Seasonal variation in seabird abundance and bycatch at artisanal bottom-set net fisheries in the southern Iberian Atlantic coast

Bycatch is a major cause of seabird mortality, yet most studies focus on industrial fisheries, with limited knowledge on artisanal fisheries. This study investigates the seasonal abundance of seabirds off the southern-eastern coast of mainland Portugal, their attendance at artisanal bottom-set net fisheries, and observed bycatch, using onboard observations and fishermen interviews. From March 2020 to November 2022, 24,643 seabirds were observed attending fishing vessels in 183 daily fishing trips (98.4 % of 186 trips), spanning 20 species across 7 families, with 25 seabirds recorded as bycatch. Lesser black-backed and Yellow-legged gulls ( Larus fuscus and Larus michahellis ) were the most abundant species, followed by Audouin's gulls ( Ichthyaetus audouinii ), Great shearwaters ( Ardenna gravis ), and Northern gannets ( Morus bassanus ). During summer, the abundance of Yellow-legged and Audouin's gulls (local breeders) and Lesser black-backed gulls (non-breeding species) increased with fishery catch per unit effort. Great shearwaters were more affected by gear type, exhibiting higher attendance and bycatch in trammel nets during autumn. 65 % of Great shearwater bycatch occurred during a single fishing event, probably from fishermen cleaning nets and discarding fish and viscera during net setting. Fishermen interviews identified Northern gannets and Great cormorants ( Phalacrocorax carbo ) as the most frequently bycaught species, particularly in winter when onboard observations were limited. These findings underscore the impact of artisanal fisheries on seabird populations and highlight the importance of combining onboard data with fishermen interviews to improve bycatch estimates and inform conservation efforts. Our study also suggests that fishermen behaviour during fishery operations significantly influence seabird bycatch. • Observations and interviews were used to study seabird attendance at fisheries. • Lesser black-backed and Yellow-legged gulls were the main species at fisheries. • Yellow-legged and Audouin's gulls likely relied on fishery subsidies during breeding. • Great shearwater had the highest bycatch, with 65 % of birds recorded in one event. • Net cleaning and discarding during fishing operations may increase bycatch risk.