Scottish Animal Welfare Commission - proximity of seals to farmed fish: response to Marine Scotland

Evidence for effects of acoustic deterrent devices on seals and non-target species

There have been a number of studies, using a range of methods that have assessed the effects of ADDs on cetaceans, seals and other species. Some studies have observed the response of wild or captive animals to the deployment of the ADDs. These studies are often problematic because of small sample sizes, especially of captive animals, and the short duration of studies, which are not appropriate for understanding long-term use of ADDs in commercially operating salmon farms. Modelling studies that relate sound propagation in water to the hearing ranges of the species or to potential geographical spread of sound have also been carried out. These theoretical studies may be useful in assessing the likely impact of ADDs, but local environmental conditions, such as water depth, sediment type, slope of the seabed, and the complexity and topography of the seabed and coastline, can affect the propagation loss of ADD outputs and hence their effectiveness against seals or their adverse effects on cetaceans. These environmental conditions are difficult to account for in modelling studies.

Additionally, many of these studies have been based on older versions of the ADDs, which were often used for prolonged periods of transmission, and used a wider range of frequencies and a longer duty cycle (duration of transmission) than current models of ADDs. 

The main welfare concerns for cetaceans with respect to ADD use are either temporary or permanent loss of hearing, and avoidance of potential habitat areas where ADDs are deployed, which may affect foraging and reproduction. Hearing is central to vital behaviours in cetaceans such as prey location, predator detection and navigation (6). Toothed cetaceans’ hearing is normally 15 to 30 dB more sensitive than pinnipeds’ hearing in the ADD frequency band of 4 to 40 kHz. Therefore, at the same distance from the source, a harbour porpoise (the most abundant and widespread in-shore species) would perceive the ADD sounds to be much louder than a seal. There are a few studies that have directly measured the effects of exposure to sound on hearing in cetaceans (7). The thresholds established in these studies are used in modelling the hearing damage that may occur following exposure to ADD transmission (8) and have indicated that hearing loss is likely after exposures ranging from 2 mins to several hours and at distances of 2 metres depending on the specifications of the ADD deployed. A range of impacts on cetaceans has also been observed from in-situ studies, from there being no response to ADD sounds to faster swimming away from ADDs. For example, in one study ADDs had a significant effect on porpoises up to 7.5 km away, which led to a reduction of detection of these animals by 52-95% within 750 metres of the device, compared with before their use, and a reduction of population density from 2.4 to 0.3/km (9).

Current ADDs produce sounds in the ultrasonic hearing range of cetaceans, which could affect their ability to hunt and navigate using their own sonar. There are limited data on the impact of ADDs on baleen whales, which do not use ultrasonic sonar, but may use sound for communication. A study on minke whales showed that they swam away directly, increasing their dive duration and speed regardless of distance from the ADDs, whereas humpback whales did not respond at all.

There have been a number of studies on the possibility that ADDs cause animals to leave areas of habitat. Studies from Canada have recorded animals retreating from ADD transmissions and lower frequencies of cetaceans in areas where ADDs were deployed (10, 11, 12).

A study in Scotland found that harbour porpoises tend to avoid areas where ADDs are active, but not exclusively, as animals were detected feeding in proximity to an ADD site13. However, all of these studies used the Airmar device which is less commonly used in Scotland than in North America and has also likely been superseded by newer models of ADDs.

Overall, there is relatively little direct data available on the effect of ADDs on cetaceans and very little from Scotland. However, researchers in this area believe that the existing evidence suggests that there is a credible risk that ADDs can cause hearing loss and affect the habitat use of cetaceans in Scotland (8, 14).