The deployment and trial of a low‑frequency acoustic startle response (A-ASR®) device designed to reduce seal depredation while minimising impacts on sensitive marine wildlife

Partners: Ace-Aquatec, University of St Andrews

Project ended: March 2025

BACKGROUND

Seal predation continues to present challenges for salmon aquaculture in Scotland. Grey seals can damage nets, stress fish, and directly kill stock, which leads to significant animal welfare and economic impacts. In severe cases, predation can cause major stock losses and costly infrastructure damage.

Producers have invested heavily in solutions, from reinforced netting and double netting to predator exclusion systems, but depredation persists at some sites. Acoustic deterrent devices (ADDs) were once widely used as part of farm protection strategies, but earlier generation systems emit mid-frequency sound that overlaps with the sensitive hearing range of harbour porpoises and other marine mammals. Regulatory reforms coupled with environmental considerations have since limited their use.

Until now, however, most studies of acoustic deterring systems have been short-term or uncontrolled, making it difficult to evaluate their real-world effectiveness on active farms. This case study focuses on the development of the Ace Aquatec RT1 A-ASR® device, designed to operate at a much lower frequency range (around 1–1.25 kHz). This project aims to deter seals whilst reducing the risk of disturbance to high-frequency cetaceans.

AIMS

This project is set out to:

1. Test whether the low-frequency RT1 (A-ASR®) device reduces seal depredation under real operational conditions on Scottish salmon farms.
2. Assess potential ecological impacts, focusing on harbour porpoises and seabirds.
3. Verify real-world acoustic output and sound propagation, comparing at-sea performance to controlled signal data.

Together, these aims support the wider goal of developing seal mitigation approaches that are both effective for farmers and responsible for the marine environment.

DESIGNING A REAL-WORLD TRIAL FOR SEAL DETERRENCE

This study was conducted on active Scottish salmon farms to test the Ace Aquatec RT1 low-frequency acoustic startle response (A-ASR®) device under real commercial conditions. The trial followed a randomised controlled design, in which periods when the A-ASR® was operating were paired with periods when it was switched off. These alternating one-week blocks enabled researchers to assess whether the presence or absence of the acoustic signal influenced seal depredation rates. Crucially, the timing of each on–off switch was controlled by independent researchers, rather than farm staff, to prevent any bias that might have occured if the A-ASR®s were activated only in response to local seal activity.

The RT1 equipment was installed with two devices on each site, placed at opposite ends of pen groups. The A-ASR®s were connected to a remote online portal, allowing researchers to monitor and change their operational status from off-site locations. The trial was initiated only at farms that were already experiencing consistent seal activity, as required by licensing conditions, ensuring that the study captured conditions where depredation was present and that meaningful data could be produced.

Daily fish mortality counts were conducted by farm personnel, using standard husbandry procedures, with the cause of death identified for each entry. These data were then combined with precise records of A-ASR® device activity to allow direct comparison between weeks when the RT1 device was active and when it was not. Statistical models were applied to both the numbers of fish lost to seals and the biomass of loss and examined trends at the individual net level and across whole sites. Additional variables, such as distance from the nearest A-ASR® and the severity of individual depredation events, were included to explore whether effects differed according to proximity or scale. In total, 35 paired on/off periods were analysed across five licensed farm sites, though initial project planning had anticipated including many more farms. Several potential sites were rejected due to regulatory concerns relating to proximity to protected areas, and one licence was revoked during the project. This reduction in site numbers, although unavoidable, substantially limited the sample size and the range of conditions tested.

Alongside the depredation trial, researchers examined the acoustic performance of the RT1 itself. Low-cost underwater recorders were attached directly to the A-ASR® devices to confirm that they were operating correctly and to document their sound characteristics throughout deployment. At one site, additional sound-mapping was conducted using calibrated hydrophones to document how the acoustic signal travelled around the pens and into the wider environment.

The project also included monitoring for potential ecological effects. Harbour porpoise activity was recorded using long-term passive acoustic moorings positioned at various distances from an active A-ASR®, and separate seabird surveys observed bird presence and behaviour around another RT1 installation. Together, these approaches allowed the study to evaluate both the farming outcomes and the environmental footprint of the technology under real farm conditions.

FURTHER INFORMATION

This is follow-on activity from an earlier project with the same partners.