Branch Line Weighting Options That Reduce the Risk of Seabird Bycatch

This paper summarises key findings from several experiments to determine the most effective branch line weighting options for pelagic longline fisheries. Initial (0-2 m deep) and overall sink rates of a range of line weighting options – each comprising a different combination of weight and leader length (distance between hook and weight) – were compared and several options identified that offer faster sink rates and hence pose reduced risk of seabird bycatch. The effect on catch rates of target and non-target species of placing weights adjacent to the hook was also tested and a new design of lead weight (‘sliding hook leads’) was developed that offers significant economic and safety advantages over traditional leaded swivels. Key findings include that: • the fastest initial and overall sink rates were achieved with weights at the hook; these sank significantly faster than the same weight with any length of leader; • 60 g weight with 1m leader also performed well, with a significantly faster sink rate than 60 g at 3.5 m from the hook; • all weights with longer leaders had slowest initial sink rates and thus pose higher risks of seabird bycatch; and • placing weight at the hook had no effect on catch rates of tuna like species or sharks. Based on this research, it is recommended that consideration be given to revising the line weighting options of CMM 2007-04 to: • require all fishers to use weighted branch lines and preferably encourage them to use a faster sinking weight option, such as 40 g or 60 g at the hook, or 60 g at 1 m; • allow the use of 40 g at no more than 0.5 m from the hook; • require 60 g weights to be used at no more than 1 m from the hook; • delete the options of 60 to 98 g within 3.5 m of the hook and greater than 98 g at 4 m from the hook; and • strongly encourage use of the new ‘sliding’ weights (including ‘safe leads’ or other proven safer methods of line weighting). Introduction This paper summarises findings from experiments conducted in Australia, to determine the effectiveness of various techniques to avoid seabird bycatch and mortalities from interactions with pelagic longline fishing operations. In particular, this paper examines the key findings on branch line weighting options to determine which options are likely to be most effective at reducing seabird bycatch and recommends revision of CMM 2007-04, the WCPFC conservation and management measure to mitigate the impacts of fishing for highly migratory fish stocks on seabirds. This review focuses on branch line weighting because the faster the hook reaches a depth at which birds can no longer see or dive to the bait directly relates to a reduction in seabird bycatch. Modifying gear to increase sink rates has been proven as an effective seabird mitigation measure in demersal longline fisheries (Agnew et al., 2000; Robertson et al., 2006; Dietrich et al., 2008; Moreno et al., 2008) and the same should apply to pelagic longline fisheries. A further motivation was the dearth of studies in the published scientific literature on