Salmon farming - use of acoustic deterrent devices: report

3. Background and definition of area of analysis

Context: The Scottish finfish industry

The majority of the finfish reared in Scotland are Atlantic salmon (Salmo salar), with 192,129 tonnes produced in 2020. Rainbow trout (Oncorhynchus mykiss) are also reared, but in smaller quantities (7,576 tonnes), with very small quantities of other species produced (43 tonnes)¹. Because of the predominance of salmon and salmon farms, this report primarily considers salmon, but refers to other farmed fish where appropriate. Farmed salmon is the UK's biggest food export, employing 2,500 people in Scotland, generating up to £614 million per year in export sales²) and plays a major role in the Scottish economy. As well as the economic value of the industry to Scotland, the salmon industry is important in socioeconomic terms as it is a major employer in remote coastal areas and islands.

In 2021, there were approximately 230 salmon production sites in Scotland, located primarily around the west coast, Highlands and Northern Isles of Scotland¹. Salmon farming has two major phases. The freshwater phase involves rearing fish in 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 occurs in sea pens. Systems vary between companies, but on average the salmon production cycle lasts around 2.5 years, with the freshwater phase lasting one year and the seawater phase about 1.5 years.

Description of the issue: seals, cetaceans and farmed fish

Seal interactions with salmon

The use of open-water marine sites for sea pens means that the fish farms are often located in the home ranges of wild species of marine mammals. For example, grey seals (Halichoerus grypus) and harbour seals (Phoca vitulina) are commonly found in these areas³. These marine carnivores 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 stated to result in the death or injury of significant numbers of fish. A study surveying fish farmers reported that 1.4 million salmon were lost to seals over a 10-year period⁴. Anecdotal evidence and the previously mentioned study suggest that seals manipulate the nets to get close to the fish and then bite them through the netting of the sea pen. As well as attacking through the netting, seals occasionally enter sea pens through holes in the nets or over the top of the handrails. The process of removing seals is difficult and can be distressing for the seals and the fish. 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 fish⁵. If a chronic stress response occurs, it may result in a reduction in immune function, making the fish more vulnerable to disease and parasites.

Seal control and deterrent measures

Because of these detrimental effects on fish welfare and the economics of fish farming, finfish farmers have used several different approaches to try to deter seals from approaching and attacking the sea pens and thereby prevent the negative effects on fish. In the past, lethal control (shooting under licence from the Scottish Government) was used to kill seals that persistently caused damage to fish or caused fish health and welfare issues (so-called 'rogue' seals). However, changes to the Marine (Scotland) Act 2010, which came into force in February 2021, removed two provisions for which Scottish Ministers could grant licences to fish farmers to take or kill seals. The purpose of these provisions was to prevent serious damage to fisheries and fish farms and to protect the health and welfare of farmed fish. This means that Scottish Government can no longer issue licences for these purposes. However, data suggest that the use of lethal control for seals was already declining. In 2020, 329 licences were granted, and 104 seals shot, which is a reduction from 2011, when 1339 licences were granted, and 359 seals were shot⁶.

Thus, farmers have adopted other methods to deter seals. Farmers more commonly use measures, such as the regular removal of dead fish from the bottom of the pens ('mort removal'), or the use of 'extra strength' netting or secondary layers of netting around the pens⁷. This denser netting or additional netting layers may adversely affect water quality, affecting fish health and welfare. However, this effect can be mitigated by regular cleaning of the netting. Scottish Government provides advice on non-lethal deterrents⁸.

Another method of seal deterrent that has attracted controversy is the use of acoustic deterrent devices (ADDs). ADDs are devices that transmit loud (170-200 db), mid-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 be deterred from approaching the sea pens, thereby reducing both attacks and seal presence. Several types of ADD with different characteristics have been developed over time, and they can be deployed in different ways. Therefore, the term "ADD" is a generic term for a group of devices that vary in their specification and use, and consequently may vary in their likely effectiveness or potential for harmful effects. A Scottish Parliamentary Report in 2021 stated that a total of 146 sites (of the 220+ finfish sites in Scotland) used ADDs in 2019³. This suggests that approximately 66% of the total number of finfish farm sites in Scotland deployed a device in this period.

It is also important to note that ADDs are also used in other contexts (such as to exclude cetaceans and seals from marine construction sites, e.g., off-shore wind farms) and a number of studies have assessed the effects of these devices on the welfare of these species⁹. As these devices are not used in aquaculture, they will not be considered here, except where the information provided is relevant.

Potential for harmful effects of ADDs on seals and other non-target species

Clearly, the sound transmission from ADDs is designed to be aversive to seals, to attempt to deter them from approaching sea pens and attacking fish or causing stress by their presence. However, transmitting sound into the marine environment at high volumes, with the intention of being aversive to seals, has the potential for adverse effects on other marine species (see below). 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 by acoustic deterrents is difficult to determine, but the most common cetacean species found on the west coast of Scotland is the harbour porpoise, with smaller numbers of the other cetacean species present there¹⁰.

The main welfare concerns for seals and cetaceans with respect to ADD use are temporary or permanent loss of hearing, and avoidance of potential habitat areas where ADDs are deployed, which may affect foraging and reproduction^3,11. Although there has not been a great deal of work done in Scottish waters, researchers believe that the existing evidence suggests that there is a credible risk that ADDs can cause hearing loss with cumulative exposure and affect the habitat use of seals and cetaceans in Scotland^11,12. The extent of these impacts on the welfare of cetaceans and seals is a major issue related to the use of ADDs by the finfish industry and forms a major part of SAWC's consideration on the validity of ADD use.

Legislation regarding disturbance of European Protected Species

All cetaceans are European Protected Species (EPS) under the The Conservation (Natural Habitats, &c.) Regulations 1994 (legislation.gov.uk). Under these regulations it is an offence to deliberately or recklessly capture, injure, kill, or harass a wild animal that is an EPS without a licence. Of particular relevance to the issue explored in this report, it is an offence to deliberately or recklessly disturb any dolphin, porpoise or whale (cetacean). The term 'disturbance' is not defined in legislation and so may potentially include stimuli that cause any change in behaviour indicating a negative experience. Marine turtles, Eurasian otters (Lutra lutra) and Atlantic sturgeon (Acipenser spp.), are also protected species and could potentially be affected by ADDs, as could other marine species without conservation protection, but no research has been carried out to assess the potential impacts on these species. The Scottish Government provides more information in relation to this provision and on farmers' responsibilities^8,13.

However, given current scientific advice, it is likely that an EPS licence will be

required for all currently available ADDs unless the fish farm operator can demonstrate that the device(s) operating at their site will not cause disturbance to cetaceans¹⁴.

The evidence outlined above suggests that the use of an ADD in an area containing cetaceans may potentially cause disturbance, and therefore, fish farms must demonstrate that no alternative solution exists and apply for a licence. However, aquaculture companies also have a legal duty to ensure the welfare of farmed fish under the Animal Health and Welfare (Scotland) Act 2006. Thus, there are legal drivers both for and against the use of ADDs to deter seals, resulting in a difficult legislative background to the issue.