NEARLY two million salmon have escaped from fish farms in Scotland over the past six years, threatening wild salmon with chemical and genetic pollution.

The pollution could be tipping the "king of fish" into an "extinction vortex" from which it may never recover, say environmentalists and anglers.

Figures released by the Scottish Executive, in response to a freedom of information request, reveal that there have been 93 escape incidents at fish farms between 2001 and October 2006 (see table).

The highest number of escapes took place last year, when the total topped one million. Damage caused by storms on 11 January, 2005, enabled 850,000 salmon to swim free from their cages in the Western Isles, the Highlands and Orkney.

Other major escapes occurred in Shetland, with 238,420 fish escaping from a farm on Christmas Eve, 2002, and 130,000 found dead on 9 February, 2006. So far this year there have been 21 incidents, including two in Strathclyde in July and August for which the number of escaped fish is not known.

The figures show that of the 1.9 million fish that have escaped since 2001, fewer than 2000 were recaptured. Some fishing experts believe that the real number of escapees may be even higher, as some incidents may not have been reported.

The Executive's figures also show six incidents in which nearly 80,000 escaping fish had been recently treated with pesticides. Environmental groups fear this could cause toxic pollution.

"Escapes are spiralling out of control and precipitating what scientists term an extinction vortex' in wild salmon," said Don Staniford, European representative for the Pure Salmon Campaign.

"The flood of escapees from salmon farms - estimated at three million per year globally - is effectively killing off wild salmon. As well as spreading genetic pollution, escapees have the capacity to spread chemical pollution."

Escaped farmed salmon can breed with their wild relatives. They produce offspring which are genetically deficient and unable to survive the long migration across the Atlantic.

Staniford said that replacing open net cages with closed containment systems could greatly reduce escapes. "The tagging of farmed salmon would identify those responsible so that companies could be fined and sanctions levied against persistent offenders," he argued.

He urged salmon farmers to "come clean" on escapes. At the moment, fish farms which have suffered escapes are not named, despite a ruling that they should be by the information commissioner, Kevin Dunion.

According to the Rivers and Fisheries Trusts of Scotland, in fish farming areas there are 10 escaped salmon for every wild salmon returning from the ocean. The juveniles produced by interbreeding are ruining the purity of the wild stock.

"By crossbreeding, the escapees will have eliminated the stock with the capacity for marine survival," said the trusts' chairman, Roger Brook.

John Webster, technical director of the Scottish Salmon Producers' Organisation, accepted that the number of escaped salmon was high, but argued that the "vast majority" failed to survive in the wild. Interbreeding with wild fish was "not a good thing", he added.

Escapes were not in farmers' commercial interests and great efforts were being made to reduce them, he said.

The Executive pointed out that the number of fish farm escapes fell between 2002 and 2004, before the "anomalies" caused by the January 2005 storms. "This demonstrates progress but we want to ensure that an overall downward trend continues," a spokeswoman said.