Aquaculture - use and efficacy of Acoustic Deterrent Devices (ADDs): report

1 Executive Summary

The aim of this study was to collate and review data on the use of Acoustic Deterrent Devices (ADDs) in the aquaculture sector to provide a better understanding of how they are being used, their efficacy and any potential for impact on sensitive non-target species. Records were provided by a range of industry sources and regulators, and a dataset developed which describes the extent of ADD use in Scotland from 2014 to 2020.

This report describes changes in ADD use across Scottish finfish farms over the period from 2014 to 2019 as well as providing a more detailed snapshot from the winter of 2019/2020. The use of ADD systems is dynamic, with changes made by finfish farm operators and manufacturers continuously. Patterns of use and device types are evolving as new technologies are developed to meet the regulatory framework and new scientific understanding. Lower frequency and lower amplitude ADDs, which potentially have a lower impact on sensitive non-target species, are now reportedly being used at an increasing proportion of farms.

Quantitative analysis was conducted to examine evidence of the efficacy of ADDs. Overall, there was a positive association between ADD use and seal depredation, with more frequent depredation and higher resulting mortality of fish on farms where ADDs were used, compared to where they were not used. This is likely to be a consequence of fish farms which experience high levels of seal depredation being more likely to use ADDs. However, despite an extensive modelling exercise designed to remove the effect of confounding variables, it was not possible to differentiate between any effect of ADDs in reducing depredation and the underlying factors that link ADD use with increased depredation. A small number of sites were identified where the use of ADDs is not permitted and depredation was found to be higher on average at these sites than sites where ADD use is permitted. If these sites could be considered representative of average background depredation levels, then this finding may indicate effectiveness of acoustic deterrence measures, but this assumption is difficult to test. Our findings illustrate the limitations of current observational data and demonstrate the importance of undertaking further experimental work to understand ADD effectiveness.

This work highlights the paucity of evidence and available data, particularly for assessment of ADD efficacy in preventing seal depredation. Systematic data collection on the extent and nature of ADD use across Scottish aquaculture sites is therefore urgently required.

Research priorities are provided based on the key knowledge gaps in relation to depredation by seals in aquaculture. Controlled experimental trials are required to understand the efficacy of ADDs in reducing depredation and to understand any effects on non-target species. This is particularly important in the context of the changing scientific understanding of the potential impacts on non-target species, and current regulatory frameworks. Research is also required to better understand the efficacy of alternative management measures, and recommendations are provided in Thompson et al. (2021), which considers the management options for pinniped predators at finfish farms in a broader context.