Salmon Farming in Scotland

The Impact on Wild Atlantic Salmon

written byHannah Kirkland - Conservation Biologist at Mossy Earth

Hannah Kirkland

Salmon is one of Scotland's biggest food exports, second only to whiskey. Unfortunately, this booming industry comes with a cost. The salmon we buy across Britain typically comes not from wild fish but farms, many of them located off the coast of Scotland. So, while wild salmon numbers reached a record low in 2018, in supermarkets, salmon was plentiful. In this article, we'd like to outline some of the issues surrounding salmon farming in Scotland and its impacts on wild fish populations and the wider ocean ecosystems.

Why are salmon important to the ecosystem?

Every year in Autumn, Scotland's crystal clear salmon rivers and streams are transformed into a frenzy of fish leaping and swimming their way upstream to spawn in freshwater gravel beds, before returning to sea. In doing so, salmon help bring nutrients from the ocean ecosystem upstream into freshwater environments. Through excretion, spawning, death and decay, salmon directly affect the ecology of their freshwater environments and increase nutrient availability for algae, invertebrates and young salmon. 

Salmon themselves are an important food source for an abundance of aquatic and terrestrial wildlife, such as otters, waterfowl and eagles. At sea, salmon are predated on by some of the ocean's top predators, such as sharks, dolphins and seals. They also have an interesting symbiotic relationship with a priority species in the UK Biodiversity Action Plan, the freshwater pearl mussel (Margaritifera margaritifera). Adult mussels filter water, improving the water quality and reproductive conditions for salmon, and salmon return the favour by providing a safe environment for mussel larvae to develop within their gills.

A white tailed eagle carries a fish in its talons
Salmon are an important source of food for a diversity of wildlife, like this white-tailed eagle.

A Difficult Journey

Today, the number of Atlantic salmon (Salmo salar) making this arduous journey is a fraction of what it once was. As a result, anglers are now engaged in a voluntary scheme to release any salmon caught in Scotland's glens, but many sources argue that the problem is out at sea. Though dams and weirs are known to obstruct migratory routes of salmon, at sea, they’re facing a combination of severe threats. Climate breakdown, a diminishing food supply and commercial salmon farming are all threatening the survival of Scotland’s iconic salmon.

A salmon leaps out of a stream
The salmon run is one of Scotland's greatest natural wonders

The Farms

Salmon farming in Scotland has overrunthe oceans off the West Coast and Western and Northern Isles of Scotland. With the longest coastline on the British Isles, Scotland's ocean habitats are precious. Now, some of the most important areas for wild salmon and sea trout in Scotland are shared with thousands of farmed salmon. They’re kept in open-net pens that allow fresh water to flow in and faeces, diseases, uneaten food, chemical treatments, medicines, and sometimes even fish themselves, to flow out into the wider ecosystem.   

Salmon pens spread across the sea
Salmon farms theaten wild populations through pollution, disease, genetic mixing and more.

Sea Lice

With thousands of fish in each pen, salmon farms can act as a breeding ground for diseases and pests, in particular, sea lice. Almost half of the salmon farms in Scotland are infested with the common salmon louse (Lepeophtheirus salmonis). The louse causes skin lesions and heavy infestations can result in death. This is a particular problem for wild salmon and trout as farms are often located in areas inhabited by these species.

A dead fish floats on the surface of the water
Severe infestations of sea lice can kill salmon and other fish species


Antibiotics and pesticides are often used to manage sea lice and other diseases. These chemicals can flow out of the pens and into the surrounding waters, threatening the health of Scotland’s coastal ecosystems and the wildlife that lives there. According to some sources, the levels of chemicals used to kill sea lice have breached environmental safety limits more than 100 times in the last 10 years. 

Wild Harvests

Farmed salmon are also often fed with small fish, such as anchovies, herring and sardines, harvested from the wild. Wild wrasse is also harvested for use as cleaner fish to control sea lice numbers. Though it's argued that farmed fish are more sustainable because they’re not harvested from wild populations, it's likely that they're still contributing to the depletion of other wild fish populations. 

A fishing boat is followed by hundreds of seabirds
Farmed fish are often fed with wild caught fish making the process environmentally unsustainable

Genetic Mixing

Occasionally, fish can escape from farms and enter the ocean ecosystems. These fish can spread disease, compete for food and habitat, and cross-breed with wild salmon. In doing so, they dilute the gene pool of wild salmon, which are adapted to local conditions, and reduce the fitness of wild populations.

Making A Difference

Many of the threats impacting wild salmon populations, like climate breakdown, will take time to rectify. Unsustainable farming, on the other hand, is comparatively easy to solve. It is vital that salmon farms address the issues outlined above and adapt quickly to safeguard the survival of salmon in Scotland. As consumers, we also have an opportunity to make a big difference, by consuming more consciously and sustainably. Along with many other species of fish, salmon are facing considerable threats, but there is a lot we can do to change things for the better.

Water flows along a wooded river in Scotland

Sources & further reading

Peer Reviewed Research Section
  1. A Global Assessment of Salmon Aquaculture Impacts on Wild Salmonids - Plos BiologyExternal linkIcon Peer Review
  2. Assessing and Managing the Impacts of Marine Salmon Farms on Wild Atlantic Salmon in Western Scotland: Identifying Priority Rivers for Conservation - Wiley Online LibraryExternal linkIcon Peer Review
  3. Wild salmonids and sea louse infestations on the west coast of Scotland: sources of infection and implications for the management of marine salmon farms - Wiley Online LibraryExternal linkIcon Peer Review
  4. Aquaculture and environmental drivers of salmon lice infestation and body condition in sea trout - int-res.comExternal linkIcon Peer Review
  5. Salmon near crisis point, says fisheries expert - BBC newsExternal link
  6. Scotland's salmon crisis: 'Anglers only want one. But it's just not happening’ - The GuardianExternal link

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