Season of the Sea Lion
Are the impacts of nutritional stress seasonally dependent?

Just as many people prefer to eat heavier foods in winter and lighter meals in summer, the Steller sea lion diet also varies with the seasons. Sea lion appetites peak during winter and again during the spring, while they resiliently adapt to smaller meals and periods of fasting in the summer.

While sea lion appetites may vary predictably, an abundance of fatty, nutritious prey is never guaranteed. Changing ocean conditions can limit the availability of key energy rich prey species such as sandlance and herring, causing prolonged food shortages and nutritional stress—a factor that may be contributing to the dramatic decline in Western Alaska’s Steller sea lion population over the past 30 years.

Scientists attempting to determine the potential effects of nutritional stress in wild Steller sea lions face a critical question: if sea lion diets vary with season, are the effects of nutritional stress also seasonally dependent? If so, what is the most reliable indicator of nutritional stress?

To answer these and other questions, graduate student Ms. Saeko Kumagai studying with Dr. David Rosen and Dr. Andrew Trites (all of the University of British Columbia) recently conducted controlled feeding trials on captive Steller sea lions at the Vancouver Aquarium, assessing their physiological response to brief shortages of different prey over four seasons.

Winter Woes
Under brief periods of nutritional stress, the sea lions in the study lost body mass faster during colder seasons than during warmer seasons. “These changes in mass coincide with the predicted food/energy requirements for the wild Steller sea lions,” the authors write, “and suggest that Steller sea lions may be more susceptible to these types of intense nutritional stress during winter.”

This finding – that changes in the sea lion food supply may have a greater impact in winter months – is particularly important for young sea lions, who allocate most of the energy they consume to growth and development. The authors note that young, nutritionally stressed sea lions may have a smaller body size or a compromised immune system; consequently, they might not respond well to abrupt changes in their environment. An unusually severe winter, for example, might be lethal to an animal that is already nutritionally stressed due to a shortage of key prey.

The study’s findings have important implications for sea lion conservation. Scientists frequently gauge an animal’s health based on its body condition, or relative lipid reserves. However, as this study has shown, this may not be the best indictor of animal ‘health’. First, an animal’s lipid stores undergo natural seasonal changes which do not necessarily reflect changes in the animal’s overall health. Second, because the sea lions use both lipids and protein during periods of nutritional stress, the relative amount of lipid (body condition) does not actually change that much even when losing large amounts of weight. For this reason, Kumagai and her co-authors suggest that alternate assessments of body condition on a seasonal basis could produce a more accurate picture of overall health.

During periods of restricted intake, the sea lions lost body mass at a significantly
higher rate in winter (1.6 ± 0.14 kg/d) than in summer (1.2 ± 0.32 kg/d).

The potential contribution of nutritional stress in Steller sea lions has not yet been resolved, but this study suggests that at least some mechanisms may be season-specific. This finding paves the way for further research into seasonal variations in sea lion diet and energetics, and the decline of Western Alaska’s Steller sea lions.

20 November 2006

Publication:
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.