Utilizing passive acoustic monitoring to deepen understanding of cetacean-fishing gear interactions in the basque gillnet fishery

The gillnet fishery in the Bay of Biscay has the highest bycatch of common dolphins (Delphinus delphis). However, the variety of vessels and fishing methods complicates understanding their susceptibility to incidental catches due to low sampling coverage. Given the voluntary nature of Remote Electronic Monitoring (REM) camera installation, passive acoustic monitoring (PAM) remains the best approach to deepen our understanding of cetacean-fishing gear interactions in the Basque gillnet fishery. In this study, two pilot deployments of hydrophones were conducted on an artisanal vessel using gillnets off the Biscayan coast in 2024. Analysis of the recordings indicated a high presence of bottlenose dolphins (Tursiops truncatus) in August, consistent with previous studies on their distribution along the Basque coast, while no other cetaceans such as common dolphins, striped dolphins (Stenella coeruleoalba), or porpoises (Phocoena phocoena) were detected. Variations in hydrophone depth and the presence of vessels in the area affected recording quality. Future studies should increase recording days and optimize recording parameters. This study demonstrated that hydrophones perform adequately when attached to fishing nets in Basque fisheries, allowing for the investigation of interactions with small cetaceans in this area. Understanding the acoustic behaviour of cetaceans near fishing nets can reveal interaction mechanisms and inform the development of more targeted mitigation measures.