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Summary
New Zealand’s surface longline fleet deploys roughly 1.2 million hooks per year in New Zealand waters, targeting high value pelagic species such as swordfish and southern bluefin, albacore, and bigeye tuna. The fishery’s impact on both seabird and turtle populations in New Zealand’s waters has caused concern amongst fisheries managers and the Department of Conservation. This has prompted the enforcement of stricter bycatch mitigation measures by Fisheries New Zealand (FNZ), with these largely being focused on reducing bycatch of seabirds during the setting phase of longlines. Despite this, seabird bycatch is still widely reported by observers reporting on the fishery, prompting concerns that bycatch events taking place during the soak period of fishing operations have been underestimated and are still threatening vulnerable populations of seabird and turtle.
This literature review provides New Zealand’s Department of Conservation (DOC) with an up-to-date synthesis of bycatch mitigation measures for seabirds and turtles during the soak period of surface longline fishing. A series of mitigation measures were collated, summarized, and analysed for their potential efficacy in reducing seabird and turtle bycatch in New Zealand, as well as any barriers to implementation that they may be associated with. The findings from the literature review, collation and analysis stages were summarized into a series of recommendations for the DOC to take forward, with the focus being on future testing of new measures to reduce soak period bycatch of these focal taxa within the surface longline fishery.
This review identified a series of key challenges in reducing seabird and turtle bycatch during the soak, as well as recommendations outlining potential candidate measures to mitigate against the bycatch associated with them. Candidate measures identified to have the ability to reduce the likelihood of mainlines shoaling include proper weighting of the mainline via weights at the base of float lines and the use of increased branchline weighting; using deep-set longlines where possible; and the potential use of line shooters where the mainline is kept out of vessels’ propeller turbulence during setting. Measures to reduce the likelihood of seabirds and turtles interacting with baited hooks where they are exposed during the soak include night soaking, bait dyeing, using longer branchlines with weights close to hooks, the use of fish bait, and the use of novel hook designs to prevent ingestion. Finally, where bycatch rates reach concerning levels the use of spatial and temporal management measures may be used to limit fishing effort in specific fisheries management areas, or during periods that are known to be associated with high bycatch rates. However, implementing these closures with any level of confidence around their expected efficacy is challenging where historical observer data on seabird and turtle bycatch is limited. Despite a paucity of literature on experimental measures, this report recommends further investigation of the potential use of automatic release mechanisms, and the use of hook timers alongside TDRs to reveal how mainlines are brought to the surface and the scale of seabird interaction with them where they do shoal to the surface.
Publication information
Peat, W., Vella, E. and Pearce, J. 2024. Literature Review of Soak Period Bycatch Mitigation Measures for New Zealand’s Surface Longline Fleet. MIT2023-02B final report prepared by MRAG Ltd. 54 p.