Preliminary assessment of the effect of seabird friendly (fast sinking) line weighting on catch rates of target and non- target fish in pelagic longline fisheries
Weights attached to pelagic branch lines increase the sink rates of baited hooks and reduce interactions with seabirds. Line weighting was introduced into the Australian pelagic longline fishery in 2007 following an experimental assessment of the effect of various gear configurations on hook sink rates (Robertson et. al., 2010). A total of 45 combinations of baits species and life status, swivel weights and leader lengths were assessed and two weighting regimes recommended for the fishery: 60 g at ≤ 3.5 m from hooks or 100 g at ≤ 4 m from hooks. The study raised two other issues, which are important in terms of future research. First, the initial sink rate was mainly influenced by the proximity of the sinker to the hook and the final sink rate was determined by the weight of the sinker. Thus, to increase both the initial and final sink rates the weight of the sinkers must be increased and sinkers must be located closer to hooks. Second, small variations in weighting regimes were difficult to detect in the shallow depth ranges (0-6 m). Examples of small variations are shortening long (e.g., 4-5 m) leaders by increments of only about 1 m, and increasing the weight of the 60 g sinkers used in the experiment by increments of, say, 20 g, 30 g or even 40 g. With both these points in mind it was concluded that to reduce seabird mortality from that associated with 60 g leaded swivels 3.5 m from hooks (the industry standard) comparative trials should include branch lines with at least 120 g ≤ 2 m from hooks (or a close approximation thereof).
This paper reports the preliminary findings of two further line weighting trials in the Australian tuna fishery. The first trial compared the industry standard (60 g at 3.5 m) line weighting with 120 g lead weights 2 m from hooks, and the second compared the industry standard with 40 g leads placed at the hook. The idea for the second trial evolved during the course of the first trial. Because both trials were conducted in an area of low seabird abundance (a collaborator could not be found in a high risk area) the research was conducted in two stages. The first stage was to determine effects of the new line weighting regimes on catch rates of target and non-target fish and the operational aspects of fishing. The second stage, which depends on the results of the first stage, will be to determine the efficacy of a new weighting regime in reducing seabird mortality from that associated with the industry standard. To date the first stage (fish catch effects) of the first trial (60 g at 3.5 m versus 120 g at 2 m) has been completed and we are currently about halfway through the first stage of the second trial (60 g at 3.5 m versus 40 hook lead).