Dr Andrew Trites
Seasonal variation in nutrient composition of Alaskan walleye pollock (Theragara chalcogramma) and its effects on the nutritional status of Steller sea lions (Eumetopias jubatus)
Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska declined since the late 1970s. Their population decline might be related to a shift in their diet fiom fatty, high-calorie fishes such as herring (Elupea palhi) to low energy density fish such as walleye pollock (Theragma chalcogrumma). I compared the nutritional value of herring with pollock and explored seasonal changes in the nutrient content of pollock.
I also compared the nutritional status of three captive Steller sea lions fed pollock and herring. Hemng was a more concentrated in dietary lipid (p<0.001) and energy source (p<0.001) than pollock. The protein of herring was also higher in digestibility (p=0.015) than pollock protein, which could indicate that even if ingested energy was equal in both diets, absorbed energy for body knctions may be reduced when pollock is eaten.
There was little difference in the protein quality of pollock and herring with the exception that valine was more abundant in herring (p=0.004). The energy content,of pollock changed seasonally, with the peak in energy concentration occurring in the summer and fall (July to November) and then declining over the winter prior to spawning. Captive Steller sea lions lost mass or increased mass at a slower rate on a pollock diet than when they consumed hemng, at which time, they all increased in mass.
The sea lions had lower levels of plasma cholesterol when fed pollock. Their red blood cells were also more susceptible to oxidation, which corresponded with lower plasma vitamin E levels. These findings suggest that consumption of predominantly pollock has nutritional consequences for the Steller sea lion. Even if they are able to increase their caloric intake to maintain their body mass, Steller sea lions may still be more susceptible to disease originating from oxidative stress.
Seasonal variation in nutrient composition of Alaskan walleye pollock (Theragara chalcogramma) and its effects on the nutritional status of Steller sea lions (Eumetopias jubatus). Azana, C.D.P. 2002. In Zoology. University of British Columbia. 109 pages (PDF)