Escaped farm salmon threaten native species

Escaped farm salmon threaten native species
Anil Ananthaswamy

New Scientist

4th June 2003

Young male salmon raised in fish farms mate more aggressively than their counterparts in the wild. This means that fish escaping from farms are likely to pose a greater threat to native species than previously thought, by depleting wild fish populations and reducing their genetic diversity.

Farmed salmon are bred to grow rapidly in crowded conditions, and this makes them more aggressive than wild fish. Environmentalists have been concerned that fish escaping from farms are displacing wild salmon from their natural habitats, but such fears have been tempered by findings that adult escapees are less successful than native fish at reproducing in the wild.

However, salmon have an alternative reproductive strategy, in which some male yearlings, known as parr, mature early and mate. These tiny fish can sneak past full-grown males up to 10 times their length and fertilise more than a third of the eggs in a female's nest.

To compare the reproductive prowess of farmed parr with those in the wild, Dany Garant of the University of Oxford and his colleagues studied three strains of Norwegian salmon: farmed, wild and hybrid strains created by crossing adult farmed fish from the country's national breeding programme with adult natives from a Norwegian river.

Paternity test

The researchers released 10 parr from each group into an artificial circular stream, designed to simulate natural breeding conditions, along with 12 adult males and 12 adult females taken from another river. At the end of the breeding season they genetically analysed fertilised eggs from 30 nests to determine their parentage.

The results revealed that wild parr were only 25 per cent as successful as the farmed parr in fertilising eggs (Ecology Letters, vol 6, p 541). Even the hybrids were twice as good at it as the wild parr. "This suggests that farmed fish have been able to displace the wild in most of the cases," says Garant.

Parr salmon can pass on traits to their offspring, so given their faster life cycles, these fish could very quickly spread their genes through wild populations.

"It's an incredibly important study," says William Muir, an expert on farmed and transgenic fish at Purdue University in West Lafayette, Indiana. Although the lighting conditions on farms tend to reduce the numbers of parr in farms, researchers suspect that escapees could actually grow faster than wild parr in natural conditions. "They could swamp the gene pools with maladapted genes and quickly cause extinction of wild fish."

Complete invasion

The conservation organisation WWF reported last week that over the past two decades, populations of wild Atlantic salmon have declined by about 45 per cent, while farmed salmon production in the North Atlantic has increased 55-fold.

Hundreds of thousands of farmed salmon escape from their pens each year. "In Norway, some rivers are completely invaded by farmed fish," says Garant. And because Atlantic salmon are being farmed on the Pacific coasts of North America and Chile, stocks of wild Pacific salmon could be threatened as well.

In 1994, seven countries signed the Oslo Resolution, agreeing to guidelines designed to minimise the impact of salmon farming on wild fish in the north Atlantic. However, the resolution does not enforce the regulations or hold member countries accountable for salmon escapes.

The WWF recommends mandatory monitoring and enforcement of the regulations, as well as creating "exclusion zones" in every country to protect wild salmon stocks in certain rivers.

Related New Scientist Stories

Gene silencing could wipe out farm pests - 10th March 2003

GM fish farming 'too risky' - 3rd September 2002

Scottish salmon in "extinction vortex" - 16th July 2002

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