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Summary
Data were collected on fishing depth during the soak period on four vessels operating in the pelagic longline fishery.
Wet Tags (ZebraTech, Nelson, New Zealand) were deployed by the crew on two vessels to measure mainline depth. The data were automatically downloaded using existing infrastructure on the vessels. This provided useful longer-term data sets at minimal cost and with minimal inconvenience for fishers. The data showed variations in fishing depth between baskets, lines, trips and vessels. This approach has the potential to provide insights into how fishing depth may influence protected species capture rates and risk profiles of different vessels.
More intensive data collection and manipulation of the fishing gear setup were completed on four at-sea trips using Time Depth Recorders (TDRs) to measure hook depth. Modifications to gear setup increased fishing depth and longer float ropes were the most effective and easiest way to increase hook depth. Hook depth throughout the soak was variable within and between baskets, between sets and between vessels. Hook availability to birds during the soak period was almost exclusively linked to fish catch on nearby branchlines, and was greater in shallower-set gear.
Increasing fishing depth is likely to reduce risk to seabirds and turtles and may be best achieved by employing longer float ropes. Additional recommendations to reduce hook availability to seabirds include; weighted gear, longer minimum branchline length, and increased branchline spacing.
As is typical in pelagic longline fisheries, catch per basket was patchy, somewhat confounding catch rate comparisons between treatments. However, there was some indication that catch rates for the deepest hooks were lower indicating that when employing longer float ropes basket size may need to be reduced, or ‘moneymaker’ floats added to larger baskets to maintain similar maximum soak depths to current gear setups. Longer-term data sets would be required to examine the effect of increasing float rope length on catch rates.
Encouraging and incentivising fishers to employ these measures proactively and/or reactively in response to captures should be explored and considered in the context of minimising protected species bycatch on a seasonal basis.
Publication information
Goad, D.; Middleton, D.A.J.; Schweder-Goad, C. 2025. Understanding and mitigating seabird and turtle bycatch during the pelagic longline soak period. MIT2023-02 and MIT 2024-02 Final Report prepared by Vita Maris 48 p.