| Trophic changes in populations of Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus) and harbor seals (Phoca vitulina) in the eastern Bering Sea and Gulf of Alaska were studied using stable isotope analysis. Declining populations of all three species of pinnipeds prompted this study to determine if changes in diet, likely resulting from food limitation, contributed to the declines. Stable carbon and nitrogen isotope ratios were analyzed in the vibrissae (whiskers) and body tissues of pinnipeds from 1993–1998 and compared with muscle tissue from prey species during the same time period to determine pinniped trophic dynamics. Vibrissae growth rate studies revealed harbor seal vibrissae are only retained for one year then replaced, while Steller sea lions maintain their vibrissae for several years. Isotopic data from all three species are consistent with diets composed of walleye pollock (Theragra chalcogramma ) at various times and locations throughout the year. Steller sea lion and northern fur seal vibrissae revealed regular oscillations along their lengths in both carbon and nitrogen isotope ratios that likely corresponded to regional isotopic differences. As these animals moved or migrated from one region to another during the year, they metabolically incorporated the different regional isotope ratios through their prey. Because these animals return to their rookery to pup, breed and molt each year, the isotope ratios in the vibrissae showed a regular pattern of enrichment and depletion. Harbor seals, which tend to stay in one geographic location, have relatively static isotope ratios in their vibrissae, while seals that moved into offshore waters had fluctuating isotope ratios that corresponded to regional differences. No trophic shifts, as evidenced by major changes in nitrogen isotope ratios, were present in any tissues from the three species over the period 1975–1998. Stable isotope ratios of bone collagen for all three species from 1950–1997 indicated no change in trophic level but did reveal a decline in the carbon isotope ratios. These data are supportive of evidence that the seasonal primary production in the North Pacific Ocean has declined and may have contributed to a decreased carrying capacity impacting these top trophic level organisms. |
