Repeated escapes of farmed salmon could drive endangered populations of wild Atlantic salmon to extinction, say scientists in the British Isles.
By Helen Briggs
BBC News Online science reporter
There has been concern over the past decade that domesticated salmon are breeding with native salmon, changing the genetic make-up of the fish and damaging their ability to survive in the natural environment.
Until now, there has been little direct scientific evidence but, according to a report published in the journal Royal Society Proceedings B, the fears of environmentalists may be justified.
Farmed fish have different genes from their wild relatives (Image: Scottish Quality Salmon)
In a 10-year study, researchers from Ireland, Northern Island and Scotland, found that wild salmon were vulnerable to extinction because of genetic and competitive pressures from farmed fish.
Experiments with wild and farmed salmon hybrids in fresh and marine water showed that the offspring of fish that had interbred had a much lower survival rate - some 70% of the fish died in the first few weeks of life.
Overall, farmed salmon were much less successful at surviving in the wild compared with native salmon and were unlikely to return to rivers to spawn.
However, they grew quicker than wild salmon and the ones that did survive displaced many of their wild cousins from the rivers.
The team, led by Dr Philip McGinnity of Ireland's national agency, the Marine Institute, and Professor Andy Ferguson of Queen's University Belfast, warn that accidental and deliberate introductions of farmed salmon could lead to extinction of vulnerable wild populations of Atlantic salmon.
They write in Proceedings B: "Our experiments, uniquely carried out over two generations, demonstrate conclusively that these intrusions lower survival and recruitment in wild populations and that repeated escapes produce a cumulative effect, which could lead to extinction of endangered wild populations."
Dr Paulo Prodohl, a co-researcher on the study, said wild salmon were the product of thousands of years of evolution, which had "fine-tuned" their genes to survive in the natural environment.
The introduction of new genes from fish that had been bred in captivity could wreak havoc on local gene pools, he said.
He told BBC News Online: "What we need is higher regulation and monitoring of the farming industry."
It is estimated that some two million Atlantic salmon escape each year from fish farms in the North Atlantic - equivalent to half the total number of wild adult salmon in the sea.
But Dr John Webster, of Scottish Quality Salmon, which represents the Scottish salmon farming industry, said farmers were doing whatever they could to keep fish in their cages.
Fish can escape during routine handling or damage to nets (Image: Scottish Quality Salmon)
He said the farmed salmon industry was working closely with the wild salmon industry to agree on strategies.
"Cooperation and collaboration is a much more effective way of dealing with these sorts of issues than improving regulations," he said.