The importance of pelagic longline fishery discards for a seabird community determined through stable isotope analysis

Seabird species within a community are expected to have distinct trophic niches according to foraging methods and body size, but some seabirds exploit fishery waste which can be quite distinct from natural foods. Even in the presence of fishery discards we hypothesize that a pelagic seabird community is structured according to body size, feeding methods, and access to discards. We measured carbon and nitrogen stable isotopes in whole blood of seabirds from the offshore wintering community of Procellariiformes (albatrosses, petrels, shearwaters and storm-petrels) in the SW Atlantic Ocean. We compared this with isotope values of potential prey items and fishery discards, to investigate the importance of discards from the tuna-shark pelagic longline fishery in the diet of these birds. Despite contrasting body masses and feeding techniques, there was extensive overlap in the range of isotopic ratios for different species. Carbon isotope values were typical of the subtropical offshore region. Nitrogen values also showed a high degree of overlap among species and clearly signify that the birds are feeding on fishery waste (especially shark liver). Recently arrived migrants from the Antarctic and sub-Antarctic, and species still rearing chicks when sampled (e.g. wandering albatross Diomedea exulans) had carbon isotopic values indicative of Antarctic and sub-Antarctic regions. All species breeding at more southerly latitudes underwent a marked shift (increase) in carbon and nitrogen isotopes, indicating a change in diet between breeding and wintering seasons. Cory's shearwater (Calonectris diomedea), the only sampled species not attending vessels, had low nitrogen values reflecting a diet of flying fish which occur naturally in the area and showed no change in isotope values between breeding and wintering grounds. Mixing models demonstrate the need to include an external food source (Antarctic krill) to recreate the isotope values obtained from seabird blood: this corresponds to the blood of wintering migrants retaining a minor component from their diet in the southern breeding grounds. The stable isotope results suggest that the availability of discards from pelagic fisheries in the SW Atlantic Ocean lead to an artificially and poorly structured seabird community, with most species utilizing the same food resource. The balance between population benefits obtained from feeding upon otherwise unavailable discards, and the costs from incidental mortality on longline fishing hooks is likely to differ among species, with positive effects for small-sized species and negative effects on albatrosses and Procellaria petrels.