Life in the Fast Food Lane

In the recent documentary film "Super Size Me," the filmmaker explores the effects of a strict diet of fast food on his own body. His health fails rapidly over a period of one month. Although he resumes a normal diet following his self-imposed ordeal, his experiences eerily parallel the plight of entire populations of Steller sea lions in Alaska, which have been in dramatic decline since the late 1970s. One of the most popular theories to explain this decline suggests that changing ocean conditions have robbed the sea lions of an abundance of fatty, nutritious fishes such as herring and sand lance. Instead, they are left to forage for low-fat fishes such as pollock, which offer less nutritional value: in effect, forcing them onto a diet of junk food.

Scientists are naturally curious about how Steller sea lions might adapt to such a change in diet, and researchers at the University of British Columbia are currently studying how strict diets of different types of fish affect Steller sea lions at the Vancouver Aquarium Marine Science Centre. A dramatic weight loss during a study of young Steller sea lions on an exclusive diet of walleye pollock (Theragra chalcogramma) raised questions. Would they have done better if they had been feed roe bearing fish? Are spawning or roe-bearing pollock more nutritious to sea lions than pre-spawning pollock? Does the nutritional value of pollock vary seasonally?

These questions led to a recent study by Dr. David Kitts of Food Nutrition and Health at the University of British Columbia, which examined the seasonal changes in the nutrient and energy content of pollock from the Bering Sea. Using pollock caught in various months through 1998-99, Kitts and UBC colleagues Drs. Minh Dieu Huynh, Chun Hu and Andrew Trites analyzed samples for energy, moisture, protein and fat (lipid) content. Among their results was the surprising conclusion that roe-bearing pollock contain less nutritional energy than pre-spawning pollock — a finding that bodes poorly for Steller sea lion populations, which depend on these fish for their survival in many regions of Alaska.

Steller sea lion appetites typically increase prior to the summer breeding season (in males) and during pregnancy and nursing (in females). Environmental changes, such as colder temperatures in winter months, also encourage sea lions to eat more, and it is especially important for young sea lions to build up sufficient fat stores to survive a northern winter. But while Kitts and colleagues found the sampled pollock to contain the highest protein and moisture content in January, the fish were also at their leanest during the winter — a time when sea lions needed to consume the most fat in order to stay warm. Conversely, the sampled pollock contained the highest energy (lipid) content in July — the height of the breeding season, when most territorial males are fasting.

The results of the present study mirrored previous research on the nutritional content of herring and capelin. After gorging on abundant food in summer months, these fishes likely use their accumulated lipid stores to meet their daily energy needs in winter (when there is less food), as well as to provide energy for gonad production prior to their March spawning. Clearly, this seasonal flux is a natural part of the reproductive success of pollock, but their variations in nutritional status appear to be ill timed to suit the needs of Steller sea lions.

This study suggests that because the energy content of pollock is lowest during important Steller sea lion feeding periods, a diet dominated by pollock may actually be detrimental to their reproductive success.

How these variations affect the survival of entire populations of Steller sea lions is the subject of further research. But one thing is clear: while humans can opt out of a super-sized junk food diet, Steller sea lions do not have that luxury. For better or for worse, they are at the mercy of the ocean’s changing bounty.

7 March 2005

Publication:
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.