INVESTIGATING TROPHIC RELATIONSHIPS OF PINNIPEDS IN ALASKA AND WASHINGTON USING STABLE-ISOTOPE RATIOS OF NITROGEN AND CARBON

We measured stable-nitrogen (delta(15)N) and stable-carbon (delta(13)C ) isotope ratios in muscle and hair from 7 northern fur seals (Callorh inus ursinus) from the Pribilof Islands, Alaska, and 27 Steller sea li ons (Eumetopias jubatus), and 14 harbor seals (Phoca vitulina) from th e Gulf of Alaska and coast of Washington Scare, in order to contrast d ietary information derived from isotopic vs. available conventional di etary studies. Stable-nitrogen-isotope analysis of muscle revealed tha t harbor seals were enriched over sea lions (mean delta(15)N = 18.6 pa rts per thousand vs. 17.5 parts per thousand) which were in Nn enriche d over northern fur seals (mean delta(15)N = 16.6 parts per thousand. Trophic segregation among these species likely results primarily from differential reliance on herring (Clupea harengus), Atka mackerel (Ple urogrdmmus monopterygius), and large us. small walleye pollock (Thereg ra chalcogramma). According to their delta(15)N values, adult male Ste ller sea lions showed a higher trophic position than adult females (me an delta(15)N: 18.0 parts per thousand vs. 17.2 parts per thousand) wh ereas adult female northern fur seals were trophically higher than juv enile male fur seals (mean delta(15)N: 16.5 parts per thousand vs. 15. 0 parts per thousand). Each of these observed differences likely resul ted from differential reliance on squid or differences in the size ran ge of pollock consumed. Three northern fur seal pups showed higher del ta(15)N enrichment over adults (mean 17.7 parts per thousand vs. 15.8 parts per thousand) due to their reliance on their mother's milk. Stab le-carbon isotope measurements of hair revealed a dine toward more neg ative values with latitude. Segregation in hair delta(13)C between Ste ller sea lions and harbor seals off the coast of Washington (mean delt a(13)C: -13.6 parts per thousand vs. -15.0 parts per thousand) reflect ed the greater association of harbor seals with freshwater input from the Columbia River. Our study demonstrates the utility of the stable i sotope approach to augment conventional dietary analyses of pinnipeds and other marine mammals.