This is the final report for MIT2018-03: Setting mitigation for small longline vessels.


Ensuring that baited books deployed in surface and bottom longline fisheries sink rapidly to depths that are beyond the typical diving depths of foraging seabirds is a key strategy for mitigating seabird captures in these fisheries. Streamer lines towed behind vessels during setting aim to prevent seabirds accessing the hooks immediatedly after deployment. As a result, there is a particular focus on measuring the depths reached by hooks at the limits of the streamer line coverage.

The time that hooks remain under the streamer line after deployment is relatively short (~ 30 seconds) and measuring sink rates in this short period is challenging. Bottle tests have been used successfully in larger vessel fisheries, but are more problematic on inshore vessels which provide a lower viewing angle and generally set lines in darkness.

Electronic time-depth recorders are the preferred measuring tool, but devices designed for research deployment on animals are unsuitable for routine deployment on fishing gear. This project assessed the use of Wet Tags, robust time and depth recorders designed for use on fishing gear, for routine deployment by vessels engaged in inshore bottom longline and surface longline fisheries. A particular aim was to provide ‘realtime’ information on sink rates to fishers to allow adaptive management of fishing behaviour.

The Wet Tags typically did not start logging until the tags reached depths of 3 - 5 m, and provided data at a coarser logging interval than the TDRs designed for wildlife deployment. Nevertheless, paired testing indicated that the estimates of line depths achieved at the streamer line extent were generally comparable. As a result, the Wet Tag data provided reasonable estimates of the sink rates and line depths achieved by a range of bottom longline vessels. Sink rates of 0.2 m s−1 to 0.4 m s−1 and estimates of 3 - 5 m depths at the extent of streamer line coverage were typical for the participating bottom longline vessels.

Surface longline data were more limited, with the majority provided by a single vessel. Further work to refine how best to account for the initial free sink period of surface longline gear is warranted. Overall, the project demonstrated that routine collection of line sink rate data from bottom and surface longline fishing is feasible, and the results are useful to fishers and vessel managers in monitoring and managing the risk of seabird captures in their fishing operations. Real-world data on sink rates achieved in day-to-day fishing operations sit alongside more focussed studies on the factors affecting sink rates, and the factors that affect seabird risk of capture, in assisting in the continuous improvement of approaches to mitigate seabird captures in inshore longline fisheries.

Publication information

Middleton, D., King, B. and Wilson, O. 2021. Development of an adaptive management tool for line setting. MIT2018-03 final report prepared by Pisces Research for the Conservation Services Programme, Department of Conservation. 34 p.


Conservation Services Programme
Department of Conservation
PO Box 10-420
Wellington 6143


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