Fears that the Atlantic salmon could become extinct were rekindled last week by a study on the effects of escaped farmed fish on wild fish populations.

The findings are expected to influence legislation in places such as Scotland, where farmed fish are often intentionally released for anglers. In addition, up to two million salmon are thought to escape from farms around the North Atlantic each year (see Nature 416, 571; 200210.1038/416571a).

Atlantic salmon are farmed mainly in Norway, Scotland, Ireland, Canada and the northeastern United States, using species bred from a few Norwegian strains for faster growth. This selection process makes the fish more economically valuable, but their unusual genes and their unfamiliarity with natural conditions make them vulnerable to predators and disease in the wild.

The new paper follows a ten-year study carried out in the Burrishoole river system in County Mayo, Ireland, a traditional centre for salmon research run by the Irish Marine Institute (P. McGinnity et al. Proc. R. Soc. Lond. B doi:10.1098/rspb.2003.2520; 2003). Researchers released known numbers of farmed salmon, along with eggs of wild– farmed hybrids, in a natural salmon-spawning river. They then recaptured fish in fresh water and at sea, and used DNA profiling to identify their parentage.

As expected, farmed fish were short-lived, surviving on average 2% as long as their wild counterparts. But those created by interbreeding became more likely to die in successive generations: hybrids survived 27–89% as long as their wild cousins, depending on their parentage and whether they were first- or second-generation offspring. The second generation fared particularly poorly: 70% of embryos died. “Additive genetic variations may finally cause the extinction of vulnerable populations,” says Andy Ferguson, a fish geneticist at Queen's University Belfast, Northern Ireland, one of the authors of the new study.

The results provide clear evidence of how interbreeding can harm wild salmon, says Eric Verspoor, head of the fish-genetics group at the Fisheries Research Services' Freshwater Laboratory in Pitlochry, Scotland. It could devastate the beleaguered Atlantic salmon, which has already been hit by pollution and overfishing — wild stocks are extinct, endangered or vulnerable in more than half the 2,000 salmon-spawning rivers around the North Atlantic.

The Edinburgh-based North Atlantic Salmon Conservation Organisation is developing international guidelines for replenishing wild populations. Next year, the group will hold talks with the farming industry about improving containment. Meanwhile, Scottish angling organizations have pledged to alert clubs to the risk of restocking rivers with juvenile farmed salmon.