Bycatch in the balance: the trawl fisheries impact on South American sea lions in San Matias Gulf, with comments on other species

Abstract

Bycatch is recognized as the primary threat to the viability of marine mammal populations. Given that bycatch levels display strong variations in space and time, knowledge of individual fishery-marine mammal interaction will likely achieve working solutions. Here, we quantify marine mammal bycatch in the trawl fishery of the San Matías Gulf, Argentina, for the first time using data from onboard observers and fishing effort records. We infer expected annual mortality, evaluate the probability of exceeding conservation thresholds, and explore the importance of relevant covariates to account for broadscale differences in bycatch rates. The South American sea lion (Otaria byronia) was the most incidentally caught species (46 individuals), while only one common dolphin (Delphinus delphis) was taken. Bycatch of sea lions was primarily associated with the catch of the target species, proximity to colonies, and the identity of the target species. The maximum estimated bycatch rate was 0.12 animals/haul and 0.08 animals/tonne of target species landed, with significantly higher bycatch rates observed in the hake fleet. For the whole period, the annual bycatch mortality were below the reference points. Our findings highlight potential hotspots of marine mammal bycatch and provide valuable insights for refining future mitigation measures and enhancing conservation efforts.