Chapter 1. Acoustic deterrent devices as a management tool within aquaculture
The Scottish Government has a vision of a clean, healthy, safe, productive and biologically diverse marine and coastal environment that meets the long terms needs of people and nature. This includes managing our seas sustainably to protect their rich biological diversity and to ensure that our marine ecosystems continue to provide economic, social and wider benefits for people, industry and society. We also recognise that there is a need for a strong, dynamic and productive economy which creates wealth and employment across Scotland, but that this must be environmentally sustainable, inclusive and benefit all of our people and communities.
The aquaculture sector is an essential component of the Scottish economy, contributing to sustainable economic growth in rural and coastal communities. In 2018, Scottish aquaculture and its wider supply chain contributed £880 million GVA to the Scottish economy and supported over 11,700 jobs (Scottish Government, 2020a).
In addition to its impact on the economy, Scotland's position at the edge of the north-west European continental shelf has a huge influence on the natural condition of our coasts and seas, creating productive and abundant marine life (Scottish Government, 2018). The seas around Scotland are internationally important for marine biodiversity, supporting a wealth of wildlife including 22 species of marine mammals, including cetaceans and two species of pinnipeds (grey and harbour seals).
However, marine mammal populations inevitably interact to some extent with marine industries, and these interactions can prove to be negative to both wild animals and the affected sectors. One of the most significant interactions which has been documented since the 1980s (Northridge et al., 2010) occurs between the aquaculture sector and grey and harbour seals, for which Scotland has an increasing population of around 122,500 grey seals and a minimum population of 26,900 harbour seals (Special Committee on Seals, 2019).
2. The challenge and need for effective solutions
Farmed fish are sentient animals and can experience physical and mental suffering. Seals interacting with finfish farms can cause serious welfare problems through direct predation on fish, injury and stress to farmed fish caused by unsuccessful attacks or damage to cages or nets, as well as loss of stock. Several literature reviews, workshops and questionnaire-based documents have addressed the issue of seal depredation at aquaculture sites (Northridge et al., 2010; Coram et al., 2014), with some providing specific context in terms of costs to the sector. Northridge et al. (2013) reported that almost 1.4 million fish were lost to seals at 87 sites over a 129 month period, equating to a conservative farm gate value of around £26,000 per site per year, while evidence from the sector has shown that in the year to May 2020 the sector lost in the region of 560,000 salmon to seal predation at a cost of £13 million.
As well as directly causing injury and mortality, the presence of seals close to nets can cause severe stress to farmed fish which is a welfare concern and can cause a significant reduction in growth rate and reduced tolerance to disease. This is of particular concern to fish farm managers who have legal duties to ensure containment, prevent escape of fish and to maintain the welfare of fish under the Aquaculture and Fisheries (Scotland) Act 2007 and Animals and Welfare (Scotland) Act 2006, respectively, while ensuring compliance with existing legal provisions on the protection of animals and wildlife.
A combination of lethal and non-lethal measures have historically been employed to address interactions between finfish farms and seals (Coram et al., 2014; Thompson et al., in prep), with earlier actions largely focused on the lethal removal of seals. However, the introduction of the Marine (Scotland) Act 2010 was an important landmark in this context, by putting in place a seal licensing regime which sought to achieve an appropriate balance between seal conservation and sustainable marine activities by allowing limited lethal removal of individual seals.
Since the introduction of the seal licensing system in 2011, there has been a decline in the number of seals granted on licences and the number of seals subsequently shot under licence by approximately 80% (Scottish Government, 2020b). This trend is most likely driven by a number of factors including the efficacy of management measures, availability of alternative non-lethal methods and technologies and increased awareness of seal-aquaculture conflicts. Indeed, records show that there has been an increase in the use of alternative non-lethal measures across the aquaculture industry since the first review of the seal licensing system in 2015 (Scottish Government, 2015) which has contributed to a reduction in numbers of seals shot in some areas (Scottish Government, 2020b).
Amendments to the Marine (Scotland) Act 2010, which came into force on 1 February 2021, have removed two licensing grounds on which Scottish Ministers can grant licences authorising the taking or killing of seals. Aimed at improving the welfare and conservation prospects for seals in Scotland, due to these amendments, the aquaculture sector can no longer obtain a licence to kill seals for the purpose of preventing serious damage to finfish farms or to protect the health and welfare of farmed fish. Alternative and more effective non-lethal measures will therefore be required to address seal interactions with farmed fish in the future.
3. Acoustic deterrent devices as a non-lethal management tool
In Scotland, one of the most widely used non-lethal measures to address the risks associated with predators is Acoustic Deterrent Devices (ADDs). ADDs operate by introducing loud, low frequency sound (usually 1 to 20 kHz) into the marine environment and are considered one of the few practical measures available to deter seals from approaching finfish farms (Coram et al., 2014; Thompson et al., in prep) therefore enabling the sector to protect the welfare of their farmed fish which can be negatively impacted by seal circling cages.
Devices vary in their design, duty cycle and acoustic properties (McGarry et al., 2020) with farms often adopting different methods of use depending on the level of predator risk. Nevertheless, the efficacy of these devices in deterring seals has not been widely studied (Northridge et al., 2010, Coram et al., 2014) with the exception of a few species-specific devices (Götz & Janik, 2016), and opinion amongst fish farm managers as to their effectiveness is largely mixed (Coram et al., in prep). However, a significant number of finfish farms in Scotland use these devices; the second seal licensing review reported ADD use at 60 to 70% of sites seeking a seal licence between 2015 and 2020 (Scottish Government, 2020b).
This is a particular concern given the increasing body of scientific evidence which shows that conventional ADDs can have unintended consequences for non-target species, particularly cetaceans (Coram et al., 2014; Sparling et al., 2015; McGarry et al., 2020) which are listed as European Protected Species (EPS) under Schedule 2 of the Conservation (Natural Habitats &c.) Regulations 1994. While the evidence for impacts of ADDs on marine mammals will be explored in more detail later in the report (Chapter 2), there would appear to be a need for more targeted and effective measures in moving forward.
In recent years, newer generation ADDs have been developed that reduce sound outputs at the frequencies most likely to cause disturbance to cetaceans. These devices work by specifically tailoring signals to produce avoidance responses in seals at lower sound pressure levels thereby greatly reducing the potential for impacts on non-target species (Götz & Janik, 2016). In addition, methods to reduce the sound output of existing ADDs have also been explored including reducing the source levels (loudness of sound) and duty cycles (changing the length of time the device is on or off), and reducing the number and duration of transmission sequences by transmitting signals only when seals are present (Thompson et al., in prep). However, there are still questions regarding the efficacy of these devices in deterring seals and the potential for impacts on non-target species, therefore further research and development is still required in this area.
4. Alternative non-lethal management measures to address the risk of seal damage and fish welfare
There is no single solution which can fully address all the challenges that exist in seal-finfish farm interactions and a combination of measures is most likely to provide the most appropriate future approach. The effectiveness of measures is also likely to vary between locations and circumstances due to the complexity of local environmental and logistical constraints.
Existing management measures
Physical exclusion can prevent predators from interacting directly with finfish farms and measures to improve fish containment such as improved nets have assisted with reducing or preventing seal damage and welfare issues (Northridge et al., 2010). However, there still remains a concern that predators can circle nets and thereby cause stress, and ultimately mortality, to the enclosed fish.
The expansion in the use of appropriately tensioned nets, anti-predator nets or more robust netting materials (e.g., Seal Pro HDPE nets) across the industry in recent years has helped to reduce seal predation in some areas. For example, Cooke Aquaculture, Scottish Sea Farms and Greig Seafoods have all reported the use of HDPE anti-predator nets at some of their sites in recent years with evidence of success (Thompson et al., in prep). This is supported by a Scottish Government report (Scottish Government, 2020b) which found that the use of anti-predator netting at Scottish finfish farms has increased from around 20% in 2016 to over 40% in 2020, although it is likely that some of this use could be attributed to more robust netting material (e.g., HDPE netting) (Thompson et al., in prep). Companies are currently expediting programmes of net replacement, although the speed with which HDPE nets can be deployed is limited by their availability from manufacturers, as well as that of additional infrastructure improvements including tensioning weights and new pen structures engineered to accommodate such equipment (pers. Comm. ScottishSalmon ProducersOrganisation). It is therefore expected that these trends will assist in further reducing the risk of seal predation.
Good husbandry practices by finfish farm managers in terms of improved attention to the frequency of the removal of dead fish ('morts') and the use of seal blinds and false-bottomed nets (on the basis that seal attack on the base of the net was twice as likely as on the wall of the net (TEP, 2010)) have also helped to reduce the potential for attracting seals and therefore predation risk. However, in some cases seal blinds and false-bottomed nets can reduce the water flow through the net base which may cause issues for fish health (Thompson et al., in prep). It is clear, however, that these measures have not eliminated the risk of seal predation and other complementary measures are required.
Management measures under development
While it is expected that the increasing trend towards the introduction of stronger nets for most sites in the future may assist in further reducing the risk of seal predation, other measures are still under development. For example, electric field deterrent systems (barriers) have been developed for use at finfish farms, but these have not been widely applied with only a single trial undertaken at a finfish farm in Scotland (Whyte, 2015).
Designed to reduce the environmental impact of finfish farms and reduce issues with sea lice, closed and semi-enclosed containment systems could also reduce seal interactions. The performance and cost effectiveness of such containment systems are under investigation (Global Aquaculture Advocate, 2018) and trials are still ongoing.
In contrast, larger cages with the same level of stock biomass offer a more immediate solution to the risk by enabling fish to remain further away from the cage extremities thereby minimising the potential for seal interactions. Larger cages can also be designed to be more robust in structure, incorporating more effective barrier systems and supportive technology. The Town and Country Planning (General Permitted Development and Use Classes) (Scotland) Amendment Order 2020, which has now been laid in Parliament and is expected to come into force on 1 April 2021, extends permitted development rights to enable the deployment of bigger finfish pens, with no increase in biomass, without having to submit a formal application for planning permission to the local planning authority. This is expected to act as a counter measure to mitigate seal predation to some extent.
While physical exclusion can help address the effects of direct mortality, stress-related effects to fish due to seal presence in the vicinity of cages cannot be immediately rectified (with the exception of some closed containment systems). As a result, alternative, or complementary measures are required. To date many options have been reviewed in terms of direct harassment, acoustic deterrents, and condition aversion, however for some of these measures further development is required in terms of their effectiveness or deployment practicalities.
A partnership project between Crown Estate Scotland (CES) and Marine Scotland has investigated the problem of seal predation in river fisheries and at finfish farms. The research (Thompson et al., in prep), which reviewed current and emerging non-lethal measures for addressing seal interactions within the aquaculture and wild fisheries sector, reported that while some measures are currently available for use, it is clear that they would benefit from further research and development and consideration by the industry in terms of their practicality, ahead of specific recommendations on measures suitable for future trials and deployment.
It is clear that further work is required across all fronts to ensure that a suite of tools are available to the industry to address risks associated with seal-finfish farm interactions while ensuring that marine wildlife are provided with protection afforded through our environmental obligations.
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