Fish farms and acoustic deterrent devices in the UK
Fish farms are frequently visited by seals and other marine predators. In an effort to prevent marine mammals from approaching fish farm sites and taking fish, Acoustic Deterrent Devices (ADDs) or ‘seal scrammers’ are often used. These devices use sound to frighten or even induce pain (due to acoustic trauma) in seals in order to keep them away from fish cages. There is growing concern about the impact of ADDs on whales and dolphins as the sound frequencies used are similar to those that they use to communicate with one another and to find food.
Fish farms are predominantly located in coastal areas. Therefore coastal species such as bottlenose dolphins and harbour porpoises are at the highest risk of exposure to disturbance from these devices. A study in British Columbia, Canada, concluded that porpoises would be excluded from a 400m radius around an ADD, and porpoise abundance would be significantly reduced within 3.5km of a device. A Scottish study found that an ADD could be detected at a distance of 14km away!
The conflict between whale and dolphin habitat and fish farm sites is particularly pertinent in the waters of western Scotland, which have a high density of fish farms. Every major sea loch in the area is occupied by at least one fish farm, and more than half of these farms use ADDs as anti-predator mechanisms. It has been calculated that harbour porpoises would be excluded from 16km2 of coastal waters and that porpoise abundance would be significantly reduced over an area of 1187km2 in western Scotland alone. Western Scotland is one of the most important porpoise habitats in Europe and aquaculture developments are on the rise. This obviously reduces the available habitat for whales and dolphins, is likely to cause wide-scale disturbance and may create barriers to their movement
Effective alternatives to ADDs include the use of properly tensioned nets. These prevent fish from escaping and from seals getting access to the fish. Other factors that can reduce seal predation, and subsequently reduce the need to use ADDs, include the removal of dead fish, putting less fish in each facility and the use of 'seal blinds'. WDC does not advocate shooting seals to reduce the problem.