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Trophic overlap between marine mammals and fisheries in subtropical waters in the western South Atlantic

Rodrigo Machado*, Larissa Rosa de Oliveira, Paulo Henrique Ott, Manuel Haimovici, Luis Gustavo Cardoso, Lucas Milmann, Maria Alejandra Romero, Roberta Aguiar dos Santos, Márcio Borges-Martins

*Corresponding author:

ABSTRACT: Marine mammals and humans are apex predators and both may compete for fish in ecosystems under continuous fishing pressure. We assessed the degree of trophic overlap between prey species found in the diet of five marine mammals (39 specimens of sea lion, Otaria flavescens, 61 fur seal, Arctocephalus australis, 76 franciscana dolphins, Pontoporia blainvillei, 25 bottlenose dolphins, Tursiops truncatus and 28 Lahille’s bottlenose dolphins, Tursiops gephyreus) and the catches of the six main commercial fishing gears used in southern Brazil (coastal gillnets, oceanic gillnets, purse seine, and demersal pair trawling, otter board trawling and double-rig trawling) between 1993 and 2016. An adjusted general overlap index indicated an overall moderate to high overlap. Specific overlap analysis (SO) showed that O. flavescens and T. truncatus presented high trophic relationships with fisheries followed by T. gephyreus. Smaller interactions were observed for A. australis and P. blainvillei, even though they also exploit commercial fishing resources. Moreover, coastal bottom gillnet and pair bottom trawling are the fisheries that most target the fish species favored by O. flavescens, T. gephyreus and T. truncatus. The information presented in this study on trophic interactions may assist decision making for both fishery management and conservation measures for these apex predators. Commercial fishing activities are a major threat to marine mammals both regionally and globally. Current levels of fishing or its intensification may lead to dramatic changes in the coastal marine food web, including additional threats to coastal marine mammal populations in southern Brazil.