Scientists are increasingly worried about the impact of escapees on native populations that are already in decline, mainly due to overfishing at sea.
In countries like Scotland, Canada, Ireland, and Norway large salmon farms containing hundreds of thousands of fish are often located in coastal waters close to salmon rivers. Atlantic salmon migrate up these rivers to breed after returning from their ocean feeding grounds.
Upwards of two million farm salmon are estimated to have escaped worldwide in 2002. Over 600,000 came from a single farm in the Faroe Islands in the North Atlantic. The incident is believed to be the world's biggest salmon escape. And in Scotland environmental groups say about a million farm salmon have escaped from their sea cages since 1998.
"Farmed fish are often detected in Scottish rivers," said Jeremy Read, director of the Atlantic Salmon Trust, a salmon conservation charity. "One study found that out of 16 rivers in northwest Scotland, 14 contained salmon of mixed farm origin."
Another study traced 80 percent of salmon in some Norwegian rivers back to fish farms. Differences between wild and farm-origin populations in Norway are thought to be halving every ten generations.
Read said: "The major problem of interbreeding is that it reduces a population's fitness and ability to survive. Native salmon have evolved to meet the particular circumstances and habitat of their river."
Garant added: "Interbreeding could disrupt the local adaptations specific to each wild population as farm fish are under very different selection pressures in an artificial habitat."
He and his colleagues say their salmon parr study suggests the rate at which farm fish genes are spreading into wild populations is greater than previously thought.
Their research also raises concerns of the possible impact of freshwater escapes from land-based farming operations where young salmon are reared before being transferred to saltwater.
If the aquaculture industry is unable to stem the flow of salmon escapes, one possible solution now being considered is for them to cultivate sterile fish known as triploids. Otherwise, the species anglers call, "the king of fish" could be headed for extinction.
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