TROPHIC RELATIONSHIPS AMONG SEABIRDS IN CENTRAL CALIFORNIA - COMBINEDSTABLE-ISOTOPE AND CONVENTIONAL DIETARY APPROACH

We used stable isotope analysis (SIA) and conventional techniques of d iet assessment to determine marine trophic relationships in the Gulf o f the Farallones, California, with an emphasis on marine birds. Stable -carbon (delta(13)C) and nitrogen (delta(15)N) isotopes were obtained from 98 tissue samples of 16 species representing primary and secondar y consumers in 1993-1994. The values of delta(13)C ranged from -20.1 p arts per thousand in whole euphausiids (krill) to -15.0 parts per thou sand in muscle of northern sea Lions. Values of delta(15)N showed step -wise trophic enrichment and ranged from 11.2 parts per thousand in eu phausiids to 19.8 parts per thousand in sea lions. SIA of egg albumen from birds indicated reliance on zooplankton by Cassin's Auklet, Commo n Murre, and Western Cull, and on fish by Brandt's and Pelagic Cormora nts, Rhinoceros Auklets, and Pigeon Guillemots during egg formation (A pril-May). However, analysis of prey brought to chicks during summer i ndicated the prevalence of fish in the diet of most seabirds, except C assin's Auklet which fed primarily on krill. Results suggest a shift i n trophic level and diet between spring and summer from krill to fish for Common Murres. delta(13)C analysis confirmed that Brandt's Cormora nts and northern sea lions feed in neritic habitats, whereas Cassin's and Rhinoceros Auklets foraged in epipelagic offshore waters. Our appr oach demonstrates the utility of combining both SIA and conventional d ietary assessments to understand trophic relationships in dynamic mari ne ecosystems.