Unravelling the deep: Assessing the bycatch of deep-sea elasmobranchs in crustacean bottom trawl fisheries in Portugal

Deep-sea elasmobranchs (DSE) play a crucial role in marine ecosystems. However, they are poorly studied and face threats from overfishing, emphasizing an urgent need for improved scientific information, monitoring, and management strategies to reduce their bycatch. This study aimed to assess DSE bycatch from crustacean bottom trawling in southern Portugal, considering depths above and below 800 m (Regulation 2016/2336). Potential bycatch of deep-sea sharks was evaluated using in situ observations from the months of February and March and extrapolated for the fishing ban period (2017–2022). A total of 1559 specimens belonging to 18 DSE species were collected from 77 hauls between June 2020 and May 2022. Despite trawlers preferences for fishing above 800 m in the South, fishing below 800 m in the Southwest resulted in increased bycatch of DSE, including protected, uncommon, and endangered species such as Deania calceus, Mitsukurina owstoni , and Centroscymnus coelolepis . Furthermore, the areas and depth strata occupied by species like Galeus melastomus and Scymnodon ringens , suggested habitat flexibility, while others showed an apparent preference for specific depths and areas. These findings highlight the complexity of managing DSE populations amidst fishing pressures and depth restrictions. Despite the ban imposed to fishing below 800 m, bottom trawling persisted, leading to a potential elevated bycatch of deep-sea sharks for the months of February and March. This study emphasizes the urgency for improved enforcement of regulations in Portuguese waters and calls for the implementation of effective bycatch mitigation and fisheries management practices to safeguard DSE populations. • In Portugal bottom trawling was conducted at prohibited depths (below 800 m). • Below 800 m, deep-sea elasmobranchs’ weight was 25–58 % the total weight of the catch. • Large, rare, protected and endangered sharks presented higher bycatch < 800 m in the SW in comparison with depths > 800 m. • Above 800 m there are smaller deep-sea elasmobranchs’ specimens and species in comparison with depths < 800 m. • Endangered species generally presented the lowest abundances.