New branch line weighting regimes reduce risk of seabird mortality in the Australian pelagic longline fishery without affecting fish catch

Concern by Australia’s pelagic longline fishing industry about the effect on target fish catch rates of the amount of weight in branch lines and the proximity of weight to the hook, has restricted adoption of gear with faster sink rates that reduces the incidental capture of seabirds. Trials of two new branch line weighting regimes involving custom‐made lead weights were conducted to determine effects on catch rates of target and non‐target fish species.

There were no statistically detectible differences in the catch rates of the main target and non‐target fish species between branch lines with 60g lead weights 3.5m from hooks (the fishing industry standard) and those with either a 120g lead weight ≤2m from the hook or a 40g lead weight placed at the hook. Branch lines with 40g weights at the hook – which have the greatest potential to be adopted in the fishery – commenced sinking immediately upon deployment and took, on average, 4.5 seconds (0.43m/s) to reach 2m depth, 33% less time than 60g at 3.5m from the hook, the industry ‘standard’. The 40g leads placed at the hook also improved crew safety, reduced the amount of time spent in gear construction and facilitated gear inspection for compliance purposes. The findings provide the fishing industry with new line weighting options that have the potential to reduce seabird bycatch without affecting target fish catch.