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Measuring
metabolic
rates
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Steller sea lion research>Nutritional
Stress>Effects of Diet Changes>Physiological
Measures
Physiological Measures and the Health of Steller Sea Lions
Researchers have been using a number of different methods to study
the effect of different diets on the physical condition and health of
Steller sea lions. One is to measure morphology, such as the length,
weight and thickness of the blubber layer. Another is to assess the nutritional
status from hormones and other elements contained within blood samples,
in the same way that doctors can tell a lot about human health by drawing
a sample of blood. Some of these studies are conducted on wild Steller
sea lions in the wild and others use a group of trained sea lions in
captivity.
Diet Manipulations
In the wild, Steller sea lions eat a number of different types of fish
and invertebrates. Recent scientific research suggests that the type
of prey available to Steller sea lions in the North Pacific may be changing.
Different prey, such as herring or pollock, provide different amounts
of energy to the animal. (seeAssessing Prey Quality)
To help determine the possible effects of diet changes, the regular diet
of the sea lions at the Vancouver Aquarium was switched from herring
to Atka Mackerel pollock and squid, which have less energy. Changes in
the food intake, body mass, and metabolism of the sea lions are then
carefully monitored. Results indicate that lower quality prey, such as
pollock, requires more energy to digest than a higher quality diet of
herring. Additionally, the sea lions appear unable to maintain body mass
when switched to a lower calorie prey such as squid or pollock, and need
to compensate by conserving energy. It appears that the sea lions do
not compensate for low energy food sources by increasing their food intake.
Instead they seem to use up more of their own energy stores (i.e., they
lose weight), and they spend less energy (i.e., they lower their resting
metabolism).
Dr. Rosen (UBC) has been assessing the metabolic adaptations employed
to combat periods of low food intake. Additional data have been collected
on how body composition changes during periods of fasting. Initial results
suggest that sea lion's physiological response to low-food conditions
may be both seasonal and age-dependent.
Morphology
One way of estimating the health or condition of a pinniped is to measure
the thickness of the layer of blubber that lies just below the skin.
Calipers measure skinfold thickness and provide a reliable estimate
of fat thickness in humans. Researchers Andrew Trites (UBC) and Remko
Jonker (UBC) sought to determine whether this simple and inexpensive
device could be used to measure the condition of Steller sea lion pups
in the wild. They measured the skinfold thickness of dead pups found
on the breeding beaches of Alaska. They discovered that heavy pups
had higher skinfold thickness than lighter pups. However, this relationship
was true for pups with blubber as well as pups that had died of starvation
(no blubber). Therefore, skinfold thickness appears to change with
body mass rather than condition. These results suggest caliper pinches
do not provide a reliable measure of condition in Steller sea lion
pups, and alternate methods must be explored.
Blood Chemistry
 A
co-operative research project with Dr. Lorrie Rea (ADF&G) investigated
the effects of fasting and reduced food intake on several physiological
parameters. Five juvenile Steller sea lions at the Vancouver Aquarium
underwent a 2-week fast and a 4-week period of 50% food intake; the pups
fasted for 3 days. The amount of food the animals received was similar
to conditions they would experience in the wild. Dr. Rea used blood samples
from these sea lions to evaluate the differences in blood chemistry reported
in wild Steller sea lions from areas of stable and declining populations.
The aim was to determine whether differences in blood biochemistry (e.g.,
blood urea nitrogen and ketone bodies) reflect nutritional status.
Hormones
Tania Zenteno-Savin (UAF) completed her Ph.D. thesis which focused on
three vasoactive hormones - AVP, ANP and ANG II - found in blood plasma.
Humans suffering from eating disorders (anorexia, bulimia) have elevated
levels of these hormones. It was therefore speculated that Steller
sea lions might exhibit similar changes in their blood hormone levels
if they are suffering from nutritional stress. To test this hypothesis,
blood samples were collected and analyzed from nearly 200 Steller sea
lions in Southeast and Northwest Alaska. A concurrent experiment was
also run with the captive Stellers at the Vancouver Aquarium to determine
how fasting or food restriction affected blood hormone levels.
In the controlled study, ANG II rose in response to food restrictions.
AVP was unaffected, while ANP seemed to decline. These feeding experiments
seem to indicate that blood hormones do provide some gauge of how well
a Steller has been eating . However, in the wild Stellers, ANG II levels
were higher in the thriving southeast population compared to the declining
Aleutian population. This is not consistent with the hypothesis that
Steller sea lions in the western population are nutritionally stressed.
Teeth
The teeth of sea lions have annual growth annuli (rings much like in
trees), which scientists have been using to age individuals. However,
Dr. David Sampson (OSU) thinks that the chemical composition of the
teeth can yield even more information. His idea is that changes in
the physical or chemical structure of the teeth can be used to date
specific life-history events, such as weaning, the onset of maturity,
or age at first lactation. Dr. Roger Nielson (OSU) has been using an
electron microprobe to measure the elemental micro-composition of teeth
obtained from dead Steller sea lions and from the National Marine Mammal
Laboratory (NMFS). This initial study will determine whether archived
teeth can be used to reconstruct changes in life history parameters
which may provide insight into what happened to Steller sea lions.
NEXT: Predation and
the decline of the steller sea lion population>>>
 
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Hormone changes indicate that winter is a critical period for food.
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Rosen, D.A.S., Kumagai, S. (in press).
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Journal of Comparative Physiology B
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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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Marine Mammals in the Lab: Tools for Conservation and Science.
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Rosen, D.A.S. and A.W. Trites. 2008.
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In North Pacific Universities Marine Mammal Research Consortium. Vancouver, BC. pp. 15 pages
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abstract
Twenty-two participants from a variety of backgrounds and interests
discussed how to improve the nature of research with marine mammals in the
laboratory and ensure its continuation as a vital scientific resource in the future.
There was agreement that captive marine mammals represent a valuable scientific
asset. Many of the pressing conservation and scientific research questions
pertaining to marine mammals cannot be carried out with their wild counterparts.
However, studying marine mammals in the laboratory incurs specific financial,
scientific, and logistical challenges. The workshop generated potential solutions to
many of these issues.
Participants expressed the need for greater cooperation and coordination between
scientists to optimize the scientific value of research with captive marine mammals,
and to minimize the costs of such research. This could be enhanced through scheduled
in-person gatherings and web-based portals for listing active and proposed research.
Better use must also be made of scientific resources and expertise, and novel
sources of revenue have to be generated. There should also be greater sharing of
documents relating to experimental design and research permitting. The effectiveness
of research will benefit from greater communication between researchers and
husbandry staff at institutions holding animals for research. Such efforts should
raise the profile (and acceptance) of captive marine mammals science within the
scientific community and for program administrators, leading to greater financial
and research opportunities.
Nine specific recommendations were forwarded that could be immediately implemented
to enhance communication and increase the value of captive marine mammal science:
1. Produce a list of research resources (animals, specialized skills and equipment);
2. Create a list of on-going captive marine mammal studies;
3. Produce a list of publications derived from research with captive marine mammals;
4. Develop a set of guidelines for communication, responsibilities, and intellectual
ownership for collaborative projects;
5. Implement means for coordination of future studies (both web-based and scheduled
workshop/meetings);
6. Design a means for sharing standard Operating Procedures;
7. Hold a workshop to increase statistical rigor and standards in experimental design;
8. Introduce the use of annual survival rates into institutions holding marine
mammals; and
9. Heighten the awareness of the value and prevalence of captive studies to the Us
Marine Mammal Commission.
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Reductions in oxygen consumption during dives and estimated submergence limitations of Steller sea lions (Eumetopias jubatus).
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Hastie, G.D., D.A.S. Rosen and A.W. Trites. 2007.
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Marine Mammal Science 23:272-286.
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abstract
Accurate estimates of diving metabolic rate are central to assessing the energy
needs of marine mammals. To circumvent some of the limitations inherent with
conducting energy studies in both the wild and captivity, we measured diving
oxygen consumption of two trained Steller sea lions (Eumetopias jubatus) in the open
ocean. The animals dived to predetermined depths (5–30 m) for controlled periods
of time (50–200 s). Rates of oxygen consumption were measured using open-circuit
respirometry before and after each dive. Mean resting rates of oxygen consumption
prior to the dives were 1.34 (±0.18) and 1.95 (±0.19) liter/min for individual sea
lions. Mean rates of oxygen consumption during the dives were 0.71 (±0.24) and
1.10 (±0.39) liter/min, respectively. Overall, rates of oxygen consumption during
dives were significantly lower (45% and 41%) than the corresponding rates measured
before dives. These results provide the first estimates of diving oxygen consumption
rate for Steller sea lions and show that this species can exhibit a marked decrease in
oxygen consumption relative to surface rates while submerged. This has important
consequences in the evaluation of physiological limitations associated with diving
such as dive duration and subsequent interpretations of diving behavior in the wild.
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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.
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Jeanniard du Dot, T. 2007.
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MSc Thesis, University of British Columbia, Vancouver. 142 pages
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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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Utilization of stored energy reserves during fasting varies by age and season in Steller sea lions.
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Rea, L.D., D.A.S. Rosen and A.W Trites. 2007.
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Canadian Journal of Zoology 85:190-200.
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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).
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Kumagai, S., D.A.S Rosen and A.W. Trites. 2006.
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Comparative Biochemistry and Physiology 179:589-598.
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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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Potential effects of short-term prey changes on sea lion physiology.
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Rosen, D.A., D.J. Tollit, A.J. Winship, and A.W. Trites. 2006.
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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.
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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.
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Rosen, D.A.S. and A.W. Trites. 2005.
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Journal of Comparative Physiology 175:265-273.
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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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Effects of body condition on resting metabolism in captive and free-ranging Steller sea lions (Eumetopias jubatus).
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Hoopes, L.A., L.D. Rea, D.A.S. Rosen and G.A.J. Worthy. 2004.
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Symposia of the Comparative Nutrition Society 2004 5:79-82.
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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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Validation of a fecal glucocorticoid assay for Steller sea lions (Eumetopias jubatus).
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Hunt, K.E., A.W. Trites, and S.K. Wasser. 2004.
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Physiology and Behavior 80:595-601.
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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.
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Rosen, D.A.S., Hastie, G.D., Trites, A.W. 2004.
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Symposia of the Comparative Nutrition Society 2004 5:145-149.
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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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Satiation and compensation for short-term changes in food quality and availability in young Steller sea lions (Eumetopias jubatus).
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Rosen, D.A.S. and Trites, A.W. 2004.
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Canadian Journal of Zoology pp. 1061-1069.
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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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Changes in metabolism in response to fasting and food restriction in the Steller sea lion (Eumetopias jubatus).
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Rosen, D.A.S. and A.W. Trites. 2002.
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Comparative Biochemistry and Physiology. 132:389-399.
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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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What is it about food? Examining possible mechanisms with captive Steller sea lions.
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Rosen, D.A.S. and A.W. Trites. 2002.
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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.
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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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The effects of food deprivation on serum lipid concentration and content in Steller sea lions (Eumetopias jubatus).
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Berman, M. and L. Rea. 2000.
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In C.L.K. Baer (ed.), Proceedings of the Third Comparative Nutrition Society Symposium. Pacific Grove, California, August 4-9, 2000. 3:13-16.
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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).
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Jonker, R.A.H. and A.W. Trites. 2000.
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Marine Mammal Science 16:757-766.
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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).
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Rea, L.D., T.R. Nagy. 2000.
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In Proceedings of the Comparative Nutrition Society. Asilomar, CA. pp. 171-175.
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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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Metabolic response to fasting in 6-week-old Steller sea lion pups (Eumetopias jubatus).
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Rea, L.D., D.A.S. Rosen and A.W. Trites. 2000.
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Canadian Journal of Zoology 78:890-894.
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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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Assessing the role of nutritional stress in the decline of wild populations: a Steller case of scientific sleuthing.
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Rosen, D.A.S. and A.W. Trites. 2000.
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In C.L.K. Baer (ed.), Proceedings of the Third Comparative Nutrition Society Symposium. Pacific Grove, California, August 4-9, 2000. 3:182-186.
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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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Morphometric measurements and body condition of healthy and starving Steller sea lion pups (Eumetopias jubatus).
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Trites, Andrew W. and Remco A.H. Jonker. 2000.
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Aquatic Mammals 26:151-157.
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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).
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Rea, L.D., D.A.S. Rosen and A.W. Trites. 1999.
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In The FASEB Journal (Federation of American Societies of Experimental Biology). Washington, D.C., April 17-21, 1999. Vol 13(5) pp. A740
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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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Metabolic effects of low-energy diet on Steller sea lions, Eumetopias jubatus.
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Rosen, D.A.S. and A.W. Trites. 1999.
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Physiological Zoology 72:723-731.
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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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Blood Chemistry and Body Mass Changes During Fasting in Juvenile Steller sea lions (Eumetopias jubatus).
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Rea, Lorrie D., David A.S. Rosen and Andrew W. Trites. 1998.
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In Proceedings of the Comparative Nutrition Society, Number 2. pp. 174-178.
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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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