The experimental trials of line weighting options for reduction of incidental mortality of seabirds in Korean tuna longline vessels

Bycatch threatens albatrosses and petrels, prompting all tuna Regional Fisheries Management Organizations to require bycatch mitigation measures in areas overlapping high seabird densities. In the Southern Hemisphere, and with one exception of the IATTC, tuna longliners south of 25°S in Indian and Atlantic oceans and south of 30°S in western Pacific Ocean must use at least two of the following three measures: night setting, bird-scaring lines, and branch line weighting. Adding weights is complex, and carries concerns about negative impacts on target species catch rates and crew safety. From 2013 to 2016, we tested the effects of Lumo Lead® on seabird bycatch, target catch rates, and safety aboard seven Korean tuna longliners. Trials involved 358,649 unweighted and 468,476 weighted lines (40–60 g, 0–200 cm from the hook) across 483 sets. The seabird bycatch rate in weighted branch lines was significantly lower than that in unweighted branch lines (p < 0.001). There was no difference in catch rate when targeting southern bluefin tuna ( Thunnus maccoyii ) and a slight difference for yellowfin tuna ( T. albacares ). The catch rate of albacore tuna ( T. alalunga , ALB) on weighted branch lines was significantly lower (p < 0.001). No safety incidents were reported during the experiment. The reduced ALB catch rate may reflect fishermen's inexperience with line weighting, but this requires further investigation.s. Results suggest the Lumo leads are a safe, effective seabird mitigation measure for Korean longliners but line weighting alone cannot eliminate bycatch and should be used with bird-scaring lines or night setting. More broadly, these findings provide lessons for pelagic longline fisheries globally, showing that safe and practical line weighting can be integrated into existing operations to support sustainable fisheries management and seabird conservation.