Revealing potential Atlantic salmon bycatch hotspots from pelagic fisheries landings across the Northeast Atlantic

Atlantic salmon (Salmo salar) marine survival has declined markedly over the past five decades, leading to concerns about future population viability. While many studies have explored the impacts of stressors such as climate change and predation, less is known about bycatch from pelagic fisheries targeting finfish species. This study, focused on the first few months of the salmon marine migration period, narrows down the space-time domains where pelagic fisheries bycatch is most likely to occur. Landings from an International Council for the Exploration of the Sea-led data call were used to assess bycatch exposure. Temporal patterns of monthly landings of six commercially important pelagic finfish species (mackerel Scomber scombrus, herring Clupea harengus, horse mackerel Trachurus trachurus, sardine Sardina pilchardus, capelin Mallotus villosus, and chub mackerel Scomber colias) within the Northeast Atlantic region from 2000 to 2022 were considered alongside reconstructed salmon migration regions. This study indicated that bycatch exposure from herring and mackerel fisheries was higher than that of other pelagic fisheries along the shared salmon migration corridor between April and August, with cumulative exposure highest over time for the herring fishery. Southern salmon populations were also exposed to the horse mackerel and sardine fisheries in April. Temporal patterns of monthly pelagic landings were compared with patterns of marine return rates in salmon index populations, but no consistent relationships were present. Quantifying salmon bycatch rates at sea remains challenging due to highly variable and sparse data but, given the severity of declines across their range, conservation measures that address factors affecting marine survival of salmon are urgently needed. Future investigations using emerging technology in the pelagic fisheries identified in this study could provide opportunities to develop more robust information on bycatch risk to Atlantic salmon stocks and to direct appropriate mitigation measures.