Geographical variation in Steller sea lion quality in Alaska

Geographical variation in Steller sea lion quality in Alaska

L. Schaufler, E. Logerwell, and J. Wollenweider

Geographical variation in Steller sea lion quality in AlaskaThis is part of Sea Lions of the World
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Nutritional stress is one of the leading hypotheses explaining the decline in Steller sea lion populations of the western stock. Central to this hypothesis is the possibility that western stock sea lions encounter prey of significantly lower quality than those from the eastern stock. We collected and analyzed over 1,200 whole fish representing species identified as sea lion prey items from the Aleutian Islands and southeastern Alaska, including species that reside in both regions. We present proximate composition and calculated mean energy densities based on the lipid and protein contents for the sampled fish. Initial comparisons of the proximate compositions and energy densities between the Aleutian Islands and southeastern Alaska fish on a species basis revealed significant differences in prey energetic content in the two regions for the sampled prey. Overall, the mean energy density for 22 forage species from southeastern Alaska (1.62 ± 0.02 kcal per g on a wet weight basis) was greater than that of 15 species from the Aleutians (1.44 ± 0.03 kcal per g), but these variations could be attributed to size differences among the fish sampled from the two regions as well as species composition and collection season differences. For example, Pacific cod sampled from the Aleutians were significantly larger (p < 0.001) than those from southeastern Alaska and had a higher energy density (p < 0.001). However, controlling for size revealed no difference in energy density between the two populations of cod (p 0.5). Similarly accounting for size, no difference was found in the energy density of walleye pollock or arrowtooth flounder from the two locations. In contrast, squid and sandfish from southeastern Alaska had higher energy densities (p < 0.01) while Aleutian rockfish had higher energy densities than those from southeastern Alaska (p < 0.001), though these may represent seasonal and species composition differences. These data reveal the importance of considering size, season, and species when making energy density comparisons of the available prey between geographical regions.

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