MEPS 342:303-310 (2007)  -  doi:10.3354/meps342303

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 δ¹⁵N and δ¹³C values of each blood component as the discriminant functions accurately classified sea lions to the appropriate foraging regions as predicted by a previous study employing scat analysis with an accuracy of 90% for serum and 95% for RBCs. The δ¹⁵N values reflect a decrease in sea lion trophic position moving east to west that is probably driven by changes in sea lion diet. The δ¹³C 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, δ¹³C values do not covary with δ¹⁵N values as a result of trophic enrichment, and are better determinants of foraging location than trophic position.

KEY WORDS: Steller sea lion · Eumetopias jubatus · Foraging ecology · Stable isotopes · Carbon · Nitrogen · Pinniped · Marine mammal