Trophic relationships among Antarctic fulmarine petrels: Insights into dietary overlap and chick provisioning strategies inferred from stable-isotope (δ15N and δ13C) analyses

Abstract

We used stable-isotope analysis (SIA) to evaluate trophic relationships in an Antarctic seabird community. We determined natural abundances of stable-nitrogen (δ15N) and stable-carbon (δ13C) isotopes from blood samples (n = 283) from adults and chicks of 4 Antarctic fulmarine petrel species (Fulmarus glacialoides, Thalassoica antarctica, Daption capense and Pagodroma nivea) during 2 consecutive breeding seasons, 1994/1995 and 1995/1996, and from representative prey items. Our objectives were to use the isotope approach to infer trophic status and diet composition within and between species, addressing interspecific and temporal variability within this seabird community, and to investigate potential age-related differences in assumed trophic position within species. Prey δ13C values ranged from -26.8‰ in amphipods to -23.9‰ in adult Antarctic silverfish. Seabird δ13C values ranged from -25.3‰ in Antarctic petrel chicks to -23.8‰ in cape petrel adults. Prey δ15N values ranged from 4.0‰ in euphausiids to 10.7‰ in adult Antarctic silverfish. Seabird δ15N values ranged from 8.4‰ in Antarctic petrel adults to 12.0‰ in snow petrel chicks. There was considerable interspecific overlap in assumed trophic positions amongst the 4 petrel species, and we conclude all species consumed fish and krill. Despite this apparent overlap, the range in δ15N values for petrels corresponded to the equivalent of 1 full trophic level, and estimated trophic level varied with both species and age. A simple trophic level model, constructed based on the δ15N data, predicted trophic levels ranging from 2.3 in krill to 4.7 in snow petrel chicks. Snow petrels and Antarctic fulmars tended to have higher δ15N values than Antarctic and cape petrels, suggesting a higher proportion of fish in their diets. Petrel chicks consistently had higher δ15N values than adults, which suggests trophic segregation between adults and chicks. We discuss advantages of selectively provisioning chicks with higher trophic level prey. Extensive overlap and a relatively narrow range of δ15N values are consistent with a food web comprised of few trophic steps.