Regional differences in foraging of young-of-the-year Steller sea lions Eumetopias jubatus in Alaska: stable carbon and nitrogen isotope ratios in blood

Abstract

Stable nitrogen and carbon isotope ratios were measured from red blood cell (RBC) and serum components from 9 mo old Steller sea lions Eumetopias jubatus captured during February and March 2000 and 2001 from the Gulf of Alaska (GOA; Region 1, n = 11), the eastern Aleutian Islands (Region 3, n = 5), and the central Aleutian Islands (Region 4, n = 5) to assess their foraging ecology. Isotope ratios from sea lions were compared with those from probable prey species and results were compared with those from a study demonstrating regional differences in the diets of juvenile and adult sea lions through the use of scat analysis. Discriminant analyses using the δ 15 N and δ 13 C values of each blood component as the discriminant functions accurately classified sea lions to the appropri- ate foraging regions as predicted by a previous study employing scat analysis with an accuracy of 90% for serum and 95% for RBCs. The δ 15 N values reflect a decrease in sea lion trophic position mov- ing east to west that is probably driven by changes in sea lion diet. The δ 13 C values demonstrated clear distinctions between foraging locations that are likely driven by known geographic carbon isotope patterns observed in the GOA and along the Aleutian archipelago. Our data present further evidence that, in the marine environment, δ 13 C values do not covary with δ 15 N values as a result of trophic enrichment, and are better determinants of foraging location than trophic position.