As with our work to minimise impact on the environment as a whole, measures to protect our salmon from other marine life – and equally, to protect marine life from harm or entrapment – are factored in as early as farm location selection and the Environmental Impact Assessment (EIA) process.
Included in these measures is a predator exclusion plan for each farm. This sets out a step-by-step guide to keeping fish stocks safe in their pens and free from the stress that predation can cause, whilst also being mindful of the need to minimise the impact on other marine life.
Predator exclusion plans aim to pre-empt and prevent predation from two primary sources: wild birds and seals.
Located around Scotland’s west coast and islands, our farms are visited by a wide range of bird species; many of them fish-eating. This can include everything from herring gulls, black-backed gulls, grey herons, cormorants, shags, and guillemots, to the more occasional gannets, sea eagles and great northern divers too.
As part of each and every planning application, we consult NatureScot for their advice on bird life local to the area. We will also include measures to:
- Protect salmon pens with top nets to provide a barrier between the fish and wild birds
- Raise top nets from the water’s surface via a floating centre support (often referred to as ‘hamster wheels’) or bird net poles, both of which help keep the netting tight and prevent it sagging from the weight of any birds
- Ensure a mesh size of between 50mm-70mm which is small enough to prevent bird access but large enough to prevent bird entanglement.
Where farms are to be located in a designated area, and at the request of the local authority, we can also:
- Change the colour of the top net – for example, from the standard black to white as we do at our Orkney farms – to increase visibility and deter birds from diving from above
- Change the mesh size of the top net to suit the predominant bird species in the particular area
- Record any incidences of birds becoming trapped in netting or pens.
Seals are an iconic sight around Scottish waters. Like any wild animal however, seals are also natural predators feeding on a variety of wild fish and other marine life as part of their natural diet.
Depending on species it’s estimated that each seal eats 3-7kg of food per day.
Working on the basis that Scotland’s common seal population consumes an average of 149 tonnes of food per day and grey seal population consumes an average of 636 tonnes per day, it adds up to something in the region of 785 tonnes of food per day.
That’s equivalent to almost three quarters of the combined daily catch of Scotland’s pelagic, demersal and shellfish fleet.
The bigger the seal population gets, the greater the demand for sources of food.
"That's equivalent to almost three quarters of the combined daily catch of Scotland's pelagic, demersal and shellfish fleet."
Protecting farmed fish from predation
Preventing seal predation of farmed fish stocks begins with good husbandry practices, including:
- Net tensioning to reduce all possible entry points
- Regular removal of any fish mortalities so as not to encourage seals
- Extra observation procedures for at least seven days following seal predation.
Until recently, there were two additional measures that Scotland’s salmon farmers could deploy to protect their stock from predation by seals.
A changing landscape
Previously, where predating seals persisted beyond all other protective measures in place and endangered the lives of farmed salmon, growers had the option of requesting that the seal be shot; a last resort measure that would be carried out under licence by an external marksman.
As of 31 January 2021, this last resort measure was removed under the Animals and Wildlife (Scotland) Act.
Separate to this, beyond March 2021, the use of acoustic deterrent devices (ADDs) – a device which emits a sound underwater designed to help keep predating seals at a safe distance – was also prohibited, unless a European Protected Species (EPS) licence had been granted.
This followed a decision by Marine Scotland to adopt the position that ADD use could be considered an offence under EPS legislation due to the potential for whales, dolphins and porpoises to be disturbed by the sound emitted by devices to deter seals.
Upholding our duty of care
While the sector continues to push for clarity from Scottish Government on what it should do in the event of a seal getting into a pen and endangering farmed salmon stocks, predation continues to have a significant impact on fish welfare.
To put this into context, from May 2019 to May 2020 more than 0.5M farmed salmon across the Scottish sector died as a result of seal attacks, with many more adversely affected by the stress of predation.
Such would appear to be the pressure for food that we’re seeing new behaviours in seals. Where once they targeted larger salmon, they are now increasingly attacking young smolts.
So, what more can Scotland’s salmon farmers do to protect farmed fish?
Next generation netting systems
Recent years have seen us introduce new high density polythylene (HDPE) netting systems – some costing three times that of traditional nylon netting – at those of our farms most likely to suffer a significant seal challenge.
Unlike softer nylon netting which hungry seals push their snouts against in the hope that salmon will swim close enough for them to grab, HDPE netting has an altogether tougher surface.
Secured in place by an intricate system of weights or circular sinker tube to increase tension, this more rigid, tougher surface is proving highly effective in deterring seal predation at many of our farms.
On some occasions where we’ve installed HDPE netting however, we’ve seen seals relocate to another farm where there had previously been no seal challenge. The goal now is to roll-out the new netting systems to each and every farm; something that’s best done ahead of each new crop so as not to risk stressing the fish.
On other occasions, we have seen persistent seals climb up and into pens so our farmers are now lacing together top nets in a new way.
We are constantly learning and continually adapting our farming practices, but the end goal remains the same – to keep farmed salmon stocks and seals safely separate.
Consolidating farms into a smaller number of larger pens could provide additional protection to HDPE nets, sinker tubes and optimal tensioning.
The thinking being that the bigger the pen, the greater the opportunity for fish to shoal nearer the middle and out of harm’s way away from the net.
Before we can make any such change to an existing farm, we must first apply for planning permission; something we’re in the process of doing in order to pilot the effectiveness of this approach in reducing the harm caused by seal predation.
Double-netting – equipping each sea pen with two layers of netting rather than one – is a fiercely debated subject.
In our farmers’ experience, however, double-netting can pose its own risks to the health of farmed salmon and local wildlife by:
- Hindering the flow of water in and out of each pen, particularly in low tidal locations, which can increase fouling and reduce oxygen levels
- Increasing the risk of entanglement of seabirds and even seals between the two nets
- Being more difficult to keep clean; clean nets being key to good fish health.
With these potential risks in mind, we’re currently trialling a new design whereby young smolts are housed in an inner mesh net, keeping them further away from predating seals on the outside of the main net, until they are of a size and weight that needs even more room to grow at which point they are released into the main net.
Investing in research
Seals are without doubt ingenious creatures and farmers have to work hard to try and stay one step ahead.
In order to do so, it’s essential that we continue to invest in the next generation prevention measures and also increase wider knowledge of the issue through applied research.
One such study we’re currently hosting – led by a Scottish university with the support of the Sustainable Aquaculture Innovation Centre and Marine Scotland, and a research EPS licence granted by NatureScot – is looking at the response of cetaceans to the low frequency RT1 ADD.