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MIT2023-01 Effects of hook and bait type on commercial longline fisheries bycatch (PDF, 5,007K)

Summary 

Hook size and bait type affect seabird and sea turtle bycatch risk in longline fisheries. This report reviews published and grey literature that presented comparisons of bycatch rates for different hook and bait types to assess the effectiveness of certain hooks (e.g., circle hooks) and baits (e.g., fish) at reducing bycatch of seabird and sea turtle species. Literature on international best practices for hook and bait type was also reviewed. Additionally, this report summarises data collected through the Conservation Services Programme (CSP) seabird necropsy project as well other data sources, such as the Centralised Observer Database. These datasets were assessed for their suitability to obtain complete and representative information about sea turtle and seabird bycatch in relation to hook size and bait type. Lastly, results from a questionnaire, administered to surface longline (SLL) fishers via the Department of Conservation, are presented to understand current gear configurations being using the New Zealand SLL fleet and the first-hand experience of fishers using different gear in response to seabird and sea turtle bycatch.

Internationally, squid, fish, or a combination are primarily used as bait in longline fisheries. Fish bait, particularly mackerel, reduced sea turtle interactions in eight studies compared to squid, although the effectiveness varies. Conversely, mackerel increased the number of shearwater, gannet, and gull captures in one study and was inconclusive in another. The impact of bait type on target species catch rates was less clear, with similarly conflicting findings reported. The effectiveness of dyed bait remained mixed across studies, with some reporting lower seabird bycatch rates.

Studies consistently showed that larger circle hooks, such as 18/0, significantly decreased the capture rates of sea turtles and seabirds compared to traditional J hooks and improved post-release survival of captured turtles. However, the effectiveness of hook type and size varied depending on factors such as fishing effort, bait type, and regional differences in fishing practices. Mitigation measures such as Hookpods, which shield the hook during setting, have shown promising results in reducing seabird andsea turtle captures and are now required for the New Zealand SLL fleet.

A review of bycatch data obtained from several sources, which includes observer-reported data, fisher-reported data, and necropsy data, revealed that data are may be insufficient to conduct robust statistical analyses on the effects of bait or hook type on protected species captures. Bait type and hook type were rarely reported, and the consistency in hook type (mostly 16/0) and bait (squid) used across the fleet in recent years could prevent a comparison of bycatch rates across different baits and hooks. It will also take considerable effort to link all the different tables across the different databases.

Based on the questionnaire responses, 17 operators in the New Zealand SLL fleet universally used circle hooks (14/0-17/0) baited with squid bait when targeting tuna and swordfish. Along with Hookpods, fishers employed various hook and line weighting and bait dying, especially during full moon phases, to mitigate seabird interaction risk. Little mitigation is focused on sea turtles at present.

Based on this review, recommendations in international guidelines adhere to the most current knowledge of the most effective hook and bait type for seabird and/or sea turtle bycatch mitigation. Typical recommendations include the use of large circle hooks (16/0 or larger) with offsets less than 10° and the use of fish bait where possible. It is also commonly suggested to use additional methods to reduce bycatch such as single hooking fish bait, reduced gear soak time, night setting, mitigation devices (e.g., tori lines, Hookpods), line weighting, and seabird/sea turtle hotspot avoidance. More research is required to determine if dying bait is effective at reducing turtle and seabird bycatch.

It is also recommended that New Zealand electronic catch and effort reporting for both surface and longline fisheries be at the trip and station level (not just associated with a protected species capture) and include additional information on hook type, size, offset, and manufacturer, along with information on bait species, hooking method, and bait state. Lastly, more research is required to understand how bait and hooks influence bycatch rates for both sea turtles and seabirds in New Zealand. Future analyses should quantify catch rates of target and non-target species using fish versus squid as bait and smaller versus larger circle hooks, considering different combinations of hooks and baits that are currently being used on longline vessels.

Publication information

Hickcox, R.P. Meyer, S., and MacKenzie, D.I. 2024. Effects of hook and bait type on commercial longline fisheries bycatch (MIT2023-01). Report for the Department of Conservation, Conservation Services Programme, Proteus Client Report: 187. Proteus, Outram, New Zealand. 98 p.