They are an identical species, but while one is lean and incredibly fit, the SAS of the fish world, the other is an obese, idle creature, a couch potato with fins. When the two interbreed, the results can be a genetic disaster.

Such a disaster looms, according to experts, after the escape of more than 100,000 farmed Atlantic salmon over the past six months on the West Coast.

In the latest incident, at the end of last week, 30,000 maturing 2.5kg (5.5lb) fish escaped from their cage in West Loch Roag, off the coast of Lewis, after a seal attack.

The escape, which was detected four days ago, comes at a time when wild salmon are approaching the rivers to spawn, meaning that there could be intermingling and genetic dilution of the wild fish. These are extremely fit creatures, swimming thousands of miles across oceans, then battling their way upstream.

If they spawn with the flabby, cage-reared fish, it is claimed that the offspring can be genetically weak and the wild salmon population, which is recovering after some very bad years, could be threatened.

The Association of Salmon Fishery Boards and the Rivers and Fisheries Trusts of Scotland are now preparing a formal complaint to the European Commission.

They say that the latest escape occurred next to the Langavat Special Area of Conservation, and close to the mouth of the Grimestra salmon river. The area is described in the EU Habitats Directive as “the best salmon system in the Western Isles”.

Andrew Wallace, of the association, said: “This could not have happened at a worse time for us, or in a worse place. The escape comes at the end of a long summer of discontent on the escapes front.”

There have now been four escapes from fish farms in the Western Isles within the past six months. The first was in March, when 18,500 fish escaped in East Loch Tarbert, off Harris, as a result of equipment failure. It is believed that a cage sank and another 25,000 fish died.

In May, 52,000 fish escaped near Lochportain, on North Uist, through a hole in a net; and in July more than 1,000 escaped from West Loch Roag.

Mr Wallace said: “How sophisticated multinational companies can afford to lose such valuable stock and continue to play Russian roulette with wild stocks is beyond comprehension. This is the fourth escape from salmon farms in the Western Isles since May and the sheer numbers involved – this latest escape being over 30 per cent of the entire Scottish rod catch – means that these incidents must be viewed with utmost seriousness.”

Roger Brook, of the Rivers and Fisheries Trusts, said: “At a time when the Scottish Salmon Producers’ Organisation has a stated intent to increase production in Scotland its members have once again shown themselves to be incapable of containing their livestock.

“This latest release is being blamed on seals, but it is the responsibility of the fish farmers to make their containment systems capable of resisting the native wildlife. It is equally their responsibility to ensure their equipment can resist the weather and conditions that are experienced off the coasts of Scotland. These continual releases of farmed fish lead me to the conclusion that all aquaculture should be brought ashore.”

The escape marks the low point in a relationship which has always been uneasy. The salmon farming industry is an important part of the Scottish economy, supporting 8,500 jobs in remote areas. Scotland is the third largest salmon producer in the world, with approximately a 10 per cent global market share, and the industry puts more than £197 million into the economy every year.

But for the salmon fishing industry, itself worth £113 million a year and the employer of 2,800 people, such success can come at too high a price. The wild salmon population is much smaller than it used to be, but is on an upward trajectory.

Sid Patten, the chief executive of the Scottish Salmon Producers’ Organisation, said: “The industry regrets all breaches of containment and we would like to reassure wild fishery interests that effective containment remains a priority.”

Scale of the problem

Wild Atlantic salmon are lean and incredibly fit, having swum thousands of miles across the ocean

— They have the drive and energy to battle upstream and spawn

— In genetic terms, they represent the survival of the fittest

— Estimated population: 3.5 million, half what it was 30 years ago

Farmed Atlantic salmon are the same length as the wild fish, but much heavier, flabbier, rounder and more heavily spotted

— They are farmed selectively to get fat

— They are not adapted to the wild and their fins are worn from being in a cage

— 70 million smolts have been put into sea cages in past two years

Source: Times database

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