Overlap between pinniped summer diet and artisanal fishery catches in Uruguay

Abstract

The similarity of the trophic ecology of Arctocephalus australis and Otaria flavescens in Uruguay was studied to help understand a possible trophic competition between the two species and its relation with artisanal fisheries. Dietary data for both pinnnipeds together with artisanal fisheries catch data were analyzed to estimate the overlap in prey species and sizes. Diet overlap was very high (Colwell and Futuyma overlap index, C = 0.80), with three fish species (Anchoa marinii, Cynoscion guatucupa, and Trichiurus lepturus) comprising more than 90% of the total number of individuals consumed by both species. In addition, the length of these three fish prey taken by fur seals and sea lions was similar, suggesting a large overlap not only in species but also in the size of prey ingested. The main difference between the two pinniped diets was that O. flavescens showed a more diverse diet than A. australis. Concerning pinniped fisheries interactions, a low level of overlap was observed between the diet of each pinniped and artisanal fisheries catches (C = 0.22 for O. flavescens, and C = 0.09 for A. australis). Possibly, sea lion diet is broader than artisanal fisheries catches. Length comparisons for C. guatucupa, the only species well represented in both pinniped species diet and fisheries catches, indicated that individuals taken by the latter were larger than those consumed by the pinnipeds. However, the use of different age classes of the same resource, at different times or spatial locations do not imply less intensity of the interaction.