Trophic relationships among six species of Icelandic seabirds as determined through stable isotope analysis

Abstract

Stable-nitrogen and carbon isotope ratios (δ15N and δ13C) were determined in liver tissue samples from six species of Icelandic seabirds. Stomach contents were primarily capelin (Mallotus villosus) in north Iceland, and primarily sandlance (Ammodytes sp.) in south Iceland. Stable isotope signatures also were measured in a range of typical prey species, including capelin and sandlance. Combined isotopic signatures in seabirds exhibited significant inter-species and inter-site variation. Similarly, combined isotope signatures differed significantly between prey species. The trophic relationships of the six seabird species differed between the two sites. Northern Fulmars (Fulmarus glacialis) exhibited a relatively depleted mean δ13C value in the north, but the reverse was true in the south. We used a dual-isotope multiple-source mixing model to estimate contributions of different prey species to the diet of seabirds based on their isotope signatures. Whereas the general patterns of proportions of different prey in seabirds' diets in the north and south produced by the model supported conventionally-collected dietary data, the importance of some relatively rare prey species tended to be overestimated.