Almost 75,000 farmed salmon in Scotland escape after Storm Amy

When Storm Amy battered the Scottish Highlands in early October, it tore through a salmon farm, releasing around 75,000 fish into open water in Loch Linnhe.

The scale of the escape has serious implications for wild Atlantic salmon, which are now classified as “endangered” in Great Britain.

Aquaculture has become one of the fastest-growing forms of food production. The most valuable of all farmed marine species is the Atlantic salmon, which accounted for 18% of global marine aquaculture production value in 2022. The UK is the world’s third largest producer, with almost all production centred around Scotland’s coast.

A salmon farm normally holds six to ten nets, each containing up to 200,000 fish. Having the nets open to strong tidal currents allows clean oxygenated water to enter and waste to be removed. However, this also means that they are vulnerable to adverse weather conditions.

Atlantic salmon farming began in the 1970s. Since then, the species has undergone intensive selective breeding, similar to chickens or dogs. Fish have been chosen for faster growth, delayed sexual maturity, disease resistance and other commercially desirable traits. Around 90% of the salmon used in Scottish aquaculture also originate from Norwegian stock.

After 15 generations of selection, farmed salmon are now one of the most domesticated fish species. They differ genetically, physiologically, morphologically and behaviourally from wild salmon, which increases their mortality when they escape into the natural environment.

However, studies show that some escapees beat the odds and not only survive in the wild but also breed. In certain Scottish and Norwegian rivers, more than 10% of salmon caught are of farmed origin, with numbers highest near intensive farming areas.

When they breed with wild salmon, their hybrid offspring inherit a mix of traits, leaving them less suited to their natural environment. This process, known as “genetic introgression”, gradually undermines the genetic integrity of wild populations.

“Wild Atlantic salmon already face multiple human-driven threats like climate change, habitat loss, pollution and invasive species. Genetic pollution from farmed escapees is yet another blow that undermines their resilience to other forms of environmental change.” Dr William Perry, Postdoctoral Research Associate

The scale of this single escape is extraordinary. Scotland’s total returning wild salmon population is estimated at ~300,000 fish. The release of 75,000 farmed salmon represents a quarter of that number.

Even if only 1% of the escapees survive and breed, that would mean around 750 fish entering rivers and potentially mixing with wild populations. A 2021 Marine Scotland report found that 23% of sites assessed already showed evidence of farmed introgression; 6% of which were in very poor condition with major genetic changes.

Without tighter regulation, better containment measures and effective genetic monitoring, these events could continue to erode what’s left of UK’s wild salmon.

The article, Almost 75,000 farmed salmon in Scotland escaped into the wild after Storm Amy – why this may cause lasting damage, is published in The Conversation.