Biological and Physical Environmental Drivers of Diet Variation in Northern Fur Seals

The eastern Bering is a productive high-latitude ecosystem characterized by high interannual variability in physical environmental conditions that impact biological communities. We investigated how the diet composition of northern fur seals (Callorhinus ursinus) breeding on the Pribilof Islands was influenced by this variation, focusing on water temperatures (surface and bottom) and an index of walleye pollock abundance within foraging areas. We also explored whether interannual variation in diet composition influenced fur seal pup mortality rates or body mass. The frequency of occurrence (FO) of all eight fur seal prey groups detected from hard parts analysis of samples collected from 1987 to 2012 was affected by interannual variation in at least one of the three environmental variables. Pollock was the predominant prey group across the study years, highlighting the importance of this species to Pribilof Island fur seals. Not only was pollock consumed more frequently as it became more abundant within fur seal foraging areas, but its relative abundance also affected how frequently other prey groups were consumed. A considerable amount of variation in FO of almost all prey groups was explained by year effects, suggesting that water temperatures alone were not sufficiently capturing the influences of regional and local physical environmental conditions on prey availability for fur seals. The summed FO of non-pollock prey groups had a small but detectable effect on the mass of male pups, indicating that the availability of prey groups beyond just pollock is somewhat beneficial for female northern fur seals early in lactation. Our results suggest that projected environmental changes in the eastern Bering Sea are likely to influence fur seal diets, but predicting the magnitude and direction of such changes is hampered until the underlying drivers of the observed temporal trends are better resolved.