Steller sea lion research > Nutritional Stress >Assessing Prey Quality

Assessing Prey Quality
All animals have individual energy and nutritional requirements that they must obtain from their food. Scientists are studying the potential effect of diet changes on Steller sea lions by measuring the nutrition and energy content of various prey items.

Nutrition and Energy Content
Scientists know that the energy contained in various fish species can differ tremendously. The proximate composition of these potential prey items can be analyzed as a first step in determining the energy that Steller sea lions obtain from different diets.

However, not all of the chemical energy (gross energy) contained in a fish is biologically available to a sea lion. A certain amount of energy is lost during the course of digestion. Consortium researchers have measured the energy lost through two major digestive processes - the heat increment of feeding and fecal energy loss. By quantifying the amount of energy lost from meals of different composition and sizes, scientists can more accurately calculate the amount of fish needed to satisfy the energy requirements of Steller sea lions.

Heat Increment of Feeding
The heat increment of feeding (HIF) is the increase in energy expended by an animal due to the work involved in breaking down, digesting, and assimilating food. Consortium scientists have measured the energy lost via HIF by feeding captive sea lions meals of different prey and size and measuring the increase in oxygen consumed (energy expended) over the following 10-16 hours.

The results of these experiments demonstrate that proportionately more energy is lost from a meal of low energy prey (e.g., Walleye pollock, squid) than from high energy prey items (e.g., herring). Additionally, proportionately more energy is lost from larger meals than smaller ones.

Digestive Efficiency and Dry-matter Digestibility
Although the digestive system of Steller sea lions is very efficient, some energy from food is lost in their feces. Scientists use a measurement known as 'digestive efficiency' to measure how little energy is lost in the feces - the less energy lost, the higher the digestive efficiency.

Consortium scientists have measured the digestive efficiency of captive Steller sea lions while they are consuming different potential prey items at different meal frequencies. In general, the sea lions have the lowest digestive efficiency with prey items that contain the least energy. In other words, proportionally more of the energy in a meal is lost in the feces from prey species such as squid, and less is lost from prey such as herring.

In the past, scientists have used the amount of material that passes through a sea lion to estimate the amount of energy lost in feces. This measure of dry-matter digestibility (also sometimes called assimilation efficiency) has also been measured by Consortium scientists with captive Steller sea lions. The results indicate that, although dry-matter digestibility is loosely related to digestive efficiency, the amount of material that passes through a sea lion's intestinal tract is more closely related to how boney the fish is than how much energy is lost in the digestive process. Therefore, the two measurements should not be used interchangeably.

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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. (in press). Aquatic Mammals 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.
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.
Population trends, diet, genetics, and observations of Steller sea lions in Glacier Bay National Park. Gelatt, T., A.W.Trites, K. Hastings, L. Jemison, K. Pitcher, and G. O’Corry-Crowe. 2007. In J.F. Piatt and S.M. Gende (eds), Proceedings of the Fourth Glacier Bay Science Symposium, U.S. Geological Survey, Juneau , Alaska. pp. 145-149. abstract We are using demographics, scat analysis, and genetic measurements of Steller sea lions (SSLs)to understand the factors affecting population status throughout Alaska. Steller sea lions are listed as threatened throughout Southeast Alaska including Glacier Bay National Park where they frequent at least five terrestrial sites, including a recently established rookery on Graves Rock. Breeding season counts in GBNP increased at ~6 percent/yr between 1989 and 2002. Brand resighting during 2003 revealed 16 western stock SSLs seen within the park. Survival to two months of age was 90 percent. Fifty pups were branded at Graves Rock in 2002. It is necessary to mark more animals to estimate annual survival rates of juveniles and adults. Sandlance and pollock were top prey items at Graves Rock and South Marble Island. Mitochondrial DNA analysis indicates that the Graves Rock rookery was established in part by females from the western sea lion stock (west of 144° W longitude).
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.
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.
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.
Killer whales, whaling and sequential megafaunal collapse in the North Pacific: a comparative analysis of the dynamics of marine mammals in Alaska and British Columbia following commercial whaling. Trites, A. W., V. B. Deecke, E. J. Gregr, J. K. B. Ford, and P. F. Olesiuk. 2007. Marine Mammal Science 23:751-765. abstract The hypothesis that commercial whaling caused a sequential megafaunal collapse in the North Pacific Ocean by forcing killer whales to eat progressively smaller species of marine mammals is not supported by what is known about the biology of large whales, the ecology of killer whales and the patterns of ecosystem change that took place in Alaska, British Columbia, and elsewhere in the world following whaling. A comparative analysis shows that populations of seals, sea lions and sea otters increased in British Columbia following commercial whaling, unlike the declines noted in the Gulf of Alaska and Aleutian Islands. The declines of seals and sea lions that began in western Alaska around 1977 were mirrored by increases in numbers of these species in British Columbia. A more likely explanation is the seal and sea lion declines and other ecosystem changes in Alaska stems from a major oceanic regime shift that occurred in 1977. Killer whales are unquestionably a significant predator of seals, sea lions and sea otters but not because of commercial whaling.
Diets of Steller sea lions (Eumetopias jubatus) in Southeast Alaska from 1993-1999. Trites, A.W., D.G Calkins and A.J. Winship. 2007. Fishery Bulletin 105:234-248. abstract Diet of Steller sea lions (Eumetopias jubatus) was determined from 1494 scats (feces) collected at breeding (rookeries) and non-breeding (haulout) sites in Southeast Alaska from 1993 to 1999. The most common prey of 61 species identified were walleye pollock (Theragra chalcogramma), Pacific herring (Clupea pallasii), Pacific sand lance (Ammodytes hexapterus), Pacific salmon (Salmonidae), arrowtooth flounder (Atheresthes stomias), rockfish (Sebastes spp.), skates (Rajidae), and cephalopods (squid and octopus). Sea lion diets at the three Southeast Alaska rookeries differed significantly from one another. Steller sea lions consumed the most diverse range of prey categories during summer, and the least diverse during fall. Diet was more diverse in Southeast Alaska during the 1990s than in any other region of Alaska (Gulf of Alaska and Aleutian Islands). Dietary differences between increasing and declining populations of sea lions in Alaska correlate with rates of population change, and add credence to the view that diet may have played a role in the decline of sea lions in the Gulf of Alaska and Aleutian Islands.
Relationship between Steller sea lion diets and fish distributions in the eastern North Pacific. Bredesen, E.L., A.P. Coombs, 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. 131-139. abstract Distributions of fish species were compared with diet information for Steller sea lions (Eumetopias jubatus) to assess the level of correspondence between potential prey availability and sea lion feeding habits. Fish distributions were compiled as part of the Sea Around Us Project at the UBC Fisheries Centre, and were based on published distributions and habitat preferences (e.g., latitude, depth). Sea lion scat samples were collected during the 1990s from seven geographic regions from Oregon to the western and central Aleutian Islands. The frequencies of occurrence of four prevalent species (walleye pollock, Theragra chalcogramma ; Pacific herring, Clupea pallasii ; Pacific cod, Gadus macrocephalus ; and North Pacific hake, Merluccius productus ) in the Steller sea lion diet were compared to their distributions in the North Pacific Ocean. The data suggest that Steller sea lion diets broadly reflect the distributions of these major prey species. However, some of the fish species that were regionally predicted to be present in high abundance were not proportionally reflected in the Steller sea lion diet, suggesting that other factors in addition to fish abundance influence their diets.
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.
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.
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.
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.
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.
Sea Lions of the World. Trites, A.W., S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea, and K. Wynne (eds). 2006. Alaska Sea Grant Alaska College Program, University of Alaska, Fairbanks. 664 pages abstract The goal of the symposium was to bring together scientists and resource managers to address knowledge of world sea lion populations in order to compare them with Steller sea lions, and to identify research needs. managers to address knowledge of world sea lion populations in order to compare them with Steller sea lions, and to identify research needs. Changes in the worldwide abundance of sea lions is of growing concern to fisheries and conservation groups, because fisheries are feared to threaten sea lions, and/or because sea lions are feared to threaten fisheries. Over the past few decades, major changes have been noted in the abundance of all five species of sea lions around the world. In the North Pacific, the Steller sea lion has been declared endangered in parts of its range and is considered threatened with extinction in others. This is in contrast to the rapid increase in populations of California sea lions in Mexico and California. Elsewhere, the Japanese subspecies of the California sea lion is probably extinct and the Galapagos subspecies is in low numbers. Numbers of New Zealand sea lions and Australian sea lions are also extremely low, with major declines recently reported in Australia. Relatively little is known about the South American sea lion. This symposium brought the world community of sea lion researchers and policy makers together to share their experiences and knowledge with each other. Interspecies comparisons can shed light on why some populations might decline while others increase. Insights might also be gained on whether trends in the abundance of sea lions are related to fishing activities through food dependencies or more directly through control or conservation measures. A better understanding of the biology of sea lions is urgently needed. The symposium significantly contributed to the understanding of fluctuating sea lion populations, especially as they compare to the Steller sea lion, by synthesizing current knowledge and forging new directions.
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.
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.
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.
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.
Suckling attempts during winter by two non-filial Steller sea lion pups (Eumetopias jubatus). Porter, B.T. and Trites, A.W. 2004. Mammalia 63:23-26. abstract Milk stealing and fostering care is rare among mammals. Among pinnipeds, the nursing of offspring that are not their own has been noted for some species of seals, but rarely for sea lions or fur seals. Thousands of hours have been spent observing Steller sea lions in the wild, but only a few successful suckling attempts have been noted. From January to March 1996, we observed two non-filial pups repeatedly suckling lactating females at a winter haulout site at Timbered Island in southeast Alaska. These two observations are noteworthy because of their rarity and the bearing they have on the poorly understood process of weaning in Steller sea lions. The timing of weaning in Steller sea lions has been speculated to occur sometime during winter or spring when pups are 6 months or older. Both mothers and pups we observed were aggressive toward intruding conspecifics and were very protective of their mother’s teats. However, there was a range of individual variation in the tolerance of both mature females and their offspring to the distance they would allow strange pups near the teats. It is undoubtedly advantageous for nutritionally stressed pups to attempt to steal milk, compared with the alternative — starvation. However the potential for injury likely out-weighs any gain in resources and probably deters most young from attempting to approach strange females. The pups we observed stealing milk did not supplement their intake with fish despite the apparent ability of this age group to capture prey. The fact that they did not suggests that they may not have been behaviourally or physiologically capable of consuming fish. Compared with milk, they may also not be physically capable of consuming enough prey to meet their daily energy needs during this period of rapid growth and development. This further suggests that weaning of Steller sea lions pups may occur much later in spring or early summer than many have previously thought.
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.
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.
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.
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.
Examining the evidence for killer whale predation on Steller sea lions in British Columbia and Alaska. Heise, K,. L.G. Bar