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

Background

The farming of finfish, primarily salmon (salmo salar), is economically important to Scotland. The salmon industry generated over £1 billion in sales in 2021 (1) and is a major employer in remote coastal areas of Scotland. Salmon farming has two major phases. The freshwater phase involves rearing fish in freshwater tanks on land from eggs through to fry, then in tanks or pens for the parr and pre-smolt phases. The second phase of the production cycle of occurs in sea pens that are located in marine coastal areas in Scotland, typically off the west coast and in waters around the Orkney and Shetland islands.

The use of open water sites for the sea pens means that the farms are often located in the home-ranges of wild species of marine mammals. Grey seals (halichoerus grypus) and harbour seals (phoca vitulina) are commonly found in these areas (2). As salmon are a natural part of their diet, these marine predators are attracted to the sea farm sites because of the presence of the fish. Predation by seals on farmed fish in sea pens has been reported to cause huge losses. It has been reported that 1.4 million fish were lost to seals over a 10-year period (3). As well as predation, seals occasionally enter sea pens through holes in the nets or over the top of the handrails, causing significant mortality and injury to the fish. The process of removing seals is difficult and can be distressing for the seal. As well as direct predation, the presence of seals near sea pens, moving through the area or actively monitoring the pens, is thought to be stressful for the fish (4).

Because of these detrimental effects, salmon farmers have used a number of different approaches to try to deter seals and prevent the negative effects on fish. These approaches include the use of secondary layers of netting, regular removal of dead fish from the bottom of pens or the use of ‘extra strength’ netting. 

These measures have not been entirely successful in reducing seal attacks. Additionally, the use of secondary netting may adversely affect water quality, which also affects fish health and welfare. Thus, farmers require other methods to deter seals. One method that has attracted some controversy is the use of acoustic deterrent devices (ADDs).

ADDs are devices that transmit loud, low-frequency sound from the farm site into the surrounding sea water. The intention is that seals will find the frequency and volume of the sound aversive and are thus deterred from approaching the sea pens, thereby reducing both attacks and seal presence.

However, transmitting sound into the marine environment can have adverse effects on other marine species. Several cetacean species are found in these marine areas, including harbour porpoises (phocoena phocoena), bottle-nosed dolphins (tursiops truncatus), minke whales (balaenoptera acutorostrata) and killer whales (orcinus orca). The numbers of individual animals affected is difficult to determine, but the most common species found on the west coast of Scotland is the harbour porpoise (5), with smaller numbers of the other cetacean species present there. Minke whales and killer whales are present in these areas but in smaller numbers. All these cetacean species are European Protected Species (EPS) which means that they cannot be deliberately disturbed by human action. Marine turtles, Eurasian otter (lutra lutra) and Atlantic sturgeon (Acipenser spp.), are also protected species and could potentially be impacted by ADDs. However, as the ADDs are designed to target seals, it is the cetaceans that are most likely to be at risk due to their auditory ranges and their inability to escape the transmitted sound by leaving the water. Therefore, this response will primarily consider the effects of ADDs on cetaceans.