6. Non-lethal measures
Information on non-lethal measures (NLMs) is considered by MS-LOT in its determination of a seal licence application. NLMs include use of particular types of management or equipment which deter seals from farms or rivers, or reduce seal interactions with farmed fish or river fisheries. NLMs typically used in fish farming include Acoustic Deterrent Devices (ADDs), net tensioning, and seal blinds. River fisheries sometimes also use ADDs, as well as ad-hoc methods such as shouting.
Applicants must demonstrate the use of the NLMs to meet the aim of the licensing system which states that shooting should be undertaken as a last resort. Most NLMs are considered not to have any negative environmental impacts. For example the use of correctly tensioned nets at fish farms can reduce the risk of seal attacks without increased environmental impacts.
6.2 Acoustic deterrent devices
ADDs are used to deter seals through the introduction of noise into the marine environment. Devices will vary in their design, intended use and acoustic properties and a full summary of these is beyond the scope of this report. Through the seal licensing regime, the usage of ADDs at fish farms and in rivers has been reported. The efficacy of devices in deterring seals has not been widely studied, however a Scottish Government-funded research project is currently seeking to assess both the use and efficacy of ADDs used within the aquaculture sector.
ADDs typically work by producing loud, low frequency sounds (usually 1 - 20kHz). Operators may set these devices to a range of duty cycles such as always on or on a timed cycle. ADD technology is continuously under improvement to focus frequency of sound to target species, and to reduce duty cycles to minimise unwanted disturbance to non-target species (Gotz & Janik, 2016).
The Animals and Wildlife (Penalties, Protections and Powers) (Scotland) Act places a statutory commitment on Scottish Ministers to lay a report before the Scottish Parliament on the use of ADDs at fish farms no later than 1 March 2021. The Scottish Government welcomes this commitment and is already actively progressing an element of this work through an ongoing review of the current management and regulation of ADD use at fish farms.
6.3 Other types of non-lethal measure
While ADDs are one of the most frequently used NLMs in aquaculture, a number of other measures can be used to reduce the incidence of seal predation events at fish farms and in rivers.
Good husbandry practices, such as adequate net tensioning and prompt removal of dead fish ('morts'), are commonly used in aquaculture to reduce predation by seals. Other pieces of equipment such as seal blinds (thicker netting which covers the bottom of nets to disguise dead fish) and anti-predator nets can also be used. Anti-predator nets can create a number of issues, which include the potential to entangle marine mammals (including seals) and birds (Northridge et al., 2013). Use of electrified nets or electric fields have been developed and trialled (MMO, 2018) although Scottish Government is not aware of many farms in Scotland employing these methods. In recent years, use of Sapphire SealPro™ nets has gained popularity in the Scottish aquaculture sector with farms reporting a significant reduction in seal predation events.
The use of NLMs in rivers tends to be more difficult than in aquaculture due to the wider spread of fishing sites and hydrological features of rivers, which are not conducive to noise transmission. River fisheries have attempted to use ADDs (either single or multiple devices) at the mouths of rivers to discourage seals from entering. Evidence suggests that seals who forage in rivers may be classed as 'specialists' (Graham et al., 2011) and seals with a strong urge to forage upriver may not be deterred by most NLMs. Electric field barriers were tested in a river in Canada but the costs were high and results were inconclusive (Forest et al., 2009). Other measures such as visual and acoustic disturbance do not provide a long-term solution in a river environment, so river fisheries are left with a limited suite of options to reduce seal predation. Mostly these involve ad-hoc use of noise to drive seals away when they are seen. Scottish Government is leading a Crown Estate Scotland-funded project to review the options available to river fisheries to deter seals and reduce interactions between river fisheries and seals. This project will consider all available options and produce options for future testing within the sector.
6.4 Use of non-lethal measures
6.4.1 Fish farms
As reported in the application survey, the number of sites reporting use of NLMs has increased over the review period (2015 – 2020). The number of farms using anti-predator nets has increased from below 20% in 2016 to over 40% in 2020 (Figure 1). The use of seal blinds and ADDs has remained fairly constant with around one-third and two-thirds of farms using the measures, respectively (Figures 2 and 3). Almost no sites (<5%) report using electrified nets. There has been a sharp increase in reported use of 'other non-lethal measures', rising from under 10% to over half of farms reporting these (Figure 4), with the majority of reported measures under this category in 2020 applications were the use of Sapphire SealPro nets.
On 2020 applications, only five out of 237 sites, across four licences, reported not using any of the NLMs described above. The reason given for the lack of measures at these sites was that seal predation events were too rare to justify use, use of ADDs were restricted by NatureScot, measures were not effective, or measures were not allowed by company policies. Licences covering these sites were granted.
Increase in use of some types of NLMs has not led to a decrease in number of seals requested or killed in the review period. The numbers of seals killed at fish farms appears to have remained relatively constant since 2015 (Figure 5). In the first five years of the licensing system, seals killed at fish farms each year reduced from over 242 animals to 66. Since 2015, numbers have fluctuated but remain similar with 61 animals killed in 2015 and 63 killed in 2019, which represents less than 27% of the total number granted each year.
6.4.2 Netting stations
The Conservation of Salmon (Scotland) Regulations 2016 introduced legislation to protect declining salmon stocks by, amongst other things, prohibiting the retention of salmon caught in coastal waters. Therefore, since 2015, only a small number of netting stations have remained on seal licences within the Moray Firth as part of the Moray Firth Seal Management Plan (Butler et al., 2008) with applications stating that sites could be required for salmon tagging studies. No seals have been reported killed at these sites in the review period.
6.4.3 River fisheries
In river fisheries, less than 10% of sites reported use of ADDs between 2016 and 2020. Reported use of other non-lethal measures was constant at around one-third of sites until 2019 but has increased to 45% in 2020 (Figure 6). Other non-lethal measures include scaring from river bank by shouting, waving arms and firing warning shots from firearms. Use of boats and general human presence are also reported to deter seals.
Only a small number of river fisheries use ADDs, likely because of the variable efficiency and difficulties of installing and maintaining devices in rivers. Over half of sites (21 out of 40) do not use any non-lethal measures. Reasons given for not using any NLMs include the expense and inefficacy of using ADDs. There is a lack of other proven options for river fisheries. Research into providing additional non-lethal options to river fisheries is discussed below.
As with fish farms, numbers of seals reported killed or taken under licence by river fisheries has remained constant since 2016 (Figure 7). Between 2011 and 2016, seals reported killed each year under licence in rivers reduced from 219 to 39. Numbers from 2016 to 2019 varied but have remained similar with 38 seals killed in 2016 and 34 seals killed under licence in rivers in 2019, which represents about 30% of the total number granted each year.
6.5 Future work to improve non-lethal measures
The Scottish Government continues to support research aimed at improving understanding of seal interactions with river fisheries and investigating NLMs to address seal predation in both aquaculture and river fisheries. Below is a summary of research supported since 2015.
- Marine Mammal Scientific Support Research Programme (2015-2020, complete).
- Improving understanding of the use, impact and efficacy of acoustic deterrent devices in Aquaculture (2019 – 2020, ongoing).
- Non-lethal measures to address seal predation at fish farms (2020, ongoing).
It is recommended that work should continue in the aquaculture and river fisheries sectors to improve the efficacy of non-lethal measures in deterring seal predation.