Assessing selectivity and bycatch of endangered, threatened and protected species in artisanal tuna fisheries in the NE Atlantic

Fisheries bycatch is a recognised pressure on marine megafauna, prompting international agreements to mandate monitoring and mitigation of impacts. The International Commission for the Conservation of Atlantic Tunas (ICCAT) specifically calls for systematic data collection on Endangered, Threatened and Protected species (ETPs), particularly in data-deficient regions. To address this need, the Spanish artisanal tuna fishery—comprising trolling and pole-and-line métiers —implemented an onboard observer programme in 2016 to document ETP bycatch in the Northeast Atlantic, an important migratory and overwintering area for marine megafauna. Our aims were to (1) assess fishery selectivity through catch and discard composition, (2) describe ETP species occurring in the fishing grounds and (3) characterise ETP bycatch events, including their frequency, the number of individuals involved per event, and the condition of animals upon release. The fishery showed high selectivity, with >99% of recorded catches consisting of target tuna species. Observers documented 18 ETP species occurring around fishing vessels (three marine mammals and 15 seabirds), illustrating substantial megafauna richness in the fishing grounds. ETP bycatch was limited to three seabird species: great shearwater Ardenna gravis , northern gannet Morus bassanus and northern fulmar Fulmarus glacialis . Great shearwaters accounted for most individuals recorded and were occasionally captured in events involving several birds during trolling operations, whereas northern gannets were recorded only in pole-and-line operations, where events consistently involved few individuals. A total of 52 individuals from three seabird species were recorded in 22 bycatch events, most of which were released alive. Spatial patterns reflected known species ecology, with shearwater bycatch concentrated in oceanic waters and gannet bycatch occurring primarily in shelf-slope areas. These findings provide the first systematic characterisation of ETP species occurrence and bycatch in this artisanal tuna fishery, supplying essential baseline information for ICCAT and supporting the development of monitoring approaches and management actions to reduce risks to ETP species in the Northeast Atlantic.