Salmon Farm Escapees Threaten Wild Salmon Stocks

James Owen in England
for National Geographic News
June 16, 2003
A recent study in Norway suggests that wild salmon lose out to sexually precocious fish-farm invaders when breeding in rivers.

It's the first time scientists have shown that escapees from salmon farms can out-compete native populations, heightening fears conservationists have for the future of wild stocks.

Although it's been known for some time that farm salmon interbreed with genetically distinct wild populations, escapees are handicapped by less-competitive breeding behavior. The same can't be said for their offspring.

While adult farm salmon have been shown to be 84 percent less successful than native fish at reproducing in rivers, the male young—or parr—they produce are four times more successful than their wild counterparts, the study found.

A proportion of male parr become sexually mature despite being at a juvenile stage of development. They are a significant factor in reproduction among Atlantic salmon. Being a fraction of the size of sea-feeding adults, the parr are able to sneak up and fertilize a female's eggs without being noticed. Up to 40 percent of hatchlings are fathered by these parr.

The Norway experiment suggests that both farm and hybrid parr are able to fertilize many more eggs than wild fish. The research team, which included scientists from Canada, Britain, Norway, and the United States, believes this is due to genetic differences resulting from artificial selection by salmon farmers who prefer big, quick-growing fish.

"They have a higher growth rate and are more aggressive than wild parr," said Dany Garant, an ecologist at the zoology department of Oxford University in England. "Also, they don't respond as much to the risk of predation."

Underwater Cameras

The breeding behavior of equal numbers of farmed, hybrid, and native parr was monitored using overhead surveillance cameras, underwater cameras, and tiny microchips inserted into the fish. They were introduced to artificial spawning beds containing wild adults collected from a local river. Relative breeding success was confirmed by DNA analysis of fertilized eggs.

Results of the experiment revealed that wild parr had a breeding success of just 25 percent compared with farm parr, and were less than half as successful as hybrid parr.

Garant and his colleagues conclude in this month's scientific journal Ecology Letters, that the presence of farmed and hybrid male parr in salmon rivers "ultimately threatens the long-term genetic integrity of native populations".

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."

Norwegian Rivers

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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