Also known as a Seal Scarer or Acoustic Harassment Device, the Acoustic Startle Device (ASD) is a type of Acoustic Deterrent Device (ADD). The device works by emitting a noise that triggers an instinctive startle reflex response in seals, causing them to swim away from fishing gear and thus reducing depredation (eating or damaging of target catch) and seal entanglement or bycatch. Several different models of ASD exist, with varying acoustic characteristics that range in decibels and kilohertz.
ASDs are anecdotally widely used to prevent seal depredation in aquaculture farms and static net fisheries. In net fisheries, ASDs are deployed from vessels. Several trials of different models of these devices have demonstrated a temporary deterrent effect, however, it is difficult to assess the effectiveness of ASDs, as seal responses are often context- and species- specific.
A study in the River Conon and River North Esk, Scotland, tested a Lofitech Seal Scarer Device and found that seal movements upstream, past the device, reduced by around 50% when the device was switched on for periods of several days. The device did not, however, affect overall seal abundance in the area – perhaps due to the limited range of the device due to the shallow, constrained river environment.
In 2020, the Marine Management Organisation (MMO) conducted at-sea trials of the Genuswave Acoustic Startle Device in an inshore, mackerel gillnet fishery in Torbay. Deployment of the ASD appeared to increase target fish catch by an average of 74% as seal depredation fell (compared to the ‘control’ net without ASDs). However, with technical difficulties during the trials, a high variability in catches and the effectiveness of the ASD was noted, with the modelled increase in catch ranging from 5% to 189%.
Furthermore, the unique characteristics of the Torbay fishery – shallow depth, overnight soak time – means that the same results may not be replicated in different fisheries. Further testing in alternative fisheries, as well as improvements in ease-of-handling, are required. A separate trial in 2020, which used the same device on the north east coast of Scotland, found that seal detection by vessel sonar fell by 97% during deployment. Seal deterrence likely reduced shoal dispersal by seals, providing longer windows for fishing. However, no statistically significant impact on the total catch was observed in this trial.
The FAO (2018) have stressed Acoustic Deterrent Devices should not be considered ‘go-to’ mitigation measures or a ‘quick fix’, as effectiveness varies from fishery to fishery, species to species, and spatially and temporally, and tend to reduce but not eliminate bycatch. Besides the risk of habituation, there are also concerns over damage to seals’ hearing, habitat exclusion, and noise pollution in the marine environment which could affect aquatic life more generally. However, as ASDs in capture fisheries are only used for as long as gear is deployed (unlike in aquaculture farms), these issues may be less pertinent. Habituation may not be realised given nets are usually deployed in different locations at different times, which may prevent seals from tolerating ADDs close to sporadic feeding opportunities at nets. Read more about this in the MMO’s 2018 Review.
This page was last updated on 20.12.22.
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