Night-time and deep setting provides an economically viable solution to seabird bycatch in an albacore tuna longline fishery

Gilman E, Evans T, Pollard I, Chaloupka M (2023) Night-time and deep setting provides an economically viable solution to seabird bycatch in an albacore tuna longline fishery. In: ACAP - 11th Meeting of the Seabird Bycatch Working Group. SBWG11 Doc 10, Edinburgh, UK

Fisheries bycatch is a major threat to albatrosses, large petrels and other pelagic seabirds. We assessed the effects of the time-of-day and relative depth of fishing on seabird and target species catch rates for a Pacific Ocean pelagic longline fishery that targets albacore tuna with an apparently high albatross bycatch rate. Using a Bayesian inference workflow with a spatially-explicit generalized additive mixed model for albacore tuna and generalized linear mixed regression models both for combined species of albatrosses and combined species of seabirds, we found that time-of-day and fishing depth did not significantly affect the target species catch rate while night-time deep setting had >99% lower albatross and seabird catch rates compared to both deep and shallow partial daytime sets. This provides the first evidence that night-time setting in combination with fishing deep reduces seabird catch risk and may be commercially viable in this and similar albacore tuna longline fisheries. Findings support evidence-informed interventions to reduce the mortality of threatened seabird bycatch species in pelagic longline fisheries.

We recommend that the Working Group:
1. Update ACAP’s review and best practice advice for reducing the impact of pelagic longline fisheries on seabirds to account for this new evidence that deep-night setting was economically viable and reduced seabird catch risk.
2. Prioritize additional studies to assess the commercial viability of deep-night setting in a broad range of pelagic longline fisheries, across target species, regions and soak durations.
3. Note that, unlike night-shallow setting, night-deep setting avoids exacerbating risks to threatened epipelagic species such as marine turtles and silky and oceanic whitetip sharks.
4. Note that the vessels included in this study have relatively long gear soak durations, allowing them to set deep at night and achieve overlap with the vertical distribution of albacore tuna, which might not be operationally feasible in all pelagic longline fisheries, in part due to spatial variability in diel vertical distributions of target species.