"The wild races of salmon off Norway are threatened by escaped fish,'' said WWF global marine program director Simon Cripps. "We suspect this is an issue for every country where salmon are farmed.''

About half of all wild Atlantic salmon are born in Norwegian rivers, with lesser populations of the prized fish found in places from Scotland to the United States.

``Around half a million farmed fish, both salmon and trout, escape from fish farms in Norway every year,'' said Cripps.

Escaped farmed fish, which make up about a quarter of salmon caught in seas off Norway and nine of 10 in some fjords, compete for food and can spread parasites to wild fish.

When farmed fish mate with their wild cousins, the hybrids dilute the genetic pool of up to 400 races of salmon in Norway's rivers.

In turn, that could make the overall species less resistant to disease, a WWF salmon report said.

Flabby farmed fish that manage to leap up rivers to lay eggs often do so after their wild cousins have done so, sometimes displacing wild eggs from river beds.

Wild salmon are born in rivers, swim out to sea and then return one to four years later to spawn.

Cripps said the WWF is worried that escapees from other fish farms, like for cod, could also hit depleted wild stocks.

The world cod catch tumbled to 890,000 tonnes in 2002 from 3.1 million in 1970.

The WWF urged Norway to tag every farmed salmon, step up security at farms and keep farms further from spawning rivers.

In Maine in the eastern United States, it said, at least one fish farm clipped identity tags to fins.

``Tagging would put pressure on the bad fish farms by showing where escaped fish come from,'' said Cripps. ``It would let consumers choose.''

He had no estimate of the extra cost.

In 2003, the percentage of escapees among salmon in Norwegian seas was 24 percent, the country's data shows. The overall figure was down from a peak above 40 percent in the late 1990s, when big storms hit.

Cripps said stricter Norwegian rules, including placing fish farms in more sheltered areas, are not enough.

Norway, the top Atlantic salmon farming nation, exported 424,000 tonnes of salmon and trout last year and promotes fish as a safer, healthier alternative to meat and a better choice than fish caught from wild stocks.