Trophic Relationships among Seabirds in Central California: Combined Stable Isotope and Conventional Dietary Approach

Abstract

We used stable isotope analysis (SIA) and conventional techniques of diet assessment to determine marine trophic relationships in the Gulf of the Farallones, California, with an emphasis on marine birds. Stable-carbon (δ 13C) and nitrogen δ 15N isotopes were obtained from 98 tissue samples of 16 species representing primary and secondary consumers in 1993-1994. The values of δ 13C ranged from -20.1% in whole euphausiids (krill) to -15.0% in muscle of northern sea lions. Values of δ 15N showed step-wise trophic enrichment and ranged from 11.2% in euphausiids to 19.8% in sea lions. SIA of egg albumen from birds indicated reliance on zooplankton by Cassin's Auklet, Common Murre, and Western Gull, and on fish by Brandt's and Pelagic Cormorants, Rhinoceros Auklets, and Pigeon Guillemots during egg formation (April-May). However, analysis of prey brought to chicks during summer indicated the prevalence of fish in the diet of most seabirds, except Cassin's Auklet which fed primarily on krill. Results suggest a shift in trophic level and diet between spring and summer from krill to fish for Common Murres. δ 13C analysis confirmed that Brandt's Cormorants and northern sea lions feed in neritic habitats, whereas Cassin's and Rhinoceros Auklets foraged in epipelagic offshore waters. Our approach demonstrates the utility of combining both SIA and conventional dietary assessments to understand trophic relationships in dynamic marine ecosystems.