Relative efficacies of alternative branchline weighting designs at mitigating seabird bycatch in longline fisheries

Robust estimates of the relative efficacies of alternative management interventions are essential for developing evidence-informed fisheries bycatch policy. Bycatch is a major threat to the conservation of albatrosses and other pelagic seabirds. Branchline weighting is one approach prescribed by regional fisheries management organizations and the Agreement on the Conservation of Albatrosses and Petrels to reduce seabird bycatch in pelagic longline fisheries. We used a Bayesian multilevel network meta-regression modelling approach to conduct the first synthesis of available evidence to assess the relative efficacies at mitigating seabird bycatch of alternative pelagic longline weighting designs. Unlike conventional pairwise meta-analysis, network meta-analysis enables the simultaneous comparison of multiple interventions within a coherent modelling framework. There was a >97% probability that all weighting designs significantly reduced seabird bycatch compared to a reference design with no weight within 5m of the hook. Nonetheless, some weighting designs were significantly more effective at reducing seabird bycatch than others — for instance, the 2 designs with weights >60g and >1m from the hook performed the best with >93% probability that those 2 designs performed significantly better than 2 more commonly used designs with less weight but attached closer to the hook. These two best performing designs reduced seabird bycatch by ca. 89% relative to the reference design. These relative efficacies and rankings, when combined with other performance criteria such as costs to commercial viability and crew safety, support robust evaluations of alternative bycatch management strategies.

RECOMMENDATIONS 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 note this new evidence of the relative efficacies of alternative branchline weighting designs derived from a multilevel network metaanalysis, an approach that produces broadly generalizable findings appropriate for informing regional policy.
2. Note that while all weighting design options prescribed by ACAP and regional fisheries management organizations (RFMOs) perform substantially better (67% to 89% significantly lower seabird catch risk) than a reference design of no weight within 5m of the hook, but not all prescribed designs perform equally. 3. When reviewing ACAP’s recommendations on branchline designs and ACAP’s RFMO advocacy, consider that designs with relatively low predicted efficacies could be eliminated as options, or designs could be categorized according to their relative efficacies. 4. Encourage Parties and other stakeholders to prioritize undertaking additional studies from a larger number of fisheries to produce more robust estimates of the relative efficacies of alternative weighting designs that explicitly account for potentially informative predictors of seabird bycatch risk.