Spatial overlap between cetaceans and gillnet fisheries in Southeastern Brazilian waters and implications for bycatch mitigation

Abstract

Abstract Human activities threaten marine mammal populations, with bycatch in gillnet fisheries being a major concern. To investigate interactions between fisheries and cetaceans and guide effective management, integrative approaches combining spatial and ecological information are essential. Recent long-term monitoring programs in Brazilian waters have generated extensive datasets on cetacean occurrence and fisheries activities, providing new opportunities to assess potential bycatch exposure and interaction risk. Using these data, we evaluated spatial overlap between cetaceans and gillnet fisheries in the Santos Basin (Brazil), a region of high marine biodiversity and intense anthropogenic pressures. We mapped core areas used by cetacean species and gillnet fisheries to identify spatial co-occurrence and infer species more exposed to bycatch. Our findings revealed that gillnet fisheries concentrate primarily along the continental shelf. Among the 13 cetacean species analysed, Guiana dolphin ( Sotalia guianensis) , Franciscana ( Pontoporia blainvillei ), and bottlenose dolphin ( Tursiops truncatus ) showed the highest spatial overlap and may be particularly exposed to bycatch. The Guiana dolphin and the Franciscana, which inhabit coastal and inner-shelf waters, appear particularly vulnerable due to their conservation status and habitat use. We also identified priority areas for bycatch risk assessment and management, including a region of high cetacean diversity. While bycatch risk is influenced by multiple factors, including seasonal variations and fishing effort, our results provide a first-order approximation of exposure based on spatial overlap, and are consistent with mortality information from beach monitoring. These findings can help prioritize monitoring efforts and support mitigation strategies aimed at reducing negative interactions between fisheries and cetaceans.