Non-Lethal Seal Deterrent in the North East Scotland Handline Mackerel Fishery

Executive Summary

Local Rosehearty fishers have observed a steady increase in seal numbers around the north east coast of Scotland in recent years with seals causing significant disruption to the inshore handline fishery for mackerel. Seals often follow boats as they leave the harbour to search for fish and when fish are located, seals interfere with fishing operations by dispersing the shoals. Fishers are keen to reduce these negative interactions with seals. A trial using Targeted Acoustic Startle Technology (TAST) to deter seals in the vicinity of fishing vessels was carried out between late July and early October 2020 on the inshore mackerel grounds in the North East Coast Regional Inshore Fishery Group (NECRIFG) area.

The trial involved five fishing vessels operating from Rosehearty Harbour. Fishers were asked to return a data sheet for each day fished, recording data on seal sightings, fishing operations and catches on days when they were either fishing normally (control) or additionally deploying the TAST device. Data analysis revealed a strong deterrence effect of TAST on seal activity directly around fishing vessels, in which seal detections on the vessels’ fish finder (sonar) decreased by 97%. Only one seal was observed under a vessel when TAST was deployed. No significant effect was evident in the visual sighting data, though fewer seals were spotted when TAST was operatiponal. This could be the result of a limited deterrence range of the device and/or a change of seal dive submersion times. Beyond the influence of TAST, visual detections of seals decreased towards the end of the season. Fishing metrics such as ‘fishing stop duration’ and ‘catch weights’ were primarily influenced by time-of-year (seasonality). However, fishing stop duration was almost twice as long when TAST was used which may be the result of a reduction in shoal dispersal caused by seals. More data are required to investigate whether TAST also leads to an increase in catch weight. Generally, statistical findings are consistent with reports from fishers, supporting the idea that TAST is effective in its primary function of deterring seals and preventing shoal dispersal caused by seals swimming under or close to the vessel.

The study suffered from an imbalance between control and TAST days and a decline in seal/vessel interactions towards the end of the fishing season. It would be helpful to collect more data during the presumed peak predation season in June and July. It would also be beneficial to investigate seal/fish shoal interactions at the behavioural level using sonar from a secondary vessel to optimise TAST use and provide fishers with improved fishing practice guidelines.

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Non-Lethal Seal Deterrent in the North East Scotland Handline Mackerel Fishery

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