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Changes in the feeding ecology of South American sea lions on the southern Brazilian coast over the last two decades of excessive fishing exploration

Abstract

In the last decades, an increasing fishing effort and a decreasing trend in fish catches have been observed in southern Brazil. Considering that marine mammals and fisheries usually compete for the same resources, it is reasonable to presume that the feeding ecology of these predators is affected by the current scenario. To evaluate this hypothesis, long-term variation in the diet of the South American sea lion (Otaria flavescens) relative to fisheries exploitation was analyzed for two periods (1993–2003 versus 2004–2014). The degree of overlap between the relative biomass of the sea lions’ diet and the target species of six types of local fishery was analyzed. An increase in prey overlap between sea lions and fisheries was observed in the more recent sampling period, along with an increase in prey diversity, richness, and niche breadth of the sea lions’ diet. These results suggest that the overfishing scenario could partly explain the modified feeding ecology of the sea lions. In this context, we recommend a review and better regulation of the current fishing effort in the region, which we believe will be an important step to maintain the fish stocks and minimize the impact of fishing on marine top predators.

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Acknowledgements

We would thank all our colleagues of the Grupo de Estudos de Mamíferos Aquáticos do Rio Grande do Sul (GEMARS) for their help during the necropsies and collection of specimens. Logistical support was provided by Universidade Estadual do Rio Grande do Sul (UERGS) and Centro de Estudos Costeiros, Limnológicos e Marinhos (CECLIMAR) da Universidade Federal do Rio Grande do Sul (UFRGS). We are grateful to Diego Janisch Alvares who kindly helped us with map editing. We would also thank all our colleagues of the Bird and Mammal Laboratory for the space provided for the analysis of the diet of the sea lions. This paper is part of the Doctoral Thesis of Rodrigo Machado in the Programa de Pós-Graduação em Biologia Animal in UFRGS. For multiple agency grants: This work was supported by the Coordenação de Aperfeiçoamento Pessoal de Nível Superior - CAPES (doctoral grant) for Rodrigo Machado; Fundo Nacional do Meio Ambiente - FNMA-MMA; and Conselho Nacional de Desenvolvimento Científico e Tecnológico – CNPq. P.H. Ott and L.R. Oliveira were supported by CNPq (no. 572180/2008-0 and 151307/2005-9, productivity grants no. 303813/2011-3 and 308650/2014-0).

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Machado, R., de Oliveira, L.R., Ott, P.H. et al. Changes in the feeding ecology of South American sea lions on the southern Brazilian coast over the last two decades of excessive fishing exploration. Hydrobiologia 819, 17–37 (2018). https://doi.org/10.1007/s10750-018-3618-8

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Keywords

  • Otaria flavescens
  • Fishing interaction
  • Overfishing
  • Trophic overlap
  • Niche breadth