Investigating Trophic Relationships of Pinnipeds in Alaska and Washington Using Stable Isotope Ratios of Nitrogen and Carbon

Authors

Keith Hobson, Prairie and Northern Wildlife Research Center, 115 Perimeter Road, Saskatoon, Saskatchewan, CanadaFollow
John Sease, National Marine Mammal Laboratory
Richard Merrick, National Marine Mammal Laboratory
John Piatt, National Biological Service, 1011 East Tudor Road, Anchorage, Alaska

Date of this Version

1-1997

Comments

Published in MARINE MAMMAL SCIENCE, 13( 1): 114-132 (January 1997).

Abstract

We measured stable-nitrogen (VN) and stable-carbon (VC) isotope ratios in muscle and hair from 7 northern fur seals (Callorhinus ursinus) from the Pribilof Islands, Alaska, and 27 Steller sea lions (Eumetopias jubatus), and 1.4 harbor seals (Phoca vitulina) from the Gulf of Alaska and coast of Washington State, in order to contrast dietary information derived from isotopic vs. available conventional dietary studies. Stable-nitrogen-isotope analysis of muscle revealed that harbor seals were enriched over sea lions (mean δ¹⁵N = l8.6% vs. 17.5%) which were in turn enriched over northern fur seals (mean 6i5N = 16.6%0). Trophic segregation among these species likely results primarily from differential reliance on herring (Clupea harengus), Atka mackerel (Pleurogrammus monopterygius), and large us, small walleye pollock (Theregra chalcogramma). According to their δ¹⁵N values, adult male Steller sea lions showed a higher trophic position than adult females (mean δ¹⁵N: 18.0% vs. 17.2 %), whereas adult female northern fur seals were trophically higher than juvenile male fur seals (mean VSN: 16.5%0 vs. 15.0%). Each of these observed differences likely resulted from differential reliance on squid or differences in the size range of pollock consumed. Three northern fur seal pups showed higher δ¹⁵N enrichment over adults (mean 17.7% vs. 15.8%) due to their reliance on their mother’s milk. Stable-carbon isotope measurements of ha.ir revealed a cline toward more negative values with latitude. Segregation in hair δ¹³C between Steller sea lions and harbor seals off the coast of Washington (mean δ¹³C: -13.6% vs. -15 .0%) reflected the greater association of harbor seals with freshwater input from the Columbia River. Our study demonstrates the utility of the stable isotope approach to augment conventional dietary analyses of pinnipeds and other marine mammals.