Over the past three decades, the Steller sea lion has suffered a mysterious population crash throughout its range in the Gulf of Alaska and Bering Sea . The causes of the population crash are suspected to involve a dwindling prey base, but remain elusive due to the difficulty of obtaining basic biological information. We use a combination of diet and stress hormone analyses of feces to determine the season that poses the greatest risk to sea lions, as well as why some populations are declining while others remain stable.
Validation studies demonstrated that physiological stress is accurately reflected by glucocorticoid hormones in sea lion feces. (Hunt, K.E., A.W. Trites and S.K. Wasser. 2004. Validation of a fecal glucocorticoid assay for Steller sea lions (Eumetopias jubatus). Physiology and Behavior 80:595-601.) We then found that stress hormones were significantly lower in winter versus summer scats from sea lion haul-outs in over a dozen locations in SE Alaska and the Aleutian Islands.
We next validated fecal thyroid hormone measures as an index of nutritional stress in Steller sea lions. Animals reduce thyroid hormone under nutritional stress to reduce metabolism, making their body more efficient at storing energy. Low thyroid hormone levels thus reflect nutritional stress, implying reduced food availability. (Glucocorticoid hormones also tend to be elevated under these circumstances. This increases low levels of circulating glucose associated nutritional stress as well as facilitates feeding behavior.)
Future studies will monitor fecal thyroid and glucocorticoids in feces to examine environmental correlates of nutritional and reproductive stress and population dynamics. Hopefully, these data will improve our understanding of how best to stem the Steller sea lion’s continued decline. This research is being conducted in collaboration with Dr. Andrew Trites of the University of British Columbia, with support from the National Marine Fisheries Service.
Funding was provided by NOAA to the North Pacific Universities Marine Mammal Research Consortium through the North Pacific Marine Science Foundation