving Steller sea lions were lower than previously reported for other species of sea lions and fur seals. I also found a significant increase in mass-specific DMR and blood volume (resulting in higher TBO) in nutritionally stressed animals that resulted in a longer cADL. Hypercapnia was found to significantly affect ventilation, but had no effect on dive behaviour―and elimination of CO₂ between dives took longer than replenishing O₂ stores. Overall, nutritional stress and hypercapnic conditions did not directly limit the diving ability of the Steller sea lions, but had an indirect effect on foraging efficiency by increasing the time they spent on the surface between dives. Accumulation of CO₂ over several dives in a foraging bout also appeared to reduce foraging efficiency, which likely ultimately limits the time a sea lion spends in apnea and therefore overall foraging duration and net energy intake.
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Sensitivity to hypercapnia and elimination of CO2 following diving in Steller sea lions (Eumetopias jubatus).
Gerlinsky, C.D., D.A.S. Rosen and A.W. Trites. 2014.
Journal of Comparative Physiology B. 184:535-544.
abstract
Marine mammal foraging behaviour inherently depends on diving ability. Declining populations of Steller sea lions may be facing nutritional stress that could affect their diving ability through changes in body composition or metabolism. Our objective was to determine whether nutritional stress (restricted food intake resulting in a 10% decrease in body mass) altered the calculated aerobic dive limit (cADL) of four captive sea lions diving in the open ocean, and how this related to changes in observed dive behaviour. We measured diving metabolic rate (DMR), blood O2 stores, body composition and dive behaviour prior to and while under nutritional restriction. We found that nutritionally stressed sea lions increased the duration of their single long dives, and the proportion of time they spent at the surface during a cycle of four dives. Nutritionally stressed sea lions lost both lipid and lean mass, resulting in potentially lower muscle O2 stores. However, total body O2 stores increased due to rises in blood O2 stores associated with having higher blood volumes. Nutritionally stressed sea lions also had higher mass-specific metabolic rates. The greater rise in O2 stores relative to the increase in mass-specific DMR resulted in the sea lions having a longer cADL when nutritionally stressed. We conclude that there was no negative effect of nutritional stress on the diving ability of sea lions. However, nutritional stress did lower foraging efficiency and require more foraging time to meet energy requirements due to increases in diving metabolic rates and surface recovery times.
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Steller sea lions (Eumetopias jubatus) have greater blood volumes, higher diving metabolic rates and a longer aerobic dive limit when nutritionally stressed.
Gerlinsky, C.D., A.W. Trites and D.A.S. Rosen. 2014.
Journal of Experimental Biology 217:769-778.
abstract
Marine mammal foraging behavior inherently depends on diving ability. Declining populations of Steller sea lions may be facing nutritional stress that could affect their diving ability through changes in body composition or metabolism. Our objective was to determine whether nutritional stress (restricted food intake resulting in a 10% decrease in body mass) altered the calculated aerobic dive limit (cADL) of four captive sea lions diving in the open ocean, and how this related to changes in observed dive behaviour. We measured diving metabolic rate (DMR), blood O2 stores, body composition and dive behaviour prior to and while under nutritional restriction. We found that nutritionally stressed sea lions increased the duration of their single long dives, and the proportion of time they spent at the surface during a cycle of four dives. Nutritionally stressed sea lions lost both lipid and lean mass, resulting in potentially lower muscle O2 stores. However, total body O2 stores increased due to rises in blood O2 stores associated with having higher blood volumes. Nutritionally stressed sea lions also had higher mass-specific metabolic rates. The greater rise in O2 stores relative to the increase in mass-specific DMR resulted in the sea lions having a longer cADL when nutritionally stressed. We conclude that there was no negative effect of nutritional stress on the diving ability of sea lions. However, nutritional stress did lower foraging efficiency and require more foraging time to meet energy requirements due to increases in diving metabolic rates and surface recovery times.

keywords     Steller sea lion, blood volume, nutritional stress, diving metabolism, oxygen stores, dive behavior
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Steller sea lions Eumetopias jubatus and nutritional stress: evidence from captive studies.
Rosen, D.A.S. 2009.
Mammal Review 39:284-306.
abstract
1. Numbers of Steller sea lions Eumetopias jubatus in the North Pacific have declined. According to the Nutritional Stress Hypothesis, this decline is due to reduced food availability. Data from studies conducted on pinnipeds in the laboratory are used here to test whether the Nutritional Stress Hypothesis can explain the decline of Steller sea lions. 2. Overall, there is strong evidence for biologically meaningful differences in the nutritional quality of major prey species. Steller sea lions can partly compensate for low-quality prey by increasing their food consumption. 3. There appear to be no detrimental effects of low-lipid prey on sea lion growth or body composition when sea lions can consume sufficient quantities of prey. However, the ability to increase consumption is physiologically limited, particularly in young animals. Overall, it is more difficult to maintain energy intake on a diet of low-quality prey than on a normal diet. 4. Under conditions of inadequate food intake (either due to decreased prey availability or quality, or increased energy requirements) the overall impacts of nutritional stress are complex, and are dependent upon season, prey quality, age, and the duration and intensity of the nutritional stress event. 5. Studies on pinnipeds in the laboratory have been instrumental in identifying the conditions under which changes in sea lion prey can result in nutritional stress, and the nature of the physiological impacts of nutritional stress events.
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Laboratory studies in wildlife conservation: The case of the Steller sea lion.
Rosen, D.A.S., A.L. Fahlman, A.W. Trites and G.D. Hastie. 2007.
Comparative Biochemistry and Physiology A Vol 146 pp. S84
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The decline of Steller sea lions in Alaska: A review of the nutritional stress hypothesis.
Trites, A.W. and C.P. Donnelly. 2003.
Mammal Review 33:3-28.
abstract

1. The decline of Steller sea lions Eumetopias jubatus in the Gulf of Alaska and Aleutian Islands between the late 1970s and 1990s may have been related to reduced availability of suitable prey. Many studies have shown that pinnipeds and other mammals suffering from nutritional stress typically exhibit reduced body size, reduced productivity, high mortality of pups and juveniles, altered blood chemistry and specific behavioural modifications.

2. Morphometric measurements of Steller sea lions through the 1970s and 1980s in Alaska indicate reduced body size. Reduced numbers of pups born and an apparent increase in juvenile mortality rates also appear to be nutritionally based. Blood chemistry analyses have further shown that Steller sea lions in the Gulf of Alaska and Aleutian Islands area exhibited signs of an acute phase reaction, or immune reaction, in response to unidentified physical and/or environmental stress. Behavioural studies during the 1990s have not noted any changes that are indicative of an overall shortage in the quantity of prey available to lactating female sea lions.

3. The data collected in Alaska are consistent with the hypothesis that Steller sea lions in the declining regions were nutritionally compromised because of the relative quality of prey available to them (chronic nutritional stress), rather than because of the overall quantity of fish per se (acute nutritional stress). This is further supported by captive studies that indicate the overall quality of prey that has been available to Steller sea lions in the declining popu-lation could compromise the health of Steller sea lions and hinder their recovery.


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Alternative models for assessing the role of nutrition in the population dynamics of marine mammals.
Donnelly, C., A.W. Trites and D.D. Kitts. 2000.
In C.L.K. Baer (ed.), Proceedings of the Third Comparative Nutrition Society Symposium. Pacific Grove, California, August 4-9, 2000. 3:41-45.
abstract
Alternative animal models are desirable to assess the role of nutrition on the population dynamics of marine mammals. If an appropriate model could be found, it might be possible to identify population consequences and risks that face sea otters forced to eat fish after depleting local invertebrates, or for sea lions which switch from a fatty fish to a lean fish. From the arguments raised above, the rat appears to be a feasible model for studying marine mammal nutrition. A preliminary study exploring the effects of nutrition on population dynamics via parameters of growth and reproductive success is feasible. Although mink and harbor seal models are superior in their similarity to other marine mammals, the difficulty and time involved in breeding them is either extremely labor intensive or prohibitive. Again, the regular, five day cycle of the rat and shorter generation time allow for parameters of fertility and offspring viability in response to different diets to be examined in a cost effective and economic way. Additionally, because of the extensive use of rats in other nutritional studies, many signs and symptoms of specific nutritional shortcomings are known and easily detected. If a reliable model can be implemented in the study of marine mammal population dynamics, research can explore aspects of physiology nor available when using captive marine mammals or mammals in the wild. Development of a model also has the potential to reduce the number of mammals taken from the wild for scientific study, thereby helping to preserve many threatened species.
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Assessing the role of nutritional stress in the decline of wild populations: a Steller case of scientific sleuthing.
Rosen, D.A.S. and A.W. Trites. 2000.
In C.L.K. Baer (ed.), Proceedings of the Third Comparative Nutrition Society Symposium. Pacific Grove, California, August 4-9, 2000. 3:182-186.
abstract
Dry-matter digestibility and energy digestive efficiency were measured in six juvenile Steller sea lions (Eumetopias jubatus) fed three diets each consisting of a single species: herring, pollock, and squid. Two of the animals were also fed pink salmon. Dry-matter digestibility (DMD) and digestive efficiency (DE) were measured using the energy and manganese concentration in fecal and food samples. DE values were high for all prey species (herring: 95.4 ± 0.7% (mean ± SD), pollock: 93.9 ± 1.4%, salmon: 93.4 ± 0.5%, squid: 90.4 ± 1.3%). Steller sea lions appear to digest prey of high energy density more efficiently than prey of low energy density. DMD values were also high for all prey species (herring: 90.1 ± 1.8%, pollock: 86.5 ± 3.4%, salmon: 87.3% ± 2.6, squid: 90.5 ± 1.2%). The low DMD value for pollock compared with herring and squid was due to the high proportion of bony material in pollock. There was a strong linear relationship between DE and DMD for each prey type, but the terms cannot be used interchangeably. DE measures are more meaningful than DMD in conveying the energetic benefits derived by sea lions from different types of prey. Species-specific measures of the digestible energy obtained from an array of prey items are a necessary component in understanding the bioenergetic consequences of consuming different prey species.
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DIET DETERMINATION

Quantitative DNA metabarcoding: improved estimates of species proportional biomass using correction factors derived from control material.
Thomas, A. C., B. E. Deagle, P. J. Eveson, C. H. Harsch and A. W. Trites. 2016.
Molecular Ecology Resources 16:714-726.
abstract
DNA metabarcoding is a powerful new tool allowing characterization of species assemblages using high-throughput amplicon sequencing. The utility of DNA metabarcoding for quantifying relative species abundances is currently limited by both biological and technical biases which influence sequence read counts. We tested the idea of sequencing 50/50 mixtures of target species and a control species in order to generate relative correction factors (RCFs) that account for multiple sources of bias and are applicable to field studies. RCFs will be most effective if they are not affected by input mass ratio or co-occurring species. In a model experiment involving three target fish species and a fixed control, we found RCFs did vary with input ratio but in a consistent fashion, and that 50/50 RCFs applied to DNA sequence counts from various mixtures of the target species still greatly improved relative abundance estimates (e.g., average per species error of 19 ± 8% for uncorrected versu s 3 ± 1% for corrected estimates). To demonstrate the use of correction factors in a field setting, we calculated 50/50 RCFs for 18 harbour seal (Phoca vitulina) prey species (RCFs ranging from 0.68 to 3.68). Applying these corrections to field-collected seal scats affected species percentages from individual samples (Δ 6.7 ± 6.6%) more than population level species estimates (Δ 1.7 ± 1.2%). Our results indicate that the 50/50 RCF approach is an effective tool for evaluating and correcting biases in DNA metabarcoding studies. The decision to apply correction factors will be influenced by the feasibility of creating tissue mixtures for the target species, and the level of accuracy needed to meet research objectives.

keywords     DNA metabarcoding, high-throughput amplicon sequencing, harbor seal, Phoca vitulina, diets, prey consumption, diet reconstruction, scats, fecal samples
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Stable carbon and nitrogen isotope trophic enrichment factors for Steller sea lion vibrissae relative to milk and fish/invertebrate diets.
Stricker, C.A., A.M. Christ, M.B. Wunder, A.C. Doll, S.D. Farley, L.D. Rea, D.A.S. Rosen, R.D. Scherer and D.J. Tollit. 2015.
Marine Ecology Progress Series. 523:255-266.
abstract
Nutritional constraints have been proposed as a contributor to population declines in the endangered Steller sea lion Eumetopias jubatus in some regions of the North Pacific. Isotopic analysis of vibrissae (whiskers) is a potentially useful approach to resolving the nutritional ecology of this species because long-term (up to 8 yr) dietary information is sequentially recorded and metabolically inert once formed. Additionally, vibrissae are grown in utero, potentially offering indirect inference on maternal diet. However, diet reconstruction using isotopic techniques requires a priori knowledge of trophic enrichment factors (TEFs), which can vary relative to diet quality and among animal species. In this study, we provide new TEF estimates for (1) maternal relative to pup vibrissae during both gestation and nursing and (2) adult vibrissae relative to a complex diet. Further, we refine vibrissa−milk TEFs based on an additional 76 animals with an age distribution ranging from 1 to 20 mo. Mother−pup vibrissae TEF values during gestation and nursing were near zero for δ13C and averaged 0.8 and 1.6‰, respectively, for δ15N. In contrast, vibrissa− fish/invertebrate TEFs averaged 3.3 (± 0.3 SD) and 3.7‰ (±0.3) for lipid-free δ13C and δ15N, respectively. Average lipid-free δ13C and δ15N vibrissa−milk TEFs were 2.5 (±0.9) and 1.8‰ (±0.8), respectively, and did not differ among metapopulations. Empirically determined TEFs are critical for accurate retrospective diet modeling, particularly for evaluating the hypothesis of nutritional deficiency contributing to the lack of Steller sea lion population recovery in some regions of Alaska.
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Diet of Steller sea lions (Eumetopias jubatus) in Frederick Sound, southeast Alaska: a comparison of quantification methods using scats to describe temporal and spatial variability.
Tollit, D. J., M. A. Wong and A. W. Trites. 2015.
Canadian Journal of Zoology 93:361-376.
abstract
We compared eight dietary indices used to describe the diet of Steller sea lions (Eumetopias jubatus (Schreber, 1776)) from 2001-2004 in Frederick Sound, Southeast Alaska. Remains (n=9 666 items) from 59+ species categories were identified from 1 693 fecal samples (scats) from 14 collection periods. The most frequently occurring prey were walleye pollock (Theragra chalcogramma (Pallas, 1814), 95%), Pacific herring (Clupea harengus (Cuvier and Valenciennes, 1847), 30%), Pacific hake (Merluccius productus (Ayres, 1855), 29%), and arrowtooth flounder (Atheresthes stomias (Jordan and Gilbert 1880), 21%). These species, along with salmon (Oncorhynchus spp.) and skate (Raja spp.), accounted for 80-90% of the reconstructed biomass and energy contribution, with pollock contributing 37-60%. Overall, 80% of fish were 14-42cm long and mainly pelagic, though 40% of scats contained benthic-associated prey. Steller sea lions switched from adult pollock to strong cohorts of juvenile pollock, and took advantage of spawning concentrations of salmon in autumn and herring in late-spring and summer, as well as a climate-driven increase in hake availability. Observed temporal and site differences in diet confirm the need for robust long-term scat sampling protocols. All major indices similarly tracked key temporal changes, despite differences in occurrence and biomass-energy based diet estimates linked to prey size and energy density effects and the application of correction factors.

keywords     Diet, scat, biomass reconstruction, hard remains, otoliths, Steller sea lion, dietary index
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Proportion of prey consumed can be determined from faecal DNA using real-time PCR.
Bowles, E., P.M. Schulte, D.J. Tollit, B.E. Deagle and A.W. Trites. 2011.
Molecular Ecology Resources 11:530-540.
abstract
Reconstructing the diets of pinnipeds by visually identifying prey remains recovered in faecal samples is challenging because of differences in digestion and passage rates of hard parts. Analyzing the soft-matrix of faecal material using DNA-based techniques is an alternative means to identify prey species consumed, but published techniques are largely non-quantitative, which limits their usefulness for some studies. We further developed and validated a real-time PCR technique using species-specific mitochondrial DNA primers to quantify the proportion of prey in the diets of Steller sea lions (Eumetopias jubatus), a pinniped species thought to be facing significant diet related challenges in the North Pacific. We first demonstrated that the proportions of prey tissue DNA in mixtures of DNA isolated from four prey species could be estimated within a margin of ~12% of the percent in the mix. These prey species included herring Clupea palasii, eulachon Thaleichthyes pacificus, squid Loligo opalescens and rosethorn rockfish Sebastes helvomaculatus. We then applied real-time PCR to DNA extracted from faecal samples obtained from Steller sea lions in captivity that were fed 11 different combinations of herring, eulachon, squid and Pacific ocean perch rockfish (Sebastes alutus), ranging from 7-75% contributions per meal (by wet weight). The difference between the average percentage estimated by real-time PCR and the percentage of prey consumed was generally less than 12% for all diets fed. Our findings indicate that real-time PCR of faecal DNA can detect the approximate relative quantity of prey consumed for complex diets and prey species, including cephalopods and fish.
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Stable isotope signal homogeneity and differences between and within pinniped muscle and skin.
Todd, S.K., B. Holm, D.A.S. Rosen and D.J. Tollit. 2010.
Marine Mammal Science 26:176-185.
abstract
Stable isotope analysis is often used to examine diet choice and tropic relationships in marine mammals. However, the technique makes a number of largely untested assumptions. For example, researchers often assume localized biopsies to be representative of the whole animal—that is, that the isotopic signal is homogenous within a tissue. Further, isotopic composition may differ across the body within the same tissue type due to differential assimilation or catabolization rates. We investigated the homogeneity of 13C and 15N values in skin and muscle across the body per individual in three pinniped species: Steller sea lions (Eumetopias jubatus, n = 5), California sea lions (Zalophus californianus, n = 6), and harbor seals (Phoca vitulina, n = 7). We also assessed if there are consistent carbon and nitrogen isotope differences between these two commonly sampled tissues. Our results show that skin tissue was significantly 13C enriched when compared to muscle tissue, and more difficult to properly process. Despite expected differences across the body in physiological processes and biochemical composition, our data indicate stable isotope signal homogeneity across the body within both muscle and skin, for both carbon and nitrogen isotopes, in all three species. These results indicate that sufficient homogeneity exists within skin and muscle tissues to suggest that point sampling can be considered representative of entire tissues, and is thus a valid technique in stable isotope studies of marine mammals.
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Determining the relative amounts of prey in Steller sea lion (Eumetopias jubatus) diet using real-time PCR.
Bowles, E. 2009.
MSc thesis, University of British Columbia, Vancouver. 54 pages
abstract
Determining diets of pinnipeds by visually identifying prey remains recovered in faecal samples is challenging because of differences in digestion and passage rates of hard parts. Analyzing the soft matrix of fecal material using DNA-based techniques is an alternative means to identify prey species consumed, but published techniques are largely non-quantitative, which limits their applicability. I developed and validated a real-time PCR technique using species-specific mitochondrial DNA primers to quantify the diets of Steller sea lions (Eumetopias jubatus). I first demonstrated that the proportions of prey tissue DNA in mixtures of DNA isolated from four prey species could be estimated within a margin of ~12% of the percent in the mix. These prey species included herring Clupea palasii, eulachon Thaleichthyes pacificus, squid Loligo opalescens and rosethorn rockfish Sebastes helvomaculatus. I then applied real-time PCR to DNA extracted from faecal samples obtained from Steller sea lions that had been fed 11 different combinations of herring, eulachon, squid and Pacific ocean perch rockfish (Sebastes alutus), ranging from 7-75% contributions to a meal mix (by wet weight). The difference between the average percentage estimated by real-time PCR and the percentage of prey consumed was generally less than 12% for all diets fed when percentages of prey consumed were corrected for differences in mtDNA density among the prey items. My findings indicate that real-time PCR can detect the quantity of prey consumed for a variety of complex diets and prey species, including cephalopods and fish.
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Development and application of DNA techniques for validating and improving pinniped diet estimates.
Tollit, D.J., A.D. Schulze, A.W. Trites, P.F. Olesiuk, S.J. Crockford, R.R. Ream T.S. Gelatt and K.M. Miller. 2009.
Ecological Applications 19:889-905.
abstract
Polymerase chain reaction techniques were developed and applied to identify DNA from >40 species of prey contained in fecal (scat) soft part matrix collected at terrestrial sites used by Steller sea lions (Eumetopias jubatus) in British Columbia and the Eastern Aleutian Islands, Alaska. Sixty percent more fish and cephalopod prey were identified by morphological analyses of hard parts compared with DNA analysis of soft parts (hard parts identified higher relative proportions of Ammodytes sp., Cottidae and certain Gadidae). DNA identified 213 prey occurrences of which 75 (35%) were undetected by hard parts (mainly Salmonidae, Pleuronectidae, Elasmobranchii and Cephalopoda), and thereby increased species occurrences by 22% overall and species richness in 44% of cases (when comparing 110 scats that amplified prey DNA). Prey composition was identical within only 20% of scats. Overall, diet composition derived from both identification techniques combined did not differ significantly from hard part identification alone, suggesting that past scat-based diet studies have not missed major dietary components. However, significant differences in relative diet contributions across scats (as identified using the two techniques separately) reflect passage rate differences between hard and soft digesta material and highlight certain hypothesized limitations in conventional morphological-based methods (e.g., differences in resistance to digestion, hard part regurgitation, partial and secondary prey consumption), as well as potential technical issues (e.g., resolution of primer efficiency and sensitivity, and scat subsampling protocols). DNA analysis of salmon occurrence (from scat soft part matrix and 238 archived salmon hard parts) provided species-level taxonomic resolution that could not be obtained by morphological identification, and showed that Steller sea lions were primarily consuming pink (Oncorhynchus gorbuscha) and chum (Oncorhynchus keta) salmon. Notably, DNA from Atlantic salmon (Salmo salar) that likely originated from a distant fish farm was also detected in two scats from one site in the Eastern Aleutian Islands. Overall, molecular techniques are valuable for identifying prey in the fecal remains of marine predators. Combining DNA and hard part identification will effectively alleviate certain predicted biases, and will ultimately enhance measures of diet richness, fisheries interactions (especially salmon related ones) and the ecological role of pinnipeds and other marine predators, to the benefit of marine wildlife conservationist and fisheries managers.
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Diets of mature male and female Steller sea lions differ and cannot be used as proxies for each other.
Trites, A.W., and D.G. Calkins. 2008.
Aquatic Mammals 34:25-34.
abstract
Disturbance of otariid breeding sites (rookeries) to determine diet from fecal remains (scats) could be eliminated if the diets of males using adjoining bachelor haulouts could be used as a proxy for diets of breeding females. We collected scats from sexually mature Steller sea lions (Eumetopias jubatus) at one male resting site (haulout) and three female dominated breeding sites (rookeries) at Forrester Island, Southeast Alaska (June-July, 1994–1999) to test whether the diets of bachelor bulls differed from that of breeding females. Female diets were fairly evenly distributed between gadids, salmon and small oily fishes (forage fish), and contained lesser amounts of rockfish, flatfish, cephalopods and other fishes. Female diet did not differ significantly between the 3 rookeries, but did differ significantly from that of males. Males consumed significantly fewer salmon, and more pollock, flatfish and rockfish compared to females. The males also consumed larger pollock compared to females. These dietary differences may reflect a sex-specific difference in foraging areas or differences in hunting abilities related to the disparity in physical sizes of males and females. The similarity of the female diets between rookeries suggests that female diets can be determined from samples collected at a single site within a rookery complex. Unfortunately, summer diets of breeding females cannot be ascertained from hard parts contained in the scats of mature male Steller sea lions.
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Quantitative analysis of prey DNA in pinniped faeces: potential to estimate diet composition?
Deagle, B.E. and D.J. Tollit. 2007.
Conservation Genetics 8:743-747.
abstract
Recent studies have shown prey DNA can be consistently recovered from faeces and effectively used to provide dietary information. We investigate the possibility of using the relative amounts of DNA recovered from different prey in faeces to obtain quantitative diet composition data. Faecal samples were obtained from captive Steller seas lions (Eumetopias jubatus) being fed a fish diet consisting of 50% Pacific herring (Clupea pallasii), 36% surf smelt (Hypomesus pretiosus) and 14% sockeye salmon (Oncorhynchus nerka) by mass. Quantitative real-time PCR was used to measure the amount of mtDNA from the three fish species in: (i) a blended tissue mix representative of the sea lion diet and (ii) the sea lion faecal samples. The percent composition of fish mtDNA extracted from the undigested tissue samples corresponded reasonably well to the mass of fish in the mixture. In the faecal samples (n = 23) the absolute amount of fish mtDNA recovered varied 100-fold, but the percent composition of the three fish was relatively consistent (57.5 ± 9.3% for herring, 19.3 ± 6.6% for smelt and 23.2 ± 12.2% for salmon). Differences between the mtDNA proportions in the tissue samples compared to the faecal samples indicate there are prey-specific biases in DNA survival during digestion. These biases may be less than those commonly observed in the conventional analysis of prey hard remains. Further investigation of this approach is warranted.
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Elemental analysis of otoliths and eye lenses in the assessment of Steller sea lion diets.
Ferenbaugh, J. 2007.
PhD, Texas Tech University. 135 pages
abstract
Steller sea lions (Eumetopias jubatus) have historically ranged along the North Pacific Rim from the coast of California to Japan, but the population has dramatically declined since the 1960s. Research has indicated that nutritional stress is likely to be the main cause of the decline. Scat analysis is the preferred technique for dietary analysis of Steller sea lions, and fish otoliths and eye lenses are routinely recovered from pinniped scat. Fisheries scientists use elemental analysis of otoliths and eye lenses to provide information on fish biology, but marine mammalogists have not incorporated this technique to study prey fish or foraging behavior. In this dissertation, I examined the use of elemental analysis of prey fish otoliths and eye lenses in dietary studies for Steller sea lions. I first examined the use of otoliths as indicators of total body burdens of metal contaminants in the fish. Then, I assessed the effects of Steller sea lion digestion on the microchemistry of otoliths. Third, I examined the microchemistry of fish eye lenses, the effects of digestion on eye lenses, and their potential use in dietary analysis. Concentrations of some metals, such as zinc and barium, in undigested otoliths are significantly correlated with concentrations found in homogenized tissues, but several factors affect this relationship, such as fish species, sampling site on the otolith, and the specific metal being analyzed. The degradation of an otolith in the sea lion digestive tract is also likely to affect correlations between otolith and tissue metal concentrations. Steller sea lion digestion has significant effects on otolith microchemistry. These effects do not prohibit the use of digested otoliths in species determination for dietary analysis, but they may preclude using otoliths recovered from sea lion scat for fish stock separation, determination of foraging locations, and fish life history analyses. Eye lenses appear to be resistant to sea lion digestion, and they form sequential growth layers that can be used to age fish. The fibrous structure of the layers may inhibit symmetrical distributions across the lens for some elements, but the distinct elemental distributions across the lens may be useful in distinguishing fish species, discriminating between fish stocks, and tracking fish movements and spatial locations.
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Comparison of fatty acid profiles of spawning and non-spawning Pacific herring, Clupea harengus pallasi.
Huynh, M.D., D.D. Kitts, C. Hu and A.W. Trites. 2007.
Journal of Comparative Biochemistry and Physiology, Part B 146:504-511.
abstract
Crude lipid and fatty acid composition from liver, intestine, roe, milt and flesh of spawning and non-spawning Pacific herring Clupea harengus pallasi were examined to determine the relative effects of spawning on the nutritional value of herring. Depletion of lipid due to spawning condition was significant (Pb0.01) in all organ tissues and flesh of spawning herring. The lipid content ranged from an average of 1.9 to 3.4% (wet weight basis) in different organ tissues of spawning herring, to 10.5 to 16% in non-spawning fish. The fatty acid profile exhibited many differences in the relative distribution of individual fatty acids among organ tissues and between the two fish groups. Oleic acid (C18:1n-9), a major monounsaturated fatty acid (MUFA) found in all tissue lipids, decreased significantly (Pb0.01) in spawning fish. The two monoenes, C20:1n-9 and C22:1n-11, occurred at high concentrations in the flesh but at only minor proportion in the digestive organs and gonads. Spawning herring also had significantly (Pb0.01) higher polyunsaturated fatty acids (PUFA) content in the organ tissues, particularly in the milt and ovary, with docosahexaenoic acid (C22:6n-3, DHA) having the greatest proportion. Among the n-6 fatty acids, only C18:2n-6 and C20:4n-6 occurred at notable amounts and were present in higher proportions in spawning fish. We concluded that although relatively higher n-3 fatty acid content was found in the organ lipids of spawning herring, they are not an energy-dense prey food source due to the fact that both flesh and gonads contain a very low amount of lipid.
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Impact of diet index selection and the digestion of prey hard remains on determining the diet of the Steller sea lion (Eumetopias jubatus).
Tollit, D.J., S.G. Heaslip, R.L. Barrick and A.W. Trites. 2007.
Canadian Journal of Zoology 85:1-15.
abstract
Abstract: Nine prey species (n = 7,431) were fed to four captive female Steller sea lions (Eumetopias jubatus (Schreber, 1776)) in eleven feeding trials over 75 days to investigate the effectiveness of different methods used to determine diet from prey hard remains. Trials aimed to replicate short (1-2 day) and long feeding bouts and consisted of single species and mixed daily diets. Overall, an average of 25.2% ± 22.2% (mean ± SD, range 0-83%) of otoliths were recovered, but recovery rates varied by species (ANOVA, P = 0.01) and were linearly related to otolith robustness (R2 = 0.88). Squid beaks were recovered at higher frequencies (mean = 96%) than the otoliths of all species. Enumerating both non-otolith skeletal structures and otoliths (together termed ?bones?) increased species recovery rates by twofold on average (P < 0.001), with increases up to 2.5 times for herring and 3-4 times for salmonids. Using bones reduced inter-specific differences (P = 0.08), but recovery ! varied among sea lions. Bones were distributed over more scats per meal (mean = 2.9 scats, range = 0-5) than otoliths (mean = 1.9 scats, range = 0-4). In three different 15-day mixed diet trials, biomass reconstruction (BR) indices performed better than frequency of occurrence indices in predicting diet fed. Applying our experimentally derived numerical correction factors (to account for species differences in complete prey digestion) further improved BR estimates, resulting in all twelve unweighted comparisons within 5% (for otoliths) and 12% (for bones) of the actual diet fed.
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Using simulations to evaluate reconstructions of sea lion diet from scat.
Joy, R., D.J. Tollit, J.L. Laake, and A.W. Trites. 2006.
In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 205-222.
abstract
Models used to describe pinniped diet can provide very different composition estimates. Occurrence indices as well as biomass reconstruction models (which use estimates of the number and sizes of prey consumed) are commonly used and increasingly utilize a variety of fish hard remains (bones) found in scats. However, the importance of any single fish can be overestimated if its bones are deposited in a succession of scats assumed to be from different fish. Similarly, the importance of a species will be underestimated relative to other species if the bones of one species are more fragile and are completely digested or if bones from different fish of the same species are contained in a single scat and assumed to be from a single fish. Species differences in the proportion of fish bones that survive digestion can be assessed from captive feeding studies where the number and species of prey consumed is known. Numerical correction factors can be calculated to take into account the levels of complete digestion. We performed computer simulations using data from captive feeding studies to investigate levels and sources of error in reconstructing simulated mixed species diets. Our simulations used different combinations of hard remains, were conducted both with and without the application of numerical correction factors, and compared four different diet indices (1. Modified frequency of occurrence, 2. Split sample frequency of occurrence, 3. Variable biomass reconstruction, 4. Fixed biomass reconstruction). Simulations indicated that levels of error were related to the MNI method of inferring fish numbers from prey remains, prey size, the number of identifiable prey structures used, and the robustness of the remains to digestive processes (recovery rate). The fewer fish fed, the higher the relative probability of counting the fish, particularly when a multiple element structure or all structure techniques are used. If recovery rates were assumed to be consistent across species, then large fish (particularly when fed in small amounts) were overestimated relative to smaller sized prey in all models, but particularly biomass reconstruction models and when using more than one paired structure. When recovery rates of a paired structure (otoliths) were varied across species (as observed in captive feeding studies) then biomass models tended to overestimate the species with high recovery rates. In contrast, frequency of occurrence models overestimated the contribution of smaller prey (particularly when fed in small amounts). Simulations also indicated correction factors can reduce levels of error in biomass reconstruction models, but cannot solve problems related to counting fish using MNI. Our work shows simulations can form a valuable component in assessing diet indices and the level (and direction) of associated errors in each.
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Estimating diet composition in sea lions: which technique to choose?
Tollit, D.J., S.G. Heaslip, B.E. Deagle, S.J. Iverson, R. Joy, D.A.S. Rosen and A.W. Trites. 2006.
In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 293-307.
abstract
Accurate estimates of diets are vital to monitor impacts of sea lion populations on their ecosystems, their interactions with fisheries and to understand the role of food to animal nutrition and health. Approaches include using: (1) prey remnants in stomach contents, spews and scats, (2) prey DNA in scats (3) fatty acid signatures in blubber and (4) stable isotope ratios in predator's tissue. Each methodology has particular advantages and limitations, many of which can be assessed and improved through controlled captive feeding trials. Analysis of prey remnants from captive sea lion scats have shown significant variability in digestion between and within prey species, which coupled with preferential regurgitation and enumeration biases, can confound accurate diet quantification, but does not prevent spatial or temporal comparisons. Correction for partial digestion and use of additional structures besides otoliths can provide accurate prey size estimates. Prey DNA can be reliably isolated from soft remains in scats from captive sea lions and with further development this approach may allow quantification of diet. Genetic methods can be expensive and representative of only one to two days foraging (like prey remnant analysis), but may be less affected by differential digestion and can identify prey in scats that could not be identified through structural remnants. Validation of fatty acid signature analysis to quantify diet at longer temporal scales in sea lions is ongoing, but this new technique promises to be particularly useful to assess biases in traditional methods, identify the onset of weaning and to highlight what prey most contribute to lipid reserves. Stable isotope analysis of predator tissues gives only trophic level data, but can provide data on diet changes on many temporal scales. Remote video monitoring of foraging events and lavage/enema techniques can provide valuable diet information, but, like many newer techniques, animal capture is required. Ideally a suite of techniques should be used to study diet. While methods and correction factors developed for Steller sea lions can likely be applied to the other five sea lion species, they should be verified experimentally.
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Food consumption by sea lions: existing data and techniques.
Winship, A.J., A.M.J. Hunter, D.A.S. Rosen, and A.W. Trites. 2006.
In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 177-191.
abstract
Knowing the quantity of prey that sea lions consume is a prerequisite for assessing the role of sea lions in aquatic ecosystems and the potential for competition to occur with fisheries. We reviewed the different approaches that have been used to estimate the food requirements for the six species of sea lions. We reviewed data on the quantity of food consumed by sea lions in captivity, and examined how consumption varied by species, body size, and season. We also reviewed and quantified available information on the energetics of sea lions and assessed the potential application of these data to parameterize an existing bioenergetic model that was developed to estimate the food requirements of Steller sea lions. Our study provided ranges of estimates of food consumption for sea lions that can be used in various modeling strategies to assess the impact of sea lions on prey populations, including commercially exploited fish species. The approaches reviewed in our study shared common difficulties arising from the quantity and quality of data, and the integration of data across scales and species. Our modeling exercise, in particular, identified the major uncertainties involved in estimating the food requirements of each sea lion species using an energetics approach. Our results provide direction for future research aimed at improving the accuracy and comparability of estimates of food consumption for sea lions.
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Molecular scatology as a tool to study diet: analysis of prey DNA in scats from captive Steller sea lions.
Deagle, B.E., D.J. Tollit, S.N. Jarman, M.A. Hindell, A.W. Trites and N.J. Gales. 2005.
Molecular Ecology 14:1831-1842.
abstract
The DNA of prey present in animal scats may provide a valuable source of information for dietary studies. We conducted a captive feeding trial to test whether prey DNA could be reliably detected in scat samples from Steller sea lions (Eumetopias jubatus). Two sea lions were fed a diet of fish (five species) and squid (one species), and DNA was extracted from the soft component of collected scats. Most of the DNA obtained came from the predator, but prey DNA could be amplified using prey-specific primers. The four prey species fed in consistent daily proportions throughout the trial were detected in more than 90% of the scat DNA extractions. Squid and sockeye salmon, which were fed as a relatively small percentage of the daily diet, were detected as reliably as the more abundant diet items. Prey detection was erratic in scats collected when the daily diet was fed in two meals that differed in prey composition, suggesting that prey DNA is passed in meal specific puls! es. Prey items that were removed from the diet following one day of feeding were only detected in scats collected within 48 hours of ingestion. Proportions of fish DNA present in eight scat samples (evaluated through the screening of clone libraries) was roughly proportional to the mass of prey items consumed, raising the possibility that DNA quantification methods could provide semi-quantitative diet composition data. This study should be of broad interest to researchers studying diet since it highlights an approach that can accurately identify prey species and is not dependent on prey hard parts surviving digestion.
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Dietary analysis from fecal samples: how many scats are enough?
Trites, A.W. and Joy, R. 2005.
Journal of Mammalogy 86(4):704-712.
abstract
Diets of mammals are increasingly being inferred from identification of hard parts from prey eaten and recovered in fecal remains (scats). Frequencies with which particular prey species occur among collections of scats are easily compiled to describe the average diet, and can be used to compare diets between and within geographic regions, and across years and seasons. Important to these analyses is the question of statistical power. In other words, how many scats should be collected to compare the diet among and between species? We addressed this problem using Monte Carlo simulations to analytically determine the consequence of sample size on the dietary analysis of scats using frequency of occurrence methods. We considered two questions: 1) how is the statistical power affected by sample size; and 2) what is the likelihood of not identifying a prey species? We randomly sampled predetermined numbers of scats (n=10–200) from computer-generated populations of scats containing prey of known species and frequencies of occurrences. We also randomly sampled a large database of field-collected scats from Steller sea lions (Eumetopias jubatus). We then used standard contingency table tests such as chi-square and Fisher’s exact test to determine whether differences between our samples and populations were statistically significant. We found a minimum size of 59 scats is necessary to identify principal prey remains occurring in >5% of scats. However, 94 samples are required when comparing diets to distinguish moderate effect sizes over time or between areas. These findings have significant implications for the interpretation of published dietary data, as well as for the design of future scat-based dietary studies for pinnipeds and other species.
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Sizes of walleye pollock (Theragra chalcogramma) consumed by the eastern stock of Steller sea lions (Eumetopias jubatus) in Southeast Alaska from 1994-1999.
Tollit, D.J., Heaslip, S.G. and Trites, A.W. 2004.
Fishery Bulletin 102(3):522-532.
abstract
Lengths of walleye pollock (Theragra chalcogramma) consumed by Steller sea lions (Eumetopias jubatus) were estimated using allometric regressions applied to seven diagnostic cranial structures recovered from 531 scats collected in Southeast Alaska between 1994-1999. Selected structural measurements were corrected for loss of size due to erosion using experimentally derived condition-specific digestion correction factors. Correcting for digestion increased the estimated length of fish consumed by 23%, and the average mass of fish consumed by 88%. Mean corrected fork length (FL) of pollock consumed was 42.4 11.6 cm (range=10.0-78.1 cm, n=909). Adult pollock (>45.0 cm FL) occurred more frequently in scats collected from rookeries along the open ocean coastline of Southeast Alaska during June and July (74% adults, mean FL=48.4 cm) than they did in scats from haulouts located in inside waters between October and May (51% adults, mean FL=38.4 cm). Overall, the contribution of juvenile pollock (20 cm) to the sea lion diet was insignificant, while adults contributed 44% to the diet by number and 74% by mass. On average, larger pollock were eaten in summer at rookeries throughout Southeast Alaska than at rookeries in the Gulf of Alaska or the Bering Sea. Overall it appears that Steller sea lions are capable of consuming a wide size range of pollock, with the bulk of fish falling between 20-60 cm. The use of cranial hard parts other than otoliths and the application of digestion correction factors are fundamental to correctly estimating the sizes of prey consumed by sea lions and for determining their overlap with commercial fisheries.
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A method to improve size estimates of walleye pollock (Theragra chalcogramma) and Atka mackerel (Pleurogrammus monopterygius) consumed by pinnipeds: digestion correction factors applied to bones and otoliths recovered in scats.
Tollit, D.J., Heaslip, S.G., Zeppelin, T.K., Joy, R., Call, K.A. and Trites, A.W. 2004.
Fishery Bulletin 102(3):498-508.
abstract
The lengths of otoliths and other skeletal structures recovered from the scats of pinnipeds, such as Steller sea lions (Eumetopias jubatus), correlate with body size and can be used to estimate the length of prey consumed. Unfortunately, otoliths are often found in too few numbers or are too digested to usefully estimate prey size. Techniques are therefore required to account for the degree of digestion of alternative diagnostic bones prior to estimating prey size. We developed a method (using defined criteria and photo-reference material) to assign the degree of digestion for key cranial structures of two prey species (walleye pollock, Theragra chalcogramma and Atka mackerel, Pleurogrammus monopterygius). The method grades each structure into one of three condition categories; good, fair or poor. We also conducted captive feeding trials to determine the extent of erosion and derive condition-specific digestion correction factors to reconstruct the original sizes of the structures consumed. In general, larger structures were relatively more digested than smaller ones. Mean size reduction varied between different types of structures (3.3-26.3%), but was not influenced by the size of the prey consumed. Results from the observations and experiments were combined to reconstruct the size of prey consumed by sea lions and other pinnipeds. The proposed method has four steps: 1) measure the recovered structures and grade the extent of digestion using defined criteria and photo-reference collection; 2) exclude structures graded in poor condition; 3) multiply measurements of structures in good and fair condition by their appropriate digestion correction factors to derive their original size; and 4) calculate the size of prey from allometric regressions relating corrected structure measurements to body lengths. This technique can be readily applied to piscivore dietary studies that use fish hard remains.
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Sizes of walleye pollock and Atka mackerel consumed by the Western stock of Steller sea lions (Eumetopias jubatus) in Alaska from 1998-2000.
Zeppelin, T. K., Tollit, D.J., Call, K.A., Orchard, T. J. and Gudmundson, C. J. 2004.
Fishery Bulletin 102(3):509-521.
abstract
Prey size selectivity by Steller sea lions (Eumetopias jubatus) is relevant for understanding the foraging ecology of this declining predator, but studies have been problematic due to the erosion or absence of prey skeletal structures and otoliths usually used to estimate fish length. We developed regression formulae to estimate fish length from seven diagnostic cranial structures of walleye pollock (Theragra chalcogramma) and Atka mackerel (Pleurogrammus monopterygius). For both species, all structure measurements were related with fork length of prey (r squared range: 0.78 - 0.99). Fork length of walleye pollock and Atka mackerel consumed by Steller sea lions was estimated by applying these regression models to cranial structures recovered from scats (feces) collected between 1998 and 2000 across the range of the Alaskan western stock of Steller sea lions. Experimentally derived digestion correction factors were applied to take into account loss of size due to digestion. Fork lengths (FL) of walleye pollock consumed by Steller sea lions ranged from 3.7 to 70.8 cm FL (mean = 1 39.3 cm, SD = 14.3 cm, n = 1 666) and Atka mackerel ranged from 15.3 to 49.6 cm FL (mean = 1 32.3 cm, SD = 5.9 cm, n = 1,685). Although sample sizes were limited, a greater proportion of juvenile (less than to 20 cm) walleye pollock were found in samples collected on summer (June - September) haul-out sites (64% juveniles, n = 1 11 scats) than on summer rookeries (9% juveniles, n = 1 132 scats) or winter (February - March) haul-out sites (3% juveniles, n = 1 69 scats). Annual changes in the size of Atka mackerel consumed by Steller sea lions corresponded to changes in the length distribution of Atka mackerel resulting from exceptionally strong year classes. Considerable overlap (> 51%) in the size composition of walleye pollock and Atka mackerel taken by Steller sea lions and the commercial trawl fishery was demonstrated.
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Quantifying errors associated with using prey skeletal structures from fecal samples to determine the diet of the Steller sea lion (Eumetopias jubatus).
Tollit, D.J., M. Wong, A.J. Winship, D.A.S. Rosen and A.W. Trites. 2003.
Marine Mammal Science pp. 724-744.
abstract
We examined the digestion and passage times of bones and other hard parts from pollock, herring, salmon, and sandlance recovered from two juvenile captive Steller's sea lions (Eumetopias jubatus) subjected to varying activity levels. Key bones that could be identified to species were distributed over an average of 3.2 scats (range 1–6) following a single meal, with pollock remains occurring in significantly more scats than other species. Relying on otoliths alone to determine the presence of prey resulted in significantly fewer prey being identified than if other structures were also used (such as vertebrae, jaw bones, and teeth), particularly for salmon. Using either technique, there were significant differences in the likelihood that bones would be recovered from the series of scats produced following a meal, with pollock recovery exceeding herring (by three-fold) and sandlance (by eight-fold). Differences between species were reduced when recovery was calculated on a per scat basis rather than over multiple scats. Active animals passed greater numbers of bones, but the overall effect on prey recovery estimates was not significant. Defecation times of prey structures from a meal were variable and ranged from an initial 2–56 h to a final 28–148 h. The time interval to pass 95% of recovered structures varied by a factor of two among prey species, and was highest for pollock due to retention beyond 65 h.
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Classifying prey hard part structures recovered from fecal remains of captive Steller sea lions (Eumetopias jubatus).
Cottrell, P.E. and A.W. Trites. 2002.
Marine Mammal Science 18:525-539.
abstract
Feces were collected from six Steller sea lions (Eumetopias jubatus) that consumed known amounts of Atka mackerel (Pleurogrammus monopterygius), Pacific herring (Clupea harengus), pink salmon (Oncorhynchus gorbuscha), walleye pollock (Theragra chalcogramma), and squid (Loligo opalacens). The goal was to determine the numbers and types of taxon-specific hard parts that pass through the digestive tract and to develop correction factors for certain abundantly occurring structures. Over 20,000 fish and squid were consumed during 267 d of fecal collection. During this period, over 119,000 taxon-specific hard parts, representing 56 different structures, were recovered. Skeletal structures and non-skeletal structures accounted for 72% and 28% of all hard parts respectively. The branchiocranium, axial skeleton, and dermocranium regions of the skeletal system accounted for the greatest number of hard parts recovered. Over 70% of all recovered hard parts were represented by one to six taxa specific structures for each prey type. The average number of hard parts (3.1-3.12) and structure types (2.0-17.7) recovered per individual prey varied across taxa and were used to derive correction factors (to reconstruct original prey numbers). A measure of the variability of hard part recovery among sea lions showed no difference for certain herring, pollock, and squid structures, however, there was a significant difference for salmon and Atka mackerel structures. Identifying all taxon-specific prey hard parts increases the likelihood of identifying and estimating the number of prey consumed.
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Temporal records of d13C and d15N in North Pacific pinnipeds: inferences regarding environmental change and diet.
Hirons, A.C., D.M. Schell and B.P. Finney. 2001.
Oecologia 129:591-601.
abstract
Sea lion and seal populations in Alaskan waters underwent various degrees of decline during the latter half of the twentieth century and the cause(s) for the declines remain uncertain. The stable carbon ( 13 C/12 C) and nitrogen ( 15 N/14 N) isotope ratios in bone collagen from wild Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) from the Bering Sea and Gulf of Alaska were measured for the period 1951–1997 to test the hypothesis that a change in trophic level may have occurred during this interval and contributed to the population declines. A significant change in d 15 N in pinniped tissues over time would imply a marked change in trophic level. No significant change in bone collagen d 15 N was found for any of the three species during the past 47 years in either the Bering Sea or the Gulf of Alaska. However, the 15 N in the Steller sea lion collagen was significantly higher than both northern fur seals and harbor seals. A significant decline in d 13 C (almost 2 ‰ over the 47 years) was evident in Steller sea lions, while a declining trend, though not significant, was evident in harbor seals and northern fur seals. Changes in foraging location, in combination with a trophic shift, may offer one possible explanation. Nevertheless, a decrease in d 13 C over time with no accompanying change in d 15 N suggests an environmental change affecting the base of the foodweb rather than a trophic level change due to prey switching. A decline in the seasonal primary production in the region, possibly resulting from decreased phytoplankton growth rates, would exhibit itself as a decline in d 13 C. Declining production could be an indication of a reduced carrying capacity in the North Pacific Ocean. Sufficient quantities of optimal prey species may have fallen below threshold sustaining densities for these pinnipeds, particularly for yearlings and subadults who have not yet developed adequate foraging skills.
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Assessing the use of hard parts in faeces to identify harbor seal prey: results of captive feeding trials.
Cottrell, P.W., A.W. Trites and E.H. Miller. 1996.
Canadian Journal of Zoology 74:875-880.
abstract
Faeces were collected from four captive harbour seals (Phoca vitulina) that consumed known amounts of herring (Clupea harengus), walleye pollock (Therugru chalcogrumma), Pacific hake (Merluccius productus), surf smelt (Hypomesus pretiosus), and juvenile chinook salmon (Oncorhynchus tshmvytschu). The goal was to determine which structures (hard parts) passed through the digestive tract (e.g., eye lenses, scales, vertebrae, otoliths), and which of these could be used to determine the type and number of fish consumed. Nearly 5000 fish were consumed, from which over 50000 hard parts were recovered from seal faeces. Scales were the most numerous of the 23 structures recovered (> 20 000), followed by vertebrae, eye lenses, and otoliths. Morphological distinctiveness and digestive erosion of the structures varied among fish taxa. Two to five structures accounted for over 90% of the taxon-specific elements recovered, depending upon the species of fish consumed. Otoliths, which are used routinely to characterize pinniped diets, accounted for only 17% of the identified taxon-specific hard parts. The variation in types of structures and rates of recovery across taxa underscores the importance of using several types of hard parts to identify prey. Identifying several different prey structures increases the likelihood of identifying a prey type.
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NUTRITION OF PREY

Season variation in nutrient composition of Alaskan walleye pollock.
Kitts, D. D., Huynhl,M. D., Hu, C. and Trites, A.W. 2004.
Canadian Journal of Zoology 82:1408-1415.
abstract
A popular hypothesis for the noted steady decline in the population of Steller sea lions in the regions from Prince William Sound through the Aleutian Islands relates to their nutritional status. Sea lion diets appear to have shifted from primarily small schooling fatty fishes to low fat fish such as walleye pollock (Theragra chalcogramma). We examined the seasonal changes in proximate nutrients of pollock collected in the Bering Sea. Mean energy density (dry-weight) of pollock peaked in October then declined and remained low throughout winter. Energy recovery occurred in the summer months with strong recovery observed in female fish caught in July. Contrary to whole fish carcass energy contents, both total protein and moisture contents were at their highest levels in winter (January) when total crude lipid content was at its lowest (p<0.05). This trend gradually declined to its lowest levels in the fall, when lipid content was high. The decline in total lipi! ds during winter seasons appeared to parallel gonad development during the pre-spawning period. Sex differences in energy densities were not found. Nor did proximate analysis data for moisture, protein, ash and lipid content show any significant variation between males and females. Protein digestibility of pollock was higher (p<0.05) in the summer than in the spring, but not different for winter or fall seasons. We conclude that the nutrient content of pollock may have some impact on the Steller sea lions that feed on them, particularly the energetic value that appears to be low during important feeding periods for this marine mammal.
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Digestive efficiency and dry-matter digestibility of Steller sea lions fed herring, pollock, salmon and squid.
Rosen, D.A.S. and A.W. Trites. 2000.
Canadian Journal of Zoology 78:234-239.
abstract
Dry-matter digestibility and energy digestive efficiency were measured in six juvenile Steller sea lions (Eumetopias jubatus) fed three diets each consisting of a single species: herring, pollock, and squid. Two of the animals were also fed pink salmon. Dry-matter digestibility (DMD) and digestive efficiency (DE) were measured using the energy and manganese concentration in fecal and food samples. DE values were high for all prey species (herring: 95.4 &amp;plusmn; 0.7% (mean &amp;plusmn; SD), pollock: 93.9 &amp;plusmn; 1.4%, salmon: 93.4 &amp;plusmn; 0.5%, squid: 90.4 &amp;plusmn; 1.3%). Steller sea lions appear to digest prey of high energy density more efficiently than prey of low energy density. DMD values were also high for all prey species (herring: 90.1 &amp;plusmn; 1.8%, pollock: 86.5 &amp;plusmn; 3.4%, salmon: 87.3% &amp;plusmn; 2.6, squid: 90.5 &amp;plusmn; 1.2%). The low DMD value for pollock compared with herring and squid was due to the high proportion of bony material in pollock. There was a strong linear relationship between DE and DMD for each prey type, but the terms cannot be used interchange-ably. DE measures are more meaningful than DMD in conveying the energetic benefits derived by sea lions from dif-ferent types of prey. Species-specific measures of the digestible energy obtained from an array of prey items are a necessary component in understanding the bioenergetic consequences of consuming different prey species.
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Effect of ration size and meal frequency on assimilation and digestive efficiency in yearling Steller sea lions, Eumetopias jubatus.
Rosen, D.A.S., L. Williams and A.W. Trites. 2000.
Aquatic Mammals 26:76-82.
abstract
Assimilation and digestive efficiencies were measured in four juvenile Steller sea lions (Eumetopias jubatus) fed three ration sizes of herring (3%, 6%, or 9% of body mass) at three frequencies (2, 3, or 4 times daily). Assimilation efficiency (dry matter digestive efficiency) was 90.0 ± 2.0% (mean ± 1SD). Digestive efficiency (efficiency of energy digestion) was 95.5 ± 1.0%. There was a strong linear relationship between digestive and assimilation efficiency, but no significant differences in either assimilation or digestive efficiency with changes in feeding frequency or changes in daily food intake within the ranges offered.
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Heat Increment of Feeding in Steller sea lions, Eumetopias jubatus.
Rosen, D.A.S. and A.W. Trites. 1997.
Comparative Biochemistry and Physiology 118A:877-881.
abstract
The heat increment of feeding (HIF) was measured in six captive, juvenile Steller sea lions (Eumetopias jubatus), fed meals of either 2 or 4 kg of herring. HIF was calculated as the post-prandial increase in metabolism above baseline levels, and was measured using open-circuit (gas) respirometry. It averaged 12.4 +/- 0.9% (SE) of ingested energy intake for the 4-kg meal trials, and 9.9 +/- 0.9% for the 2-kg meal size. The effect lasted 8-10 hr for the larger meal size. Metabolism peaked 3.7 hr after feeding, and was 2.13 times higher than baseline levels. For the 2-kg meal size, the effect lasted 6-8 hr, with metabolism peaking 2.8 hr after ingestion at 1.76 times baseline levels. Our estimates of HIF for Steller sea lions are at the lower end of estimates for terrestrial mammals, and are consistent with estimates for other marine mammals.

keywords     digestion, heat increment of feeding, pinnipeds, specific dynamic action, Steller sea lion
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A forage fish is what? Summary of the symposium.
Springer, A.M. and S.G. Speckman. 1997.
In Forage Fishes in marine ecosystems. Univ. of Alaska Sea Grant Program. Report 97-01:773-806.
abstract
The conference was organized around a number of themes that emerged as papers concerning one or more of the interrelated issues of forage fish basic biology, their role as predators and prey, causes of population fluctuations, assessment methodologies, and management considerations. The papers in this volume are grouped according to subject, but many of them contain information on a variety of aspects of forage fish biology and ecology that can only be discovered by examining them all.
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ENERGY NEEDS

Quantifying the costs of dive behaviours and foraging strategies in Steller sea lions (Eumetopias jubatus).
Goundie, E.T. 2015.
MSc Thesis, University of British Columbia, Vancouver, B.C. 80 pages
abstract
Air-breathing divers, such as marine mammals, should adjust their diving behaviours in relation to the depth and density of their prey to minimize the energetic costs and maximize the benefits of foraging. However, there is little experimental data to test these predictions or to develop models to predict the responses of marine mammals to changes in prey availability. The objectives of my study were to 1) determine how changes in prey availability affect dive behaviour and foraging efficiency in Steller sea lions (Eumetopias jubatus) and 2) develop models with data from free-diving captive Steller sea lions to estimate foraging costs in wild animals and evaluate energetic trade-offs between different foraging strategies. I measured the diving metabolic rate, dive durations, and food intake of 4 trained sea lions diving in the open ocean on simulated prey patches of high- or low-densities at 10 m and 40 m. I also measured diving metabolic rates of sea lions performing 4 controlled dive types that allowed me to estimate the separate costs of different dive components (i.e., surface time, bottom time, and transiting to and from depth). I found that animals diving on prey patches with low prey density altered their dive behaviours and spent proportionally less time actively foraging, which ultimately decreased their foraging efficiency. I also found that making single, longer dives were less energetically costly than making multiple shorter dives in a bout, but that the sea lions replenished oxygen stores more efficiently when making a bout of dives. Finally, I determined the metabolic cost of transiting to and from depth (20.5Ä…13.0 ml O2 min-1 kg-1) was greater than the cost of foraging during the bottom portion of a dive (13.5Ä…4.1 ml O2 min-1 kg-1). With these values, I generated a predictive equation to estimate the diving costs of free-ranging animals. Overall, my results indicate that Steller sea lions do alter their dive behaviour in relation to prey availability and that different foraging strategies have different energetic costs. These results can be used to understand how changes in prey availability affect the overall energy balance and health of Steller sea lions.

keywords     Steller sea lion, foraging energetics, diving physiology
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Evidence of partial deferment of digestion during diving in Steller sea lions (Eumetopias jubatus).
Rosen, D.A.S., C.D. Gerlinsky and A.W. Trites. 2015.
Journal of Experimental Marine Biology and Ecology 469:93-97.
abstract
Past foraging success of diving air-breathing vertebrates can adversely affect future foraging capabilities and costs through changes in circulation or increased metabolic costs associated with digestion that are incompatible with efficient diving. This study tested the physiological interaction between digestion and diving by comparing the cost of diving in fasted and pre-fed trained Steller sea lions foraging under controlled conditions in the open ocean. Pre-dive and post-dive surface metabolism and diving metabolic rate were all higher in the pre-fed animals than the fasted animals, indicating an effect of digestion on metabolism. However, the sea lions displayed a significant reduction in the apparent additive effect of digestion during diving. The increase in rate of oxygen consumption associated with digestion was reduced by 54% during diving compared to the increase observed in pre-dive metabolism. This truncation of the additional cost of digestion rapidly disappeared following cessation of diving. The results suggest that Steller sea lions diving to depth demonstrate a partial deferment of digestion while actively foraging and that the classically held view that digestion and diving are incompatible processes may be much more variable and adaptable to specific diving conditions and behaviors than previously thought.
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Identification of prey captures in Australian fur seals (Arctocephalus pusillus doriferus) using head-mounted accelerometers: Field validation with animal-borne video cameras.
Volpov, B.L., A.J. Hoskins, B. Battaile, M. Viviant, K.E. Wheatley, G.J. Marshall, K. Abernathy and J.P.Y. Arnould. 2015.
PloS One Vol 10(6): e0128789
abstract
This study investigated prey captures in free-ranging adult female Australian fur seals (Arctocephalus pusillus doriferus) using head-mounted 3-axis accelerometers and animal-borne video cameras. Acceleration data was used to identify individual attempted prey captures (APC), and video data were used to independently verify APC and prey types. Results demonstrated that head-mounted accelerometers could detect individual APC but were unable to distinguish among prey types (fish, cephalopod, stingray) or between successful captures and unsuccessful capture attempts. Mean detection rate (true positive rate) on individual animals in the testing subset ranged from 67-100%, and mean detection on the testing subset averaged across 4 animals ranged from 82-97%. Mean False positive (FP) rate ranged from 15-67% individually in the testing subset, and 26-59% averaged across 4 animals. Surge and sway had significantly greater detection rates, but also conversely greater FP rates compared to heave. Video data also indicated that some head movements recorded by the accelerometers were unrelated to APC and that a peak in acceleration variance did not always equate to an individual prey item. The results of the present study indicate that head-mounted accelerometers provide a complementary tool for investigating foraging behaviour in pinnipeds, but that detection and FP correction factors need to be applied for reliable field application.

keywords     Arctocephalus pusillus doriferus, accelerometer, prey capture success, foraging behavior, foraging success, pinniped
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Validating the relationship between 3-dimensional body acceleration and oxygen consumption in trained Steller sea lions.
Volpov, B.L., D.A.S. Rosen, A.W. Trites and J.P.Y. Arnould. 2015.
Journal of Comparative Physiology B 185:695-708.
abstract
We tested the ability of overall dynamic body acceleration (ODBA) to predict the rate of oxygen consumption (sVO2) in freely diving Steller sea lions ( Eumetopias jubatus/) while resting at the surface and diving. The trained sea lions executed three dive types―single dives, bouts of multiple long dives with 4-6 dives per bout, or bouts of multiple short dives with 10-12 dives per bout葉o depths of 40 m, resulting in a range of activity and oxygen consumption levels. Average metabolic rate (AMR) over the dive cycle or dive bout calculated was calculated from sVO2. We found that ODBA could statistically predict AMR when data from all dive types were combined, but that dive type was a significant model factor. However, there were no significant linear relationships between AMR and ODBA when data for each dive type was analyzed separately. The potential relationships between AMR and ODBA were not improved by including dive duration, food consumed, proportion of dive cycle spent submerged or number of dives per bout. It is not clear whether the lack of predictive power within dive type was due to low statistical power, or whether it reflected a true absence of a relationship between ODBA and AMR. The average percent error for predicting AMR from ODBA was 7-11%, and standard error of the estimated AMR was 5-32%. Overall, the extensive range of dive behaviours and physiological conditions we tested indicated that ODBA was not suitable for estimating AMR in the field due to considerable error and the inconclusive effects of dive type.

keywords     Steller sea lion, oxygen consumption, overall dynamic body acceleration, activity, oxygen depletion, diving physiology
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Steller sea lions (Eumetopias jubatus) have greater blood volumes, higher diving metabolic rates and a longer aerobic dive limit when nutritionally stressed.
Gerlinsky, C.D., A.W. Trites and D.A.S. Rosen. 2014.
Journal of Experimental Biology 217:769-778.
abstract
Marine mammal foraging behavior inherently depends on diving ability. Declining populations of Steller sea lions may be facing nutritional stress that could affect their diving ability through changes in body composition or metabolism. Our objective was to determine whether nutritional stress (restricted food intake resulting in a 10% decrease in body mass) altered the calculated aerobic dive limit (cADL) of four captive sea lions diving in the open ocean, and how this related to changes in observed dive behaviour. We measured diving metabolic rate (DMR), blood O2 stores, body composition and dive behaviour prior to and while under nutritional restriction. We found that nutritionally stressed sea lions increased the duration of their single long dives, and the proportion of time they spent at the surface during a cycle of four dives. Nutritionally stressed sea lions lost both lipid and lean mass, resulting in potentially lower muscle O2 stores. However, total body O2 stores increased due to rises in blood O2 stores associated with having higher blood volumes. Nutritionally stressed sea lions also had higher mass-specific metabolic rates. The greater rise in O2 stores relative to the increase in mass-specific DMR resulted in the sea lions having a longer cADL when nutritionally stressed. We conclude that there was no negative effect of nutritional stress on the diving ability of sea lions. However, nutritional stress did lower foraging efficiency and require more foraging time to meet energy requirements due to increases in diving metabolic rates and surface recovery times.

keywords     Steller sea lion, blood volume, nutritional stress, diving metabolism, oxygen stores, dive behavior
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Drag, but not buoyancy, affects swim speed in captive Steller sea lions.
Suzuki, I., K. Sato, A. Fahlman, Y. Naito, N. Miyazaki and A. W. Trites. 2014.
Biology Open 3:379-386.
abstract
Swimming at an optimal speed is critical for breath-hold divers seeking to maximize the time they can spend foraging underwater. Theoretical studies have predicted that the optimal swim speed for an animal while transiting to and from depth is independent of buoyancy, but is dependent on drag and metabolic rate. However, this prediction has never been experimentally tested. Our study assessed the effects of buoyancy and drag on the swim speed of three captive Steller sea lions (Eumetopias jubatus) that made 186 dives. Our study animals were trained to dive to feed at fixed depths (10–50 m) under artificially controlled buoyancy and drag conditions. Buoyancy and drag were manipulated using a pair of polyvinyl chloride (PVC) tubes attached to harnesses worn by the sea lions, and buoyancy conditions were designed to fall within the natural range of wild animals (,12–26% subcutaneous fat). Drag conditions were changed with and without the PVC tubes, and swim speeds were recorded and compared during descent and ascent phases using an accelerometer attached to the harnesses. Generalized linear mixed-effect models with the animal as the random variable and five explanatory variables (body mass, buoyancy, dive depth, dive phase, and drag) showed that swim speed was best predicted by two variables, drag and dive phase (AIC=-139). Consistent with a previous theoretical prediction, the results of our study suggest that the optimal swim speed of Steller sea lions is a function of drag, and is independent of dive depth and buoyancy.
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Seasonal resting metabolic rate and food intake of captive Pacific white-sided dolphins (Lagenorhynchus obliquidens).
Rechsteiner, E.U., D.A.S. Rosen and A.W. Trites. 2013.
Aquatic Mammals. 39:241-252.
abstract
Like many marine mammals, Pacific white-sided dolphins (Lagenorhynchus obliquidens) consume prey that change seasonally in numbers, distribution, and energy density. However, it is not known whether these ecological factors are associated with underlying seasonal changes in energy requirements. We investigated these potential seasonal shifts in physiology by measuring resting metabolic rate (a conserved physiological trait) and recording associated daily food energy intake of three captive adult Pacific white-sided dolphins over 12 consecutive months. Two dolphins that met the criteria for measuring resting metabolism had a mean (± SE) mass-specific rate of 0.31 ± 0.0047 MJ kg-1 day- 1 (~34 MJ day-1), which was higher than that of other species of small cetaceans. Resting metabolic rates of Pacific white-sided dolphins did not vary seasonally and, hence, were not related to observed seasonal changes in water or air temperature, total energy intake, or body mass. Overall, resting metabolism accounted for ~70% of total energy intake. However, total food energy intake changed seasonally and was highest during the fall (October to December). While levels of food intake were not predicted by resting metabolic rate, body mass, or water and air temperatures, the increased intake in the fall resulted in the seasonal increase in body mass exhibited by all three dolphins. Our estimates of resting metabolic rates and relative changes in total energy intake can be used to parameterize bioenergetic models needed to estimate the ecological impacts and energetic requirements of Pacific white-sided dolphins in the wild, which will have conservation implications.

keywords     energetics, oxygen consumption, Pacific white-sided dolphins, season, food intake, metabolic rate
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Seasonal oscillations in the mass and food intake of Steller sea lions.
Allen, P.C. 2009.
MSc thesis, University of British Columbia, Vancouver. 154 pages
abstract
Morphometric measurements and daily feeding records of 62 captive Steller sea lions (Eumetopias jubatus) were analyzed to provide information about seasonal growth and food consumption that has been impossible to collect from wild animals. Data from nursing pups, intact and castrated males, pregnant, lactating and non-reproductive females were also used to determine differences in rates of maturity between males and females, and the effects that climate, sexual maturity, castration and pregnancy and lactation have on growth and food intake. Data were fit with seasonal (sine function) and annual (von Bertalanffy, logistic, Gompertz, Richard’s and maturity) growth models, and showed that males achieved larger body sizes than females by undergoing a growth spurt during puberty and by extending their growth throughout adulthood. Annual increases in the length and mass of females slowed significantly following sexual maturity. Males and females both experienced seasonal oscillations in body mass, but the seasonal fluctuation in male mass peaked later (April) and was far more dramatic than that of females. The mass of lactating and non-reproductive females peaked in early spring (March), while increases in the mass of pregnant females paralleled fetal growth, reaching a maximum before parturition. Changes in mass did not parallel changes in consumption. Fish intake by males and females peaked during winter and bottomed during late spring, while seasonal changes in body mass reached their high and low 3 to 4 months later than food intake. Pregnant and non-reproductive females differed little in the amount of prey they consumed, unlike lactating females that significantly increased their consumption during summer and winter. The differences between females highlight the relatively low additional energetic requirements of pregnancy and the high costs of lactation. Differences between neutered and intact males further suggest that testosterone affected overall male growth, but had smaller effects of seasonal oscillations in mass and did not affect food intake. The reproductive cycle and thermoregulatory requirements appeared to drive seasonal changes in body mass and food intake of male and female Steller sea lions but at different time scales. Our findings also indicate that mass is not a simple reflection of food intake, which has important implications for future nutritional research and bioenergetic modeling of wild pinnipeds.
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Seasonal differences in biochemical adaptation to fasting in juvenile and subadult Steller sea lions (Eumetopias jubatus).
Rea, L.D., M. Berman-Kowalewski, D.A.S. Rosen, and A. W.Trites. 2009.
Physiological and Biochemical Zoology 82:236-247.
abstract
Nine Steller sea lions (Eumetopias jubatus) aged 1.756 yr were experimentally fasted for 714 d during the breeding and nonbreeding seasons to identify changes in plasma metabolites that are indicative of fasting and to determine whether the ability of sea lions to fast varies seasonally or with age. Although some animals approached the limit of their protein-sparing ability by the end of our fasting experiments, there was no sign of irreversible starvation biochemistry. Plasma blood urea nitrogen (BUN) concentrations decreased in all animals within the first week of fasting, reflecting a shift to a fasting-adapted state; however, significant increases in plasma BUN concentration at the end of the nonbreeding season fasts suggest that subadult Steller sea lions were not able to maintain a protein-sparing metabolism for a full 14 d during the nonbreeding season. In contrast, juveniles were able to enter protein sparing sooner during the nonbreeding season when they had slightly higher initial percent total body lipid stores than during the breeding season. Subadult and juvenile sea lions had low circulating ketone body concentrations compared with young sea lion pups, suggesting an age-related difference in how body reserves are utilized during fasting or how the resulting metabolites are circulated and catabolized. Our data suggest that metabolite concentrations from a single blood sample cannot be used to accurately predict the duration of fast; however, threshold metabolite concentrations may still be useful for assessing whether periods of fasting in the wild are unusually long compared with those normally experienced.
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Fasting affects the surface and diving metabolic rates of Steller sea lions (Eumetopias jubatus).
Svärd, C., A. Fahlman, D.A.S. Rosen, R. Joy and A.W. and Trites. 2009.
Aquatic Biology 8:71-82.
abstract
Changes in metabolic rates were measured in 3 captive female Steller sea lions (Eumetopias jubatus) that experienced fasts during summer and winter. Metabolic rates were measured (via O2 consumption) before (MRs, surface) and after (DMR, dive + surface interval) the sea lions dove to 10–50 m depths. Measurements were obtained prior to 9-10 day fasts, and following a 14 day recovery period. The sea lions lost significantly more body mass (Mb) during the winter fast (10.6%), compared with the summer (9.5%). Mass-corrected dive metabolic rate (cDMR = DMR • Mb-0.714) was not affected by dive depth or duration, but increased significantly following the winter fasts (13.5 ± 8.1%), unlike the decrease during summer (-1.1 ± 3.2%). However, mass-corrected surface metabolic rate (cMRs) decreased significantly after both the summer (-16.4 ± 4.7%) and winter (-8.0 ± 9.0%) fasts. Consequently, the ratio between cDMR and cMRc was significantly higher in winter, suggestive of an increased thermal challenge and convective heat loss while diving. Increased cDMs following the fast indicated that digestion began during foraging and was not deferred, implying that access to ingested energy was of higher priority than optimizing diving ability. cDMR was elevated throughout the recovery period, independent of season, resulting in a 12% increase in foraging cost in winter and a 3% increase in summer. Our data suggest that Steller sea lions are more sensitive to changes in body condition due to food shortages in the winter compared with the summer.
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Hormone changes indicate that winter is a critical period for food shortages in Steller sea lions.
Rosen, D.A.S., Kumagai, S. 2008.
Journal of Comparative Physiology B 178:573-583.
abstract
Given that many marine mammals display seasonal energetic priorities, it is important to investigate whether the impact of unexpected food restriction differs during the year. Steller sea lions (Eumetopias jubatus) fed restricted diets for up to 9 days during spring, summer, fall, and winter lost an average of 10% of their initial body mass. We tracked changes in the levels of three hormones (cortisol, total thyroxine—TT4, total triiodothyronine—TT3) and one blood metabolite (blood urea nitrogen—BUN) following a food restriction in relation to season, body mass, body composition, and metabolism. Degree of changes in cortisol, TT3, and BUN after food restriction was significantly affected by season. The greatest changes in cortisol (+231%), BUN (+11.4%), TT4 (-23.3%), and TT3 (-35.6%) occurred in the winter (November/December) when rates of body mass loss were also greatest. Changes in cortisol levels were positively related to total body mass loss, while changes in TT3 levels were negatively related. While greater increases in BUN were related to greater rates of mass loss, the use of BUN levels as an indicator of metabolic state is complicated by the type and level of food intake. The observed changes in hormone levels support morphological data suggesting Steller sea lions may be more strongly impacted by short-term, reduced energy intake during winter than at other times of the year.
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Effects of body condition on resting metabolism in captive and free-ranging Steller sea lions (Eumetopias jubatus).
Hoopes, L.A., L.D. Rea, D.A.S. Rosen and G.A.J. Worthy. 2004.
Symposia of the Comparative Nutrition Society 2004 5:79-82.
abstract
The objectives of this study were to compare Resting Metabolic Rate (RMR) from animals in the eastern and western Alaskan populations to discern whether there is any evidence of nutritional stress. Oxygen consumption data were collected from captive Steller sea lions held at the Vancouver Aquarium, Vancouver, BC and from free-ranging Steller sea lions captured from western and eastern Alaskan stocks. In water, RMR ranged from 33.3 to 56.7 MJ/day for sub-adult animals (109-158 kg, 2.9-4.6 times predicted for an adult animal) and from 20.0 to 26.6 MJ/day for pups (57-59 kg, 3.3-4.3 times predicted) at 2°C. RMR, generally decreased with increasing water temperature, but the relationship was not statistically significant. Reduced body condition had a noticeable impact on RMR in juvenile sea lions at colder water temperatures. The results of the present study suggest that young sea lions would be subject to even greater thermoregulatory demands if their body condition were reduced.
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No evidence for bioenergetic interaction between digestion and thermoregulation in Steller sea lions, Eumetopias jubatus.
Rosen, D.A.S. and A.W. Trites. 2003.
Physiological and Biochemical Zoology 76(6):899-906.
abstract
The increase in metabolism during digestion—the heat increment of feeding—is often regarded as an energetic waste product. However, it has been suggested that this energy could offset thermoregulatory costs in cold environments. We investigated this possibility by measuring the rate of oxygen consumption of four juvenile Steller sea lions (Eumetopias jubatus) before and after they ingested a meal in water temperatures of 2-8 degrees C. Rates of oxygen consumption of fasted and fed animals increased in parallel with decreasing water temperature, such that the apparent heat increment of feeding did not change with water temperature. These results suggest that Steller sea lions did not use the heat released during digestion to offset thermoregulatory costs.
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Prey consumption of Steller sea lions (Eumetopias jubatus) off Alaska: how much prey do they require?
Winship, A.J. and A.W. Trites. 2003.
Fishery Bulletin 101:147-163.
abstract
The effects of seasonal and regional differences in diet composition on the food requirements of Steller sea lions (Eumetopias jubatus)were estimated by using a bioenergetic model. The model considered differences in the energy density of the prey, and differences in digestive effciency and the heat increment of feeding of different diets. The model predicted that Steller sea lions in southeast Alaska required 45–60% more food per day in early spring (March) than after the breeding season in late summer (August) because of seasonal changes in the energy density of the diets (along with seasonal changes in energy require ments).The southeast Alaska population,at 23,000 (±1660 SD)animals (all ages), consumed an estimated 140,000 (±27,800) of prey in 1998. In contrast, we estimated that the 51,000 (±3680) animals making up the western Alaska population in the Gulf of Alaska and Aleutian Islands consumed just over twice this amount (303,000 [±57,500 ] t). In terms of biomass removed in 1998 from Alaskan waters,we estimated that Steller sea lions accounted for about 5% of the natural mortality of gadids (pollock and cod) and up to 75% of the natural mortality of hexagram mids (adult Atka mackerel).These two groups of species were consumed in higher amounts than any other.The predicted average daily food require ment per individual ranged from 16 (±2.8)to 20 (±3.6)kg (all ages com bined). Per capita food requirements differed by as much as 24% between regions of Alaska depending on the rel ative amounts of low–energy-density prey (e.g.gadids)versus high–energy density prey (e.g. forage fish and salmon)consumed. Estimated require ments were highest in regions where Steller sea lions consumed higher proportions of low—energy-density prey and experienced the highest rates of population decline.
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Modeling the energetics of Steller sea lions (Eumetopias jubatus) along the Oregon coast.
Malavear, M. Y. G. 2002.
M.Sc thesis, Newport, Oregon, USA. 124 pages
abstract
A dynamic bioenergetic model for Steller sea lions (Eumetopias jubatus) was built using the STELLA simulation modeling system. The model is intended as an aid for the exploration of ecological questions regarding growth and survival of immature Steller sea lions (ages 1-3) living along the Oregon coast under different nutritional scenarios. The ultimate goals were: 1) to identify features of the Oregon ecosystem that could contribute to the growth of the Steller sea lion population in contrast to the declining population in Alaska and 2) to provide a basis for examining the various hypotheses that have been put forward regarding the causes of the Steller sea lion decline in Alaska. The dynamic energetic model was composed of coupled submodels, created or adapted from the literature, that describe the energetic inputs and outputs of the animal. It is a mechanistic model based on biological principles that attempts to describe the connections and feedbacks between the different components and the allocation of energy to them under suboptimal nutrition. The model predicted that both changes in prey abundance and quality would have a more pronounced effect in one-year-old animals than in two- and three-year-old sea lions. A reduction in prey density could delay the attainment of sexual maturity, and this could have a significant negative effect on the population rate of increase. The seasonal migration of Pacific whiting was shown to be very important as a biomass influx into the system. In general, the model predictions were consistent with observations on the declining population of Steller sea lions in Alaska.
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A bioenergetic model for estimating the food requirements of Steller sea lions (Eumetopias jubatus) in Alaska.
Winship, A.J., A.W. Trites and D.A.S. Rosen. 2002.
Marine Ecology Progress Series 229:291-312.
abstract
A generalized bioenergetic model was used to estimate the food requirements of Steller sea lions <i>Eumetopias jubatus</i> in Alaska, USA. Inputs included age and sex-specific energy require-ments by date, population size and composition, and diet composition and energy content. Error in model predictions was calculated using uncertainty in parameter values and Monte Carlo simulation methods. Our model suggests that energy requirements of individuals were generally lowest in the summer breeding season (June to August) and highest in the winter (December to February) and spring (March to May) mainly due to changes in activity budgets. Predicted relative daily food requirements were highest for young animals (12 ± 3% SD and 13 ± 3% of body mass for 1 yr old males and females respectively) and decreased with age (5 ± 1% and 6 ± 1% of body mass for 14 yr old males and 22 yr old females respectively). The mean daily food requirement of pregnant females predicted by the model was only marginally greater than the predicted mean daily food requirement of non-pregnant females of the same age. However, the model suggested that the mean daily food requirement of females nursing pups was about 70% greater than females of the same age without pups. Of the 3 sets of model parameters (diet, population, and bioenergetic), uncertainty in diet and bioenergetic parameters resulted in the largest variation in model predictions. The model provides a quantitative estimate of the Steller sea lion population’s food requirements and also suggests directions for future research.
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Estimates of basal metabolic and feeding rates for marine mammals from measurements of maximum body length.
Hunter, A.M.J., A.W. Trites and D. Pauly. 2000.
In C.L.K. Baer (ed.), Proceedings of the Third Comparative NutritionSociety Symposium. Pacific Grove, California, August 4-9, 2000. 3:103-106.
abstract
Compared to terrestrial mammals, marine mammals are generally perceived as having elevated metabolic rates and insatiable appetites, attributable to maintaining their high body core temperatures in a cold aquatic environment. The perception that marine mammals have higher metabolic rates than terrestrial mammals of similar body size is reinforced by a substantial body of literature that dates over half a century (Sergeant, 1973; Lavigne, 1982) and is further supported by reports of captive marine mammals ingesting large quantities of food (Sergeant, 1969, 1973; Bonner, 1982). However, within the past two decades, this convention has been challenged. Lavigne et al. (1986) failed to reject the hypothesis that physically mature phocids (true seals) have similar basal metabolic rates (BMRs) as terrestrial mammals of similar body weight, when measured under standard conditions. Innes et al. (1987) found similar results when comparing feeding rates (FRs) of seals and whales. However, much research has been conducted on the FRs and BMRs of marine mammals since these studies were completed. In our study, we re-investigated whether basal metabolic and feeding rates of marine mammals are similar to those predicted for terrestrial mammals. We also explored relationships between taxa and were able to predict the basal metabolic rates of species of marine mammals not previously studied. These estimates can also be used to assess the amount of prey consumed by species of marine mammals whose metabolisms have never been determined in the field or in the lab.
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EFFECT OF CHANGES IN PREY QUANTITY

Quantifying the costs of dive behaviours and foraging strategies in Steller sea lions (Eumetopias jubatus).
Goundie, E.T. 2015.
MSc Thesis, University of British Columbia, Vancouver, B.C. 80 pages
abstract
Air-breathing divers, such as marine mammals, should adjust their diving behaviours in relation to the depth and density of their prey to minimize the energetic costs and maximize the benefits of foraging. However, there is little experimental data to test these predictions or to develop models to predict the responses of marine mammals to changes in prey availability. The objectives of my study were to 1) determine how changes in prey availability affect dive behaviour and foraging efficiency in Steller sea lions (Eumetopias jubatus) and 2) develop models with data from free-diving captive Steller sea lions to estimate foraging costs in wild animals and evaluate energetic trade-offs between different foraging strategies. I measured the diving metabolic rate, dive durations, and food intake of 4 trained sea lions diving in the open ocean on simulated prey patches of high- or low-densities at 10 m and 40 m. I also measured diving metabolic rates of sea lions performing 4 controlled dive types that allowed me to estimate the separate costs of different dive components (i.e., surface time, bottom time, and transiting to and from depth). I found that animals diving on prey patches with low prey density altered their dive behaviours and spent proportionally less time actively foraging, which ultimately decreased their foraging efficiency. I also found that making single, longer dives were less energetically costly than making multiple shorter dives in a bout, but that the sea lions replenished oxygen stores more efficiently when making a bout of dives. Finally, I determined the metabolic cost of transiting to and from depth (20.5Ä…13.0 ml O2 min-1 kg-1) was greater than the cost of foraging during the bottom portion of a dive (13.5Ä…4.1 ml O2 min-1 kg-1). With these values, I generated a predictive equation to estimate the diving costs of free-ranging animals. Overall, my results indicate that Steller sea lions do alter their dive behaviour in relation to prey availability and that different foraging strategies have different energetic costs. These results can be used to understand how changes in prey availability affect the overall energy balance and health of Steller sea lions.

keywords     Steller sea lion, foraging energetics, diving physiology
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Assessment of competition between fisheries and Steller sea lions in Alaska based on estimated prey biomass, fisheries removals and predator foraging behaviour.
Hui, T.C.Y., R. Gryba, E.J. Gregr and A.W. Trites. 2015.
PLoS ONE Vol 10(5): e0123786
abstract
A leading hypothesis to explain the dramatic decline of Steller sea lions (Eumetopias jubatus) in western Alaska during the latter part of the 20th century is a change in prey availability due to commercial fisheries. We tested this hypothesis by exploring the relationships between sea lion population trends, fishery catches, and the prey biomass accessible to sea lions around 33 rookeries between 2000 and 2008. We focused on three commercially important species that have dominated the sea lion diet during the population decline: walleye pollock, Pacific cod and Atka mackerel. We estimated available prey biomass by removing fishery catches from predicted prey biomass distributions in the Aleutian Islands, Bering Sea and Gulf of Alaska; and modelled the likelihood of sea lions foraging at different distances from rookeries (accessibility) using satellite telemetry locations of tracked animals. We combined this accessibility model with the prey distributions to estima te the prey biomass accessible to sea lions by rookery. For each rookery, we compared sea lion population change to accessible prey biomass. Of 304 comparisons, we found 3 statistically significant relationships, all suggesting that sea lion populations increased with increasing prey accessibility. Given that the majority of comparisons showed no significant effect, it seems unlikely that the availability of pollock, cod or Atka mackerel was limiting sea lion populations in the 2000s.

keywords     Eumetopias jubatus, walleye pollock, Pacific cod, Atka mackerel, accessibility, prey distribution, CPUE, linear mixed-effects models
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A model to predict fasting capacities and utilization of body energy stores in weaned Steller sea lions (Eumetopias jubatus) during periods of reduced prey availability.
Noren, D.P., L.D. Rea, and T.R. Loughlin. 2009.
Canadian Journal of Zoology 87:852-864.
abstract
The population decline of Steller sea lions (Eumetopias jubatus (Schreber, 1776)) may be linked to a decline in juvenile survivorship. Limitations in prey availability may contribute to the decline, thus it is important to understand fast- ing capacities of Steller sea lions. For most mammals, fat catabolism is the preferred energetic pathway to ensure that pro- tein is spared. However, marine mammals also have a conflicting requirement to conserve fat because the main site of fat storage is the blubber layer, which is also their primary thermal barrier when at sea. We developed a dynamic state varia- ble model to demonstrate how protein and fat reserve utilization and maximum fasting duration are influenced by body condition and time spent foraging. This model was parameterized with respect to conditions faced by juvenile and subadult Steller sea lions foraging unsuccessfully during a period of reduced prey availability. The model accurately predicted changes in fat and protein mass of juvenile and subadult Steller sea lions fasting in captivity. Furthermore, the model demonstrated that body lipid content, body mass, and the proportion of time spent in water influence energy reserve catabolism and maximum fasting durations. Consequently, small, lean individuals are particularly susceptible to reductions in prey availability.
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Steller sea lion foraging response to seasonal changes in prey availability.
Sigler, M.F., D.J. Tollit, J.J. Vollenweider, J.F. Thedinga, D.J. Csepp, J.N. Womble, M.A. Wong, M.J. Rehberg and A.W. Trites. 2009.
Marine Ecology Progress Series 388:243-261.
abstract
We hypothesized that: (1) Steller sea lion Eumetopias jubatus diet choice is a function of prey availability, (2) sea lions move to take advantage of times and locations of seasonal prey concentrations and (3) the number present depends on the amount of prey available (numerical response). Over 3 yr, typically on a quarterly basis, in Frederick Sound, SE Alaska, multiple measurements were taken of Steller sea lion abundance (aerial surveys), diet (scats), dive behavior (satellite telemetry)and prey availability and caloric density (nearshore, pelagic and demersal fish surveys). We found that Steller sea lions shifted diet composition in response to changes in prey availability of pollock Theragra chalcogramma, hake Merluccius productus, herring Clupea pallasi and salmon Oncorhynchus spp. They selected intermediate-sized fish and avoided small (<10 cm) and large (>60 cm) fish, and moved between areas as prey became available seasonally. The number of sea lions present depended on the amount of prey available; a standing biomass of 500 to 1700 t of prey in a nonbreeding area such as Frederick Sound, depending on species composition, can attract and sustain about 500 sea lions. Pollock was more frequent in sea lion diet in inside waters of SE Alaska including Frederick Sound, Stephens Passage and Lynn Canal than anywhere else in Alaska and contributed about one-third of the dietary energy in Frederick Sound. This finding implies that a diet with substantial year-round contributions from less nutritious, but abundant prey such as pollock can form part of a healthy diet as long as more nutritious prey such as herring, salmon or eulachon Thaleichthys pacificus also are consumed. Our study supports the conclusion that the Steller sea lion is an opportunistic marine predator with a flexible foraging strategy that selects abundant, accessible prey and shifts among seasonally available species.
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Utilization of stored energy reserves during fasting varies by age and season in Steller sea lions.
Rea, L.D., D.A.S. Rosen and A.W Trites. 2007.
Canadian Journal of Zoology 85:190-200.
abstract
Nine captive Steller sea lions (Eumetopias jubatus (Schreber, 1776), 1.75–6 years of age) were fasted for 7–14 d to test the effect of short-term fasting on changes in body mass and body condition. Trials were repeated during both the summer breeding season and the nonbreeding season in seven animals to elucidate whether there was a seasonal component to the ability of Steller sea lions to adapt to limited food resources. Mean percent mass loss per day was higher during the breeding season in juveniles (1.8% ± 0.2%·d–1) than in subadults (1.2% ± 0.1%·d–1), but there were no significant age-related differences during the nonbreeding season (juveniles, 1.5% ± 0.3%·d–1; subadults, 1.7% ± 0.3%·d–1). A decrease in the rate of mass loss occurred after the first 3 d of fasting only in subadults during the breeding season. Percent total body lipid ranged from 11% to 28% of total body mass at the initiation of fasting trials. Animals with lower initial percent total body lipid exhibited higher subsequent rates of mass loss and a lower percentage of tissue catabolism derived from lipid reserves. There was no evidence of metabolic adaptation to fasting in juveniles, which suggests that juvenile sea lions would be more negatively impacted by food limitation during the breeding season than would subadults.
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Body mass and composition responses to short-term low energy intake are seasonally dependent in Steller sea lions (Eumetopias jubatus).
Kumagai, S., D.A.S Rosen and A.W. Trites. 2006.
Comparative Biochemistry and Physiology 179:589-598.
abstract
Steller sea lions (Eumetopias jubatus) were fed restricted iso-caloric amounts of Pacific herring (Clupea pallasi) or walleye pollock (Theragra chalcogramma) for 8-9 days, four times over the course of a year to investigate effects of season and prey composition on sea lion physiology. At these levels, the sea lions lost body mass at a significantly higher rate during winter (1.6 ± 0.14 kg d-1), and at a lower rate during summer (1.2 ± 0.32 kg d-1). Decreases in body fat mass and standard metabolic rates during the trials were similar throughout the seasons and for both diet types. The majority of the body mass that was lost when eating pollock derived from decreases in lipid mass, while a greater proportion of the mass lost when eating herring derived from decreases in lean tissue, except in the summer when the pattern was reversed. Metabolic depression was not observed during all trials despite the constant loss of body mass. Our study supports the hypothesis that restricted energy intake may be more critical to Steller sea lions in the winter months, and that the type of prey consumed (e.g., herring or pollock) may have seasonally-specific effects on body mass and composition.
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Seasonal differences in physiology of captive Steller sea lions (Eumetopias jubatus) in response to short-term low energy intake.
Kumagai, S. 2004.
University of British Columbia, Vancouver BC. 95 pages
abstract

Steller sea lions (Eumetopias jubatus) were fed restricted iso-caloric amounts of Pacific herring (Clupea pallasi) or walleye pollock (Theragra chalcogramma) for 8-9 days, four times a year. At these levels, the sea lions lost an average of 10.1% of their initial body mass while on both experimental diets for up to nine days, but at a significantly higher rate in winter and at a lower rate in summer. Decreases in body fat mass and standard metabolic rates during the trials were similar throughout the seasons and for both diets. Metabolic depression was not always observed during the trials despite the constant loss of body mass. Changes in cortisol, triiodothyronine and blood urea nitrogen (BUN) were seasonally dependent. Over the course of the trials, serum levels of cortisol and BUN increased and total triiodothyronine decreased the most in winter. Serum cortisol levels correlated negatively with both body mass and body condition suggesting that cortisol may play an important role in body fat regulation in Steller sea lions. The mean ghrelin level in Steller sea lions correlated negatively with body mass, but ghrelin did not correlate with serum leptin. My findings support the hypothesis that restricted energy intake at different times of the year differentially affects Steller sea lions, and that diet type (herring or pollock) may have seasonally-specific effects on body mass and composition. Steller sea lions may be more severely impacted by reduced energy intake in winter than at other times of the year.

Changes in iron binding capacity were significantly greater in the herring-fed group than in the pollock-fed group, and a significantly greater decrease occurred in winter and spring compared to summer and fall. Iron saturation increased in the herring-fed group and decreased in the group fed pollock. These results suggested a potential anemia from a reduced diet of pollock in Steller sea lions. Serum iron, phosphorus, hematocrit and gamma glutamyltransferase showed consistent changes during food restriction, suggesting that these may serve as indicators of nutritional stress in Steller sea lions.


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The effects of prey availability on pup mortality and the timing of birth of South American sea lions (Otaria flavescens) in Peru.
Soto, K., A.W. Trites, and M. Arias-Schreiber. 2004.
Journal of Zoology 264:419-428.
abstract
Pup mortality and the timing of birth of South American sea lions Otaria flavescens were investigated to determine the possible relationship between fluctuations in prey availability in the Peruvian upwelling ecosystem and current and future reproductive success of sea lions during six consecutive breeding seasons. Our study from 1997 to 2002 encompassed the strongest El Nino on record and one La Nina event. Pup mortality ranged from 13% before El Nino to 100% during El Nino, and was negatively correlated with prey availability. Abortions were also more frequent when prey availability was low. However, pup mortality remained high following El Ni~no due to the punctuated short-term effects it had on population dynamics and subsequent maternal behaviour. Births occurred later in the season after years of low food availability and earlier following years of high food availability. The peak of pupping coincided with the peak of mortality in all years, and may have ! been the product of intensive competition between bulls at the peak of the breeding season. The stronger and more frequent El Ninos that appear to be occurring along the Peruvian coast may produce significant stochastic changes in future births and pup mortality, which may place the vulnerable South American sea lion population in Peru at greater risk.
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Changes in metabolism in response to fasting and food restriction in the Steller sea lion (Eumetopias jubatus).
Rosen, D.A.S. and A.W. Trites. 2002.
Comparative Biochemistry and Physiology. 132:389-399.
abstract
Many animals lower their resting metabolism (metabolic depression) when fasting or consuming inadequate food. We sought to document this response by subjecting five Steller sea lions to periods of: (1) complete fasting; or (2) restricting them to 50% of their normal herring diet. The sea lions lost an average of 1.5% of their initial body mass per day (2.30 kg y d )during the 9 –14-day fast, and their resting metabolic rates decreased 31%, which is typical of a ‘fasting response ’. However, metabolic depression did not occur during the 28-day food restriction trials,despite the loss of 0.30% of body mass per day (0.42 kg y d). This difference in response suggests that undernutrition caused by reduced food intake may stimulate a ‘hunger response ’, which in turn might lead to increased foraging effort. The progressive changes in metabolism we observed during the fasts were related to, but were not directly caused by, changes in body mass from control levels. Combining these results with data collected from experiments when Steller sea lions were losing mass on low energy squid and pollock diets reveals a strong relationship between relative changes in body mass and relative changes in resting metabolism across experimental conditions.While metabolic depression caused by fasting or consuming large amounts of low energy food reduced the direct costs from resting metabolism, it was insufficient to completely overcome the incurred energy deficit.
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Metabolic response to fasting in 6-week-old Steller sea lion pups (Eumetopias jubatus).
Rea, L.D., D.A.S. Rosen and A.W. Trites. 2000.
Canadian Journal of Zoology 78:890-894.
abstract

Four Steller sea lions (Eumetopias jubatus) aged 6 weeks were fasted for 2.5 d to determine how young pups mobilize energy reserves during short periods of fasting similar to those experienced in the wild. At 6 weeks of age, the pups lost 5.1 ± 0.3% of their body mass during 2 d of fasting, with an average daily mass loss of 0.7 ± 0.1 kg·d –1 . Plasma blood urea nitrogen (BUN) concentration increased significantly from 3.0 ± 0.1 mM, after an over-night fast, to 4.8 ± 0.5 mM, after 2.5 d of fasting. It is apparent that BUN levels are quickly depressed, since after only an overnight fast, these pups showed BUN levels 2- to 4-fold lower than those measured after the same pups, when 9 months of age, had recently been fed fish. Plasma ketone body (b-HBA) concentrations of the 6-week-old pups increased significantly from 0.32 ± 0.08 to 0.42 ± 0.08 mM between 0.5 and 1.5 d of fasting. There was no significant change in mean plasma concentration beyond 1.5 d, owing to variable individual responses to extended fasting. Plasma b-HBA levels at 9 months of age ranged from 0.07 to 0.18 mM. Six-week-old Steller sea lion pups showed blood chemistry consistent with metabolic adaptation to fasting within 16 h but were unable to sustain a protein-sparing metabolism for a prolonged period. The pups appeared to revert to protein catabolism after only 2.5 d of fasting. This infers a decrease in lipid catabolism that might be due to the depletion of available lipid resources.


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Morphometric measurements and body condition of healthy and starving Steller sea lion pups (Eumetopias jubatus).
Trites, Andrew W. and Remco A.H. Jonker. 2000.
Aquatic Mammals 26:151-157.
abstract
The thickness and weight of skin, blubber, and body core were measured from 12 dead Steller sea lion pups (Eumetopias jubatus). These necropsied pups represented a wide range of body sizes and conditions (small to large, and fat to no-fat), and were chosen to compare the relative body conditions of healthy and starved pups. Seven of the pups lacked blubber and were significantly lighter for a given length compared to the five that had fat at their time of death. Volume exceeded mass by a factor of 1.3% with density averaging 0.987g cm-3. Skin and blubber were not uniformly thick over the body surface. Skin was thinnest on the head and around the flippers (3mm), and became thicker towards the rump (5mm). Skin thickness did not differ between dorsal and ventral sides, unlike blubber, which was thickest on the ventral side, increasing from the snout (1.5mm)to midtrunk (7mm) and decreasing posteriorly (5mm at the tail). Along the back, blubber increased from 1 mm at the snout to about 4.5mm at mid-trunk. The five pups that died of trauma had about 13% skin and 10% blubber (expressed as a proportion of total body mass). Starvelings lost an estimated 43% of their body mass before dying (10% blubber, and 33% body core). Morphometric measurements applied to three proposed indices of body condition suggest that girth is not a good predictor of body condition for Steller sea lion pups. Only the ratio of observed to predicted body mass derived from standardized mass-length relationships could distinguish starvelings from pups with body fat.

keywords     morphometric measurements, body condition, Steller sea lions, pups, skin, volume, density, starvation, #2
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Seasonal differences in adaptation to prolonged fasting in juvenile Steller sea lions (Eumetopias jubatus).
Rea, L.D., D.A.S. Rosen and A.W. Trites. 1999.
In The FASEB Journal (Federation of American Societies of Experimental Biology). Washington, D.C., April 17-21, 1999. Vol 13(5) pp. A740
abstract
Five juvenile Steller sea lions (Eumetopias jubatus) between the ages of 3 and 4 years were experimentally fasted for 9 to 14 d to assess changes in mass and in key plasma metabolites indicative of biochemical adaptation to fasting. The 5 sea lions lost 20.4 to 35.1 kg each, at a rate of 1 to 2% of their initial body mass per day. Two animals fasted during the natural breeding season (June) exhibited a mean daily loss of 1.6 +/- 0.1kg d-1. This was significantly lower than the mean 2.8 +/- 0.1kg d-1 lost by sea lions fasted outside the normal breeding season in April, October and November (p<0.001). The two sea lion studied in June maintained low BUN concentrations throughout the remainder of the study, while the remaining 3 animals showed significant increases after 7 d of fasting. Only the two juveniles fasted during the breeding season maintained a protein sparing metabolism, typical of the species adapted to long-term fasting. With the exception of the smallest female (after 12 d of fasting), ketone body levels ranged from 0.03 to 0.17 mM. Seasonal differences in how sea lions adapt to fasting suggests that these animals would be more severely impacted by limited food resources during the non-breeding season.
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Changes in metabolism in response to varying energy intake in a marine mammal, the Steller sea lion.
Rosen, D.A.S. and A.W. Trites. 1998.
In Proceedings of the Comparative Nutrition Society, Number 2. pp. 182-187.
abstract
When faced with decreases in energy intake, an animal has two conditions, hunger stimulates increased foraging activity, a strategy of short-term expenditure off-set by a reasonable-expectation of foraging success. However, when faced wit increased energy h periods of predictable or prolonged shortages of energy intake (although not necessarily energy availability), the animal should limit energy expenditures. The most common response to experimental undernutrition or fasting in homeotherms is metabolic depression. Invoking such physiologic responses that limitenergy expenditures limits tissue loss and delays death by starvation. Some species of marine mammals have exhibited metabolic depression, although its occurrence, scope, and triggers are still unclear. This study was designed to document the extent of metabolic depression in Steller sea lions. It investigated the role of energy and food intake on metabolic depression, and the relationship between changes in body mass and the scope of metabolic depression.
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EFFECT OF CHANGES IN PREY QUALITY

Low prey abundance leads to less efficient foraging behaviour in Steller sea lions.
Goundie, E.T., D. A. S. Rosen and A.W. Trites. 2015.
Journal of Experimental Marine Biology and Ecology 470:70-77.
abstract
Breath-hold divers should adjust their dive behaviors to maximize the benefits and minimize the costs of foraging on prey patches of different densities at different depths. However, few studies have quantified how animals respond to changes in prey availability (depth and density), and how this affects their foraging efficiency. We tested the effects of changes in prey availability on the foraging behavior and efficiency of Steller sea lions (Eumetopias jubatus) by measuring diving metabolic rate, dive durations, and food intake of 4 trained sea lions diving in the open ocean on controlled prey patches of different densities at different depths. Sea lions completed bouts of 5 consecutive dives on high- or low-density prey patches at two depths (10m and 40m). We found that the rate of energy expenditure did not change under any of the imposed foraging conditions (meanąSD: 0.22ą0.02 kJ min−1 kg−1), but that the proportion of time spent consuming prey increased with prey patch density due to changes in diving patterns. At both depths, sea lions spent a greater proportion of the dive bout foraging on prey patches with high prey density, which led to high rates of energy gain (4.3 ą 0.96 kJ min−1 kg−1) and high foraging efficiency (cost:benefit was 1:20). In contrast, the sea lions spent a smaller proportion of their dive bout actively feeding on prey patches with low prey density, and consequently had a lower energetic gain (0.91 ą 0.29 kJ min−1 kg−1) and foraging efficiency (1:4). The 5-fold differences in foraging efficiency between the two types of prey patches were greater than the 3-fold differences that we expected based on differences in food availability. Our results suggest that sea lions faced with reduced prey availability forage less efficiently and therefore would have greater difficulty obtaining their daily energy requirements.

keywords     Dive behavior, Diving energetics, Foraging efficiency, Optimal foraging, Steller sea lion
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Energy reallocation during and after periods of nutritional stress in Steller sea lions: low-quality diet reduces capacity for physiological adjustments.
Jeanniard du Dot, T., D.A.S Rosen and A.W. Trites. 2009.
Physiological and Biochemical Zoology 89:516-530.
abstract
Two groups of female Steller sea lions (Groups H and P) were subjected to periods of energy restriction and subsequent re-feeding during winter and summer to determine changes in energy partition among principal physiological functions and the potential consequences to their fitness. Both sea lion groups consumed high-quality fish (herring) before and after the energy restrictions. During restrictions, Group H was fed a lower quantity of herring and Group P a caloric equivalent of low-quality fish (pollock). Quantitative estimates of maintenance and production energies and qualitative estimates of thermoregulation, activity and basal metabolic rate were measured. During summer, all animals compensated for the imposed energy deficit by releasing stored energy (production energy). Group H also optimized the energy allocation to seasonal conditions by increasing activity during summer when fish are naturally abundant (foraging effort) and by decreasing thermoregulation capacity when waters are warmer. During winter, both groups decreased the energy allocated to overall maintenance functions (basal metabolic rate, thermoregulation and activity together) in addition to releasing stored energy, but preserved thermoregulatory capacity. Group H also decreased activity levels in winter when foraging in the wild is less efficient, unlike Group P. Overall, sea lions fed pollock did not change energy allocation to suit environmental conditions as readily as those fed herring. This implies that low energy density diet may further reduce fitness of animals in the wild during periods of nutritional stress.
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Steller sea lions show diet-dependent changes in body composition during nutritional stress and recover more easily from mass loss in winter than in summer.
Jeanniard du Dot, T., Rosen, D. A. S. , Trites, A. W. 2008.
Journal of Experimental Marine Biology and Ecology 367(1):1-10.
abstract
Controlled feeding experiments were undertaken with captive Steller sea lions (Eumetopias jubatus) to assess seasonal (winter vs. summer) physiological responses of individual animals to reduced quantities and qualities of food that are hypothesised to occur in the wild. Eight animals were randomly divided into two experimental groups fed isocaloric diets: Group H ate Pacific herring (Clupea pallasi) throughout the experiment while Group P was switched to walleye pollock (Theragra chalcogramma) during a 28-day food restriction (after a 28-day baseline) and back to herring during a 28-day controlled re-feeding. Diet type did not impact the rates of body mass lost when food was restricted, but did influence the type of internal energy reserve (protein vs lipids) the sea lions predominantly used. In both summer and winter, Group H lost significantly more lipids and less lean mass than Group P that was fed pollock during the restriction phase. The response of Group H was consistent with the predicted pattern of nutritional stress physiology (i.e. protein sparing and utilization of lipid reserves). Group P lost a surprisingly high proportion of body protein while consuming restricted levels of pollock, which could lead to muscle impairment and vital organ failure on a long-term basis. When given increased amounts of herring during the controlled re-feeding phase, the capacity of both groups to compensate for the previous mass loss was found to depend on season and was independent of previous diet. All of the sea lions increased their rates of mass gain and returned to their pre-experimental weight during winter, but not during summer. Some intrinsic energetic plasticity related to seasonal adaptation to the environment may render winter an easier period than summer to recover from nutritional stress.
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Diet quality and season affect physiology and energetic priorities of captive Steller sea lions during and after periods of nutritional stress.
Jeanniard du Dot, T. 2007.
MSc Thesis, University of British Columbia, Vancouver. 142 pages
abstract
The ability of animals to contend with unpredictable seasonal shifts in quality and quantity of prey has implications for the conservation of wildlife. Steller sea lions (Eumetopias jubatus) were subjected to different quantities and qualities of food to determine what physiological and endocrine responses would occur and whether they differed between season (summer and winter) or diet (high-lipid Pacific herring Clupea pallasi vs. low-lipid walleye pollock Theragra chalcogramma). Eight females were divided among two groups. One (Group H) were fed herring for 28 days (baseline), then received a reduced caloric intake for a subsequent 28 days (restriction) to induce a 15% loss of body mass. The second (Group P) were also fed herring during the baseline followed by a reduced isocaloric diet of pollock during the restriction. Both groups subsequently returned to their baseline intake of herring for a 28-day controlled re-feeding. The two groups of sea lions lost identical mass during restrictions independent of species eaten, but did differ in the type of internal energy reserve (protein vs. lipids) they predominantly used. Group H lost significantly more lipids and less lean mass than Group P in both seasons. In summer, Group H also increased activity levels and decreased thermoregulation capacity to optimize energy allocation. No such changes were observed for Group P whose capacity to adjust to the reduced caloric intake seemed to have been blocked by the pollock diet. During winter, the sea lions spared energy allocated to activity (especially Group H) and preserved thermoregulation capacity. Changes in body mass was negatively related to free cortisol and positively related to IGF-1 in winter, but only IGF-1 was related to changes in mass in summer when lean mass regulation seemed more important. Levels of IGF-1 were associated with changes in protein metabolism in both seasons for both groups, but changes in body condition were never explained by the measured metabolites or hormones. The cap! acity to compensate for mass loss was seasonally dependent with sea lions displaying compensatory growth (by restoring lipid stores) in winter but not in summer. Summer appears to be a more difficult season for sea lions to recover from mild nutritional stress. These physiological findings can be used to refine bioenergetic models needed for the conservation of Steller sea lion populations.
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Potential effects of short-term prey changes on sea lion physiology.
Rosen, D.A., D.J. Tollit, A.J. Winship, and A.W. Trites. 2006.
In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 103-116.
abstract
hanges in the proximate composition of prey can result in a nutritional imbalance in individual animals, regardless of total energy intake. This mechanism has been hypothesized to have contributed to the decline of Steller sea lions (Eumetopias jubatus). Yet little is known about how otariids react physiologically to short-term changes in prey quality and availability. A series of studies with young captive Steller sea lions tested several potential links between prey quality and sea lion health. Body composition (fat to total mass ratio) of animals fed constant, maintenance-level, isocaloric diets of high- or low-lipid prey changed with season, but overall was not aff ected by prey composition. The sea lions appeared to prioritize maintaining core growth rates even when energy was limited, electing to deplete lipid reserves to fulfi ll energy defi cits, resulting in changes in relative body condition. In contrast, sea lions subject to short- term, sub-maintenance diets of high- or low-lipid prey utilized a greater portion of their lipid reserves when losing body mass on low lipid prey. Experiments with diff erent ad libitum feeding regimes indicated that sea lions are readily able to alter food intake levels to compensate for diff erences in prey energy content and, to a lesser degree, prey availability. However, the results also suggest that decreases in prey quality and/or foraging opportunities can readily combine to require food intake levels that are greater than the digestive capacity of the individual. This is particularly true for young animals that may already be living ?on the edge? energetically.
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Examining the potential for nutritional stress in young Steller sea lions: physiological effects of prey composition.
Rosen, D.A.S. and A.W. Trites. 2005.
Journal of Comparative Physiology 175:265-273.
abstract
The effects of high- and low-lipid prey on the body mass, body condition, and metabolic rates of young captive Steller sea lions (Eumetopias jubatus) were examined to better understand how changes in prey composition might impact the physiology and health of wild sea lions and contribute to their population decline. Results of three feeding experiments suggest that prey lipid content did not significantly affect body mass or relative body condition (lipid mass as a percent of total mass) when sea lions could consume sufficient prey to meet their energy needs. However, when energy intake was insufficient to meet daily requirements, sea lions lost more lipid mass (9.16±1.80 kg±SE) consuming low-lipid prey compared with eating high-lipid prey (6.52±1.65 kg). Similarly, the sea lions lost 2.7±0.9 kg of lipid mass while consuming oil-supplemented pollock at maintenance energy levels but gained 5.2±2.7 kg lipid mass while consuming identical energetic levels of herring. Contrary to expectations, there was a 9.7±1.8% increase in metabolism during mass loss on submaintenance diets. Relative body condition decreased only 3.7±3.8% during periods of imposed nutritional stress, despite a 10.4±4.8% decrease in body mass. These findings raise questions regarding the efficacy of measures of relative body condition to detect such changes in nutritional status among wild animals. The results of these three experiments suggest that prey composition can have additional effects on sea lion energy stores beyond the direct effects of insufficient energy intake.
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Satiation and compensation for short-term changes in food quality and availability in young Steller sea lions (Eumetopias jubatus).
Rosen, D.A.S. and Trites, A.W. 2004.
Canadian Journal of Zoology pp. 1061-1069.
abstract
Foraging theory predicts that animals should proportionately increase their food intake to compensate for reduced energy content and/or prey availability. However, the theoretical intake levels will – at some point – exceed the digestive capacity of the predator. We tested the ability of Steller sea lions (Eumetopias jubatus, Schreber, 1776) to compensate for short-term changes in prey energy density and availability, and quantified the maximum amount of food a young sea lion could consume. Five 1-2 year old captive Steller sea lions were alternately offered herring (high-energy) or capelin (low-energy) each day or every second day. When prey were available on a daily basis the sea lions compensated for differences in the energy content of herring and capelin by consuming sufficient quantities of each (8.3 vs. 14.0 kg d-1, respectively) to maintain an equivalent gross energy intake. When herring was available only on alternate days, the sea lions increased their consumption by 52% to 11.5 kg d-1, which was not sufficient to maintain an average gross intake equal to when herring was available every day. When capelin was available only on alternate days, some animals increased their intake for a few days, but average intake (15.2 kg d-1) was far below levels observed during daily feeding. Generally, the sea lions appeared to reach their digestive limit at a level equivalent to 14-16% of their body mass. Our findings suggest that Steller sea lions can alter their food intake in response to short-term changes in prey quality or availability, but that these variables can quickly combine to necessitate food intake levels that exceed the physiological digestive capacities of young animals.
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Possible effects of pollock and herring on the growth and reproductive success of Steller sea lions: insights from feeding experiments using an alternative animal model, Rattus novegicus.
Donnelly, C.P., A.W. Trites and D.D. Kitts. 2003.
British Journal of Nutrition 89:71-82.
abstract
The decline of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska appears to have been associated with a switch of diet from one dominated by fatty forage fishes (such as her-ring; Clupea pallasi ) to one dominated by low-fat fish (such as pollock; Theragra chalco-gramma). Observations made during the decline include reduced body size of sea lions, low pregnancy rates, and high mortality. We used the general mammalian model, the laboratory rat (Rattus norvegicus ), to test whether changing the quality of prey consumed could cause changes in size and reproductive performance. Five groups of twelve fiale, weanling rats were fed diets composed of herring (H), pollock (P), pollock suppliented with herring oil (PH), pollock suppliented with pollock oil (PP), or a sii-purified diet (ICN). Mean body weights were greatest for H, followed by PH, P, PP and finally ICN, although ICN was the only group significantly different from the others (P 0·05). Food intakes before mating were 10 % higher for groups on the lower-fat diets (P and ICN), resulting in similar energy intakes in all groups. The protein efficiency ratio was highest for the H diet, slightly lower for all pollock diets, and significantly lower for ICN (P 0·05). The fetal weights for mothers fed P were significantly reduced (P 0·05). The present study shows that the energy content was a major limiting factor in the nutritional quality of pollock. When food intake was adjusted to meet energetic requirients, there were no detrimental consequences from eating pollock. However, supplientation of pollock meal with additional pollock oil may reduce growth and reproductive performance, although the reasons for this were not apparent.
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What is it about food? Examining possible mechanisms with captive Steller sea lions.
Rosen, D.A.S. and A.W. Trites. 2002.
In D. DeMaster and S. Atkinson (eds), Steller sea lion decline: Is it food II. University of Alaska Sea Grant, AK-SG-02-02, Fairbanks. pp. 45-48.
abstract
Changes in the quality or quantity of food can have a dramatic effect on the population status of wild animals. Unfortunately, it is difficult to assess (or define) whether nutritional stress is a contributing factor to the decline of any particular species.The “nutritional quality ” of a diet to an animal is a complex matter to assess given the range of components that can influence its value.The effects of different diets on animal health are equally complex, and are particularly difficult to assess in large, wild animals. Research by the North Pacific Universities Marine Mammal Research Consortium with captive Steller sea lions is evaluating the possible mechanisms by which dietary changes might adversely affect the nutritional or health status of individual animals, and ultimately the population as a whole. The research investigates the three potential proximate mechanisms by which changes in diet might impact Steller sea lions:a decrease in energy intake, a decrease in the intake of some essential element, and the over-consumption of an element detrimental to sea lion health.
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Pollock and the decline of Steller sea lions: testing the junk-food hypothesis.
Rosen, D.A.S. and A.W. Trites. 2000.
Canadian Journal of Zoology 78:1243-1258.
abstract
The decline of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska and the Aleutian Islands may be the result of them eating too much pollock (a gadid fish) instead of a more balanced and diverse diet containing fattier fishes, such as herring or sandlance. We sought to test this junk-food hypothesis by feeding six captive Steller sea lions (ages 0.9–4.5 years) only pollock or herring. All sea lions gained mass while eating herring. However, eating only pollock for short periods (11–23 d) caused the study animals to lose an average of 6.5% of their initial body mass (0.6 kg/d) over an average feeding trial of 16 d (initial mass averaged 125 kg). The animals were allowed to eat as much pollock as they wanted but did not increase their food intake to compensate for the low energy they were receiv-ing. The sea lions showed progressive metabolic depression while losing body mass on a pollock-only diet. The loss of body mass while eating pollock was due to the lower gross energy content of pollock versus herring, the higher cost of digesting pollock, and the increased energy loss from digesting the larger quantity of fish needed to compensate for the lower energy content of pollock. Thus, our sea lions would have had to eat 35–80% more pollock than herring to maintain similar net energy intakes. Results from our captive-feeding studies are consistent with the junk-food hypothe-sis and have serious implications for Steller sea lions that have been eating primarily pollock in the Gulf of Alaska and the Aleutian Islands.
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Metabolic effects of low-energy diet on Steller sea lions, Eumetopias jubatus.
Rosen, D.A.S. and A.W. Trites. 1999.
Physiological Zoology 72:723-731.
abstract
Diets of six Steller sea lions (Eumetopias jubatus) were switched between a high (herring) and a low (squid) energy density food for 14 d to determine the effects on ingested prey mass, body mass, resting metabolic rate, and the heat increment of feeding. Body mass was measured daily, and resting metabolism was measured weekly by gas respiro-metry. Ingested food mass did not differ significantly be-tween the squid diet and the control or the recovery herring diet periods. As a result of differences in energy density, gross energy intake was significantly lower during the squid diet phase than during either the control or recovery pe-riods. As a result, sea lions lost an average of 1.1 kg/d, totaling 12.2% of their initial body mass by the end of the experimental period. The heat increment of feeding for a 4-kg squid meal was significantly lower than for a similarly sized meal of herring. Decreases in both absolute (24.0 to 18.0 MJ/d, 224%) and mass-corrected (903 to 697 kJ/d/ kg 0.67 , 220%) metabolism were observed by the end of the squid feedings. This study suggests that sea lions can depress their resting metabolism in response to decreases in energy intake or body mass, regardless of satiation level.
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DETECTING NUTRTIONAL STRESS

Normal reference ranges, and age-related and diving exercise effects on hematology and serum chemistry of female Steller sea lions (Eumetopias jubatus).
Gerlinsky, C. D., M. Haulena, A. W. Trites and D. A. S. Rosen. (in press).
Journal of Zoo and Wildlife Medicine
abstract
Decreased health may have lowered the birth and survival rates of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska and Aleutian Islands over the past 30 yr. Reference ranges for clinical hematology and serum chemistry parameters needed to assess the health of wild sea lion populations are limited. Here, blood parameters were serially measured in 12 captive female Steller sea lions ranging in age from 3 wk to 16 yr to establish baseline values and investigate age-related changes. Whether diving activity affects hematology parameters in animals swimming in the ocean compared with animals in a traditional aquarium setting was also examined. Almost all blood parameters measured exhibited significant changes with age. Many of the age-related changes reflected developmental life history changes, including a change in diet during weaning, an improvement of diving capacity, and the maturity of the immune system. Mean corpuscular hemoglobin and mean corpuscular volume were also higher in the ocean diving group compared with the aquarium group, likely reflecting responses to increased exercise regimes. These data provide ranges of hematology and serum chemistry values needed to evaluate and compare the health and nutritional status of captive and wild Steller sea lions.

keywords     Diving, Eumetopias jubatus, hematology, marine mammal, serum chemistry, Steller sea lion
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Fecal triiodothyronine and thyroxine concentrations change in response to thyroid stimulation in Steller sea lions (Eumetopias jubatus).
Keech, A.L., D.A.S. Rosen, R.K. Nelson Booth, A.W. Trites and S.K. Wasser. 2010.
General and Comparative Endocrinology 166:180-185.
abstract
Variation in concentrations of thyroid hormones shed in feces may help to identify physiological states of animals, but the efficacy of the technique needs to be validated for each species. We determined whether a known physiological alteration to thyroid hormone production was reflected in hormone concentrations in the feces of Steller sea lions (Eumetopias jubatus). We quantified variation of triiodothyronine (T3) and thyroxine (T4) concentrations in feces following two intramuscular injections of thyrotropin (thyroid-stimulating hormone, TSH) at 24 h intervals in four captive female sea lions. We found fecal T3 concentrations increased 18-57% over concentrations measured in the baseline sample collected closest to the time of the first TSH injection (p=0.03) and 1-75% over the mean baseline concentration (p=0.12) for each animal of all samples collected prior to injections. The peak T3 response occurred 48 h post injection in three animals and 71 h in the fourth. Post-injection T4 concentrations did not differ between the baseline sample collected closest to the time of the first TSH injection (p=0.29) or the mean baseline concentration (p=0.23) for each animal. These results indicate that induced physiological alterations to circulating thyroid hormone concentrations can be adequately detected through analyses of fecal T3 concentrations and that the technique may provide a means of non-invasively detecting metabolic changes in Steller sea lions.
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Seasonal influence on the response of the somatotropic axis to nutrient restriction and re-alimentation in captive Steller sea lions (Eumetopias jubatus).
Richmond, J.P., T. Jeanniard du Dot, D.A.S. Rosen and S.A. Zinn. 2010.
Journal of Experimental Zoology 311A:144-156.
abstract
Fluctuations in availability of prey resources can impede acquisition of sufficient energy for maintenance and growth. By investigating the hormonal mechanisms of the somatotropic axis that link nutrition, fat metabolism, and lean tissue accretion, we can assess the physiological impact of decreased nutrient intake on growth. Further, species that undergo seasonal periods of reduced intake as a part of their normal life history may have a differential seasonal response to nutrient restriction. This experiment evaluated the influence of season and age on the response of the somatotropic axis, including growth hormone (GH), insulin-like growth factor (IGF)-I, and IGF-binding proteins (BP), to reduced nutrient intake and re-alimentation in Steller sea lions. Eight captive females (five juveniles, three sub-adults) were subject to 28-day periods of food restriction, controlled re-feeding, and ad libitum recovery in summer (long-day photoperiod) and winter (short-day photoperiod). Hormone concentrations were insensitive to type of fish fed (low fat pollock vs. high fat herring), but sensitive to energy intake. Body mass, fat, and IGF-I declined, whereas GH and IGFBP-2 increased during feed restriction. Reduced IGF-I and IGFBP with increased GH during controlled re-feeding suggest that animals did not reach positive energy balance until fed ad libitum. Increased IGF-I, IGFBP-2, IGFBP-3, and reduced GH observed in summer reflected seasonal differences in energy partitioning. There was a strong season and age effect in the response to restriction and re-alimentation, indicating that older, larger animals are better able to cope with stress associated with energy deficit, regardless of season.
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Changes in glucocorticoids, IGF-I and thyroid hormones as indicators of nutritional stress and subsequent refeeding in Steller sea lions (Eumetopias jubatus).
Jeanniard du Dot, T., D.A.S. Rosen, J.P. Richmond, A.S. Kitaysky, S.A. Zinn and A.W Trites. 2009.
Comparative Biochemistry and Physiology, Part A 152:524-534.
abstract
Physiological responses to changes in energy balance are tightly regulated by the endocrine system through glucocorticoids, IGF-I and thyroid hormones. Changes in these hormones were studied in eight captive female Steller sea lions that experienced changes in food intake, body mass, body composition, and blood metabolites during summer and winter. During a period of energy restriction, one group of sea lions was fed reduced amounts of Pacific herring and another was fed an isocaloric diet of walleye pollock, after which both groups returned to their pre-experimental diets of herring. Cortisol was negatively and IGF-I was positively associated with changes in body mass during periods of energy restriction (mass loss associated with increase in cortisol and decrease in IGF-I) and refeeding (body mass maintenance associated with stable hormone concentrations in summer and compensatory growth linked to decrease in cortisol and increase in IGF-I in winter). Cortisol and IGF-I were also correlated with changes in lipid and lean mass, respectively. Consequently, these two hormones likely make adequate biomarkers for nutritional stress in sea lions, and when combined provide indication of the energetic strategy (lipid vs lean mass catabolism) animals adopt to cope with changes in nutrient intake. Unlike type of diet fed to the sea lions, age of the animals also impacted hormonal responses, with younger animals showing more intense hormonal changes to nutritional stress. Thyroid hormones, however, were not linked to any physiological changes observed in this study.
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Fecal triiodothyronine assay validation using captive Steller sea lions (Eumetopias jubatus) and subsequent application to free-ranging populations to examine nutritional stress.
Keech, A.L. 2009.
MSc Thesis, University of British Columbia, Vancouver. 97 pages
abstract
Reduced availability of high energy-content prey (nutritional stress) is a predominant hypothesis to explain the decline of Steller sea lion (Eumetopias jubatus) populations in western Alaska from the late 1970's to the late 1990's. Animals may respond to eating insufficient prey by increasing stress levels and decreasing metabolic rates. It may thus be possible to identify nutritional stress by measuring concentrations of GC metabolites (stress) and thyroid hormones (metabolism) shed in the feces of Steller sea lions. However, techniques to measure thyroid hormone concentrations from sea lion feces have not been developed. We quantified variation of triiodothyronine (T3) and thyroxine (T4) concentrations in Steller sea lion feces following two injections of thyrotropin (TSH) at 24 h intervals into four captive animals. Glucocorticoid (GC) metabolites were also assayed to examine any relationship to stimulated thyroid hormone secretion. We found that fecal T3 peaked 48 h post-injection and increased 25-57% in three sea lions (all animals, p=0.03). Pre-injection GC increases indicated stress from isolation for baseline fecal collections, but post-injection increases could not be confirmed as a response to TSH injections or as a product of the study design. The results demonstrated that pre- and post-injection changes in fecal GC and T3 concentrations were consistent with predictions of an increased stress response and metabolic rate within the animals. We then measured T3 and GC concentrations in 834 Steller sea lion fecal samples collected in 2005 and 2006 from 15 resting (haulout) and breeding (rookery) sites between British Columbia and the Central Aleutian Islands. Overall, GC concentrations did not differ between haulout populations (western 2006 pre-pupping and eastern 2005 post-pupping). Fecal hard-part analyses revealed a lower energy-content diet in the western population, suggesting that diet quality is a relevant hypothesis to explain slightly higher GC concentrations found in the western population, specifically the Aleutian Islands region. However, the nutritional stress hypothesis could not be substantiated through T3 concentrations. The rookeries possessed the highest energy-content diets, but also exhibited a nutritional stress response with a significantly higher GC and lower T3 concentration than either haulout population (possibly related to lactation or decreased leptin levels), but T3 comparisons performed at scales of site and region were inconclusive.
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Effects of prey composition on the endocrine response to nutrient restriction and re-alimination in Steller sea lions (Eumetopias jubatus).
Richmond, J. P., T. Jeanniard du Dot, D. A. S. Rosen and S. A. Zinn. 2006.
Symposia of the Comparative Nutrition Society 63:136-141.
abstract
Little is known about the mechanism in which decreased nutrient intake influences the physiology of Steller sea lions. By investigating the factors that link nutrition, fat metabolism and lean tissue accretion, we can assess the impact of decreased nutrient intake on energy storage and lean tissue growth, which may have implications for survival. Captive Steller sea lion females (n = 8, 2 to 5 year of age) were used to examine changes in the somatotropic axis in response to decreased nutrient intake. Animals were placed on a normal herring maintenance diet for 1 month. After this 1 month ‘baseline’ period four animals were placed on a herring submaintenance diet and four animals were fed an isocaloric Pollock submaintenance diet for 1 month. During the 1 month submaintenance period, the animals lost 10 to 15% of their body mass. In the 1 month re-alimentation period, only three animals increased mass to their initial value. Concentrations of IGF-I followed the expected pattern paralleling changes in intake. Concentrations of GH were more variable than IGF-I. Concentrations of IGFBP generally followed the expected response based on domestic animal research. The overall concentration of IGFBP-3 declined with decreased nutrient intake. In contrast, IGFBP-2 increased with decreasing nutrient intake.
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Validation of a fecal glucocorticoid assay for Steller sea lions (Eumetopias jubatus).
Hunt, K.E., A.W. Trites, and S.K. Wasser. 2004.
Physiology and Behavior 80:595-601.
abstract
The Steller sea lion (Eumetopias jubatus) is listed as endangered in parts of its range and is suspected of suffering from ecological stressors that may be reflected by fecal glucocorticoid hormones. We validated a fecal glucocorticoid assay for this species with an adrenocorticotropic hormone (ACTH) challenge. Feces were collected from captive Steller sea lions (two males and two females) for 2 days before injection with ACTH, and for 4 or more days postinjection. Feces were freeze-dried, extracted with a methanol vortex method, and assayed for glucocorticoids. The assay demonstrated good parallelism and accuracy. All animals showed the expected peak of fecal glucocorticoid excretion after ACTH injection. However, the two males had higher baselines, higher peaks, and more delayed peaks than the females. Peak glucocorticoid excretion occurred at 5 and 28 h postinjection for the two females, and at 71 and 98 h for the two males. Correction for recoveries by the addition of tritiated hormones produced ACTH profiles that were virtually identical in pattern to uncorrected data, but with higher within-sample coefficients of variation. Based on these results, we conclude that this fecal glucocorticoid assay accurately reflects endogenous adrenal activity of Steller sea lions, and that recovery corrections are not necessary for this species when using the methanol vortex extraction method. More research is needed to address possible sex differences and other possible influences on fecal glucocorticoid concentrations.
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Searching for stress: Hematological indicators of nutritional inadequacies in Steller sea lions.
Rosen, D.A.S., Hastie, G.D., Trites, A.W. 2004.
Symposia of the Comparative Nutrition Society 2004 5:145-149.
abstract
This experiment examined the response of a suite of hematologic parameters to experimentally induced nutritional stress in a group of captive Steller sea lions. The goal was to identify a suite of parameters that could be used to diagnose comparable conditions among wild Steller sea lions. Previous studies, many with ruminant mammals, have shown that there are significant changes in blood characteristics with nutritional status. However, it is equally clear that there is no overwhelming choice of blood parameter to indicate nutritional stress across different species. Therefore, species-specific empirical tests such as the one carried out in the current study are essential to place results from wild studies in a biologically meaningful context.
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The effects of food deprivation on serum lipid concentration and content in Steller sea lions (Eumetopias jubatus).
Berman, M. and L. Rea. 2000.
In C.L.K. Baer (ed.), Proceedings of the Third Comparative Nutrition Society Symposium. Pacific Grove, California, August 4-9, 2000. 3:13-16.
abstract
The western Alaska population of Steller sea lions has significantly declined over the past thirty-five years. A population estimate of 180,000 individuals in 1965 declined to a current estimate of 50,000. A widely accepted hypothesis for the cause of decline is from indirect competition with the commercial fishing industry. Analysis of Steller sea lion censuses have determined that decline is most evident in the juvenile portion of the population. This could be explained by a decrease in prey availability for juveniles which are physiologically and behaviorally limited in their ability to forage further and deeper for food. Although Steller sea lions naturally fast during their summer breeding season, they are not as biochemically adapted to handle food deprivation at other times of the year (Rea et al. 1999). This study addresses the physiological implications of food deprivation by analyzing the effects of fasting on serum lipid composition and content. Additionally, the breeding and non-breeding seasons were compared to determine if seasonality affects serum lipid composition and content.
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The reliability of skinfold-calipers for measuring blubber thickness of Steller sea lion pups (Eumetopias jubatus).
Jonker, R.A.H. and A.W. Trites. 2000.
Marine Mammal Science 16:757-766.
abstract
Twelve dead Steller sea lion pups (Eumetopias jubatus) aged 3-14 d were recovered from rookeries in Southeast Alaska. They had a wide range of body sizes and conditions (small to large and fat to no fat). The ability of calipers to estimate the thickness of their blubber layer was assessed with a set of skinfold calipers. Average error of measurement for skin and blubber thickness was an acceptable 5.4%, but the skin and blubber of the pups were highly compressible. Skinfold thickness increased with body mass but did not necessarily reflect the development of blubber, given that pups with no blubber also showed an increase in skinfold thickness with increases in body mass. Skinfold thickness of sea lion pups appears to predict body size better than it predicts blubber thickness, making it difficult if not impossible to develop a simple index of body condition or a calculation of percent body fat for Steller sea lion pups from skinfold caliper measurements.
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Changes in serum leptin levels during fasting and food limitation in Steller sea lions (Eumetopias jubatus).
Rea, L.D., T.R. Nagy. 2000.
In Proceedings of the Comparative Nutrition Society. Asilomar, CA. pp. 171-175.
abstract
Leptin, also commonly known as the ob protein, is a peptide hormone secreted by adipocytes which has been shown to have a role in energy metabolism and food intake in rodents and man (Campfield et al. 1996); Although the specific molecular and biochemical pathways of action of this hormoneare still the-focus of intensive study, it is thought that leptin acts as a negative feedback signal to satiety centers in the hypothalmus to regulate body energy stores. When adipose reserves are abundant, high levels of leptin are secreted and signal the brain to regulate energy balance (i.e. decrease food intake). The role of leptin in other animal systems has received much less attention to date, thus we chose to investigate how serum leptin concentrations change in response to food deprivation in an animal which is known to undergo periods of voluntary natural fasting in the wild. Female Steller sea lions fast for 1 to 2 weeks during the summer breeding season in order to give birth and nurse their young. Males are also thought to fast while defending territory during the breeding season. By simulating these fasting bouts in a captive environment the effect of complete fasting and body condition (i.e. total fat content) on circulating leptin levels could be addressed.

In rodents and humans, food intake has been shown to increase leptin production and fasting consistently decreased leptin secretion by the adipocytes (Saladin et al. 1995, Pratley et al. 1997). To address the related, but individual effects of fasting and decrease in body reserves on leptin production we also held sea lions on a low plane of nutrition (food limitation) for 28 d such that body mass loss was similar to that experienced during 14 d fasting experiments.

In several species studied to date, a close correlation between serum leptin concentrations and total body fat mass has been demonstrated. If a close correlation between leptin and body fat content could also be established for Steller sea lions, this hormone could provide an index of body condition that could be more easily monitored in free-ranging animals. Presently the best method for determining body fat content in these animals involves holding captured individuals under an aesthesia during the two hour equilibration period necessary for the dilution of deuterium.

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Blood Chemistry and Body Mass Changes During Fasting in Juvenile Steller sea lions (Eumetopias jubatus).
Rea, Lorrie D., David A.S. Rosen and Andrew W. Trites. 1998.
In Proceedings of the Comparative Nutrition Society, Number 2. pp. 174-178.
abstract
Fasting in bears, penguins and phocid seals is accompanied by predictable changes in plasma metabolite concentrations related to alterations in the body reserves that are catabolized and illustrate a species’ ability to limit protein degradation during long-term fasting (see review in Castellini and Rea 1992, Nordoy et al. 1993, Rea 1995). Steller sea lions (Eumetopias jubatus) also undergo periods of fasting in their natural environment; adult females fast while nursing pups on the rookery, males defend breeding territories and young pups fast on the rookeries while their mothers are at sea foraging. Five juvenile Steller sea lions were fasted ‘in captivity (with free access to fresh water) for 9 to 14 days to test the hypothesis that juvenile Steller sea lions also exhibit changes in key plasma metabolites indicative of biochemical adaptation to fasting. The secondary objective of this study was to determine if blood metabolite concentrations could be used as biochemical indicators of nutritional status in free-ranging juvenile Steller sea lions.

keywords     blood chemistry, body mass, fasting, juvenile Steller sea lions, #2
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Plasma angiotensin II, arginine vasopressin and atrial natriuretic peptide in free ranging and captive seals and sea lionsPlasma angiotensin II, arginine vasopressin and atrial natriuretic peptide in free ranging and captive seals and sea lions.
Zenteno-Savin, T., and M.A. Castellini. 1998.
Comparative Biochemistry and Physiology 116c(1):1-6.
abstract
We used radioimmunoassay methods to quantify arginine vasopressin (AVP), atrial natriuretic peptide (ANP), and angiotensin II (Ang 11) in plasma samples from harbor seals (Phoca vitulina richardsii), Weddell seals (Lepconychotes wedellii), northern elephant seals (Mirounga angustirostris), ringed seals (Phoca hispida), California sea lions (Zalophus californianus), and Steller sea lions (Eumetopius jubatus). Plasma concentrations of AVP, ANP, and Ang II in these pinniped species were within the ranges reported for other vertebrates under resting conditions. However, there were species, geographic and developmental variations in these hormones: Levels of AVP in plasma samples from adult Steller sea lions and harbor seals were higher than in pups of the same species; higher levels of plasma ANP were found in wild captured Alaskan Steller sea lions and in hunted ringed seals; differences in plasma levels of all three hormones were found throughout the geographic distribution of harbor seals and Steller sea lions in Alaska. This is the first report on circulating concentrations of vasoactive hormones in pinnipeds, and demonstrates that further studies are needed to ascertain the natural variability in these levels with the impact of molting, fasting, diving and environmental factors in seals and sea lions.
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Reliability of calipers to measure the blubber thickness in Eumetopias jubatus.
Jonker, R.A.H. 1996.
In Faculty of Veterinary Medicine. University of Utrecht, the Netherlands, 23 pages.
abstract
The accuracy of measnring skinfold thickness using a Slimguide caliper was compared to the actual sculp thickness (subcutaneous fatlayer and skin) on 12 Steller sea lion pups @umelopius jubalu). The correlation between skinfold thickness and actual sculp thickness on the intact animals was 0.63 (r2 = 0.4, p<0.005). After correcting for variation in the mean fat thickness between animals and places of measurement, the correlation was only 0.32 (p<0.005). On dissected sculps, the correlation between skinfold thickness and actual sculp thickness was 0.72 (? = 0.52, p<0.005) and 0.41 (p<0.005) after correcting for variation between animals and places of measurement. Differences in the correlation betireen skinfhld thickness and actual sculp thickness for tlie corrected and uncorrected data can be attributed to the lo\v sensitivity of the caliper method. Variation of the actual fat layer thickness becomes smaller, as does the correlation between skinfold thickness and actual sculp thickness, when differences in fat layer thickness between the animals is accounted for. This makes measurcments of skinfold thickness, done with a caliper on Steller sea lion pups to determine the hluhher thickness, not reliable enough yet. Factors, such as the compressibility of the skinfold. the structure ,11'tlit. subcutaneous fat tissue and the technical error of measurement could l~a\reb een responsible for the 1-ar-iatiori in measured skinfold thickness when spots with a equal sculp thickness were compared.
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