Brazilian gillnets overlap most with habitats used by three dolphin species

Mapping in the Santos Basin found especially strong overlap for Guiana dolphins, franciscanas and bottlenose dolphins.

Editorial illustration — not from the study.

Researchers combined long-term records of cetacean sightings and fishing activity to map core-use areas in the Santos Basin, a region with high marine biodiversity and substantial human activity. They found that gillnet fisheries were concentrated mainly along the continental shelf, where several cetacean species also occur.

Guiana dolphins, franciscanas and bottlenose dolphins had the highest spatial overlap with the fisheries. The researchers identified priority areas for assessing bycatch risk and managing interactions, including a region with high cetacean diversity.

Where dolphins and nets overlap

Among the 13 cetacean species analysed, Guiana dolphins, franciscanas and bottlenose dolphins showed the greatest spatial overlap with gillnet fisheries. The Guiana dolphin and franciscana, which use coastal and inner-shelf waters, may be particularly exposed because their habitats overlap with areas where gillnets are concentrated. The study also identified priority locations for bycatch risk assessment and management, including an area with high cetacean diversity. The spatial patterns were consistent with mortality information from beach monitoring.

Priorities for reducing bycatch

Gillnet bycatch is a major concern for marine mammals. Identifying places where fishing activity and cetacean habitats overlap can help direct monitoring and mitigation efforts, especially for the Guiana dolphin and franciscana, which have conservation concerns and use coastal waters. The maps provide a basis for prioritising areas, although reducing bycatch will require considering other factors such as seasonal changes and fishing effort.

What the maps can show

The study used spatial data from long-term monitoring of cetacean occurrence and fisheries activity in the Santos Basin. Its conclusions are based on co-occurrence: overlap indicates potential exposure to bycatch, not confirmed interactions or deaths caused by the fisheries. The researchers describe the results as a first-order approximation, because bycatch risk also varies with season and fishing effort. The findings were consistent with beach-monitoring mortality information, but the abstract does not report a direct measurement of bycatch rates.

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