Sea lions suffer from “fast food” diet
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, reports the North Pacific Universities Marine Mammal Consortium.
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.
| Posted 03.08.05 at 5:13 pm