Opportunities to reduce bycatch mortality of threatened species in a data-limited tuna longline fishery

Marine megafauna exposed to fisheries bycatch belong to some of the most threatened taxonomic groups and include apex and mesopredators that maintain ecosystem functions and stability. To identify evidence-informed bycatch management interventions for a data-limited albacore tuna longline fishery, this study analyzed a short time series of electronic monitoring data. The fishery requires the employment of additional seabird bycatch mitigation methods when fishing in the southwestern Pacific Ocean to bring it into compliance with regional management measures such as by adjusting the branchline weighting design or the time of day of setting. Based on fitting the data to a generalized linear mixed regression model, nighttime deep setting did not significantly affect the target species catch rate and had a >99% lower seabird catch rate compared to partial-daytime deep and shallow sets. This suggests that nighttime deep setting, which avoids diurnal-foraging seabirds as well as threatened epipelagic sharks and marine turtles, may be commercially viable for albacore tuna longline fisheries. The fishery has high compliance with regional management measures for sharks and rays. Findings identified approaches to reduce catch and mortality rates of marine turtles and elasmobranchs. This included modifications to gear designs such as having shallowest hooks fish below 100 m to reduce turtle catch risk, reducing retention of shark species with low at-vessel mortality rates, and improving handling practices for species such as the pelagic stingray for which at-vessel mortality rates were lower than release mortality rates. Tradeoffs from multispecies conflicts of fishing depth and using circle hooks are discussed. Findings support evidence-informed interventions to reduce the mortality of threatened bycatch species in data-limited pelagic longline fisheries.

Refer also to: Gilman E, Evans T, Pollard I, Chaloupka M (2023) Adjusting time-of-day and depth of fishing provides an economically viable solution to seabird bycatch in an albacore tuna longline fishery. Sci Rep 13:2621. https://doi.org/10.1038/s41598-023-29616-7