for National Geographic News
The Atlantic salmon is much admired. Flavorsome, high in protein and a rich source of vitamins and omega-3 oils, it ranks among the world's most popular foods. As a sport fish the Atlantic salmon's reputation is unmatched. Then there's its incredible spawning run, when the fish navigates across 1,500 miles (2,400 kilometers) of ocean, before fighting up rapids and falls, to reach the very same waters in which it was born.
But scientists say Salmo salar, the "leaper" in Latin, is now losing this ability, which could lead to the extinction of wild populations throughout Europe and North America. The reason, they say, is the creation of a new race of mongrel salmonthe product of escaped farmed salmon breeding with wild fish.
The warning follows a ten-year experiment in County Mayo, western Ireland, where researchers monitored successive generations of hybrid offspring produced by wild and farmed salmon. Reporting their findings earlier this month in the scientific journal Royal Society Proceedings B, they found these hybrid fish have poor survival rates at sea and are unable to find their way back to freshwater to spawn.
While it has been known for some time that escaped farmed salmon are breeding in rivers with native fish, scientists say the research provides the first scientific evidence that long-term interbreeding could lead to the extinction of wild populations.
"As repeated escapes [of farmed salmon] are now a common occurrence in some areas, a cumulative effect is produced generation on generation, which could lead to extinction of endangered wild populations," said Andy Ferguson, from the School of Biology and Biochemistry, Queen's University, Belfast, who was joint leader of the study.
Leading fish geneticist, Kjetil Hindar, from the Norwegian Institute for Nature Research, describes the research as "seminal," adding: "A long-term study of the fitness of cultured salmon in the wild is unique and the results will be of major importance to those dealing with the biology and management of salmonid fishes throughout the world."
The experiment, carried out at Ireland's Marine Institute research facility on the Burrishoole River System, was designed to simulate the impacts of fish farm escapees now present in rivers throughout northwest Europe and northeast North America.
Two Million Escapees
An estimated two million farm salmon are escaping from marine cages in the North Atlantic each year, equivalent to about 50 percent of all wild adult salmon in the sea. The worst single incident occurred last year, when 600,000 fish were lost during a storm in the Faroe Islands. Research suggests as many as one-third of adult salmon entering Norwegian rivers are farmed fish, while it's estimated they now outnumber native salmon by ten to one in some North American rivers.
The Burrishoole research team planted known numbers of farm and wild-origin salmon eggs on spawning beds. Young fish were also reared in hatchery tanks and released as smolts, the stage at which salmon head to sea before traveling to northerly feeding grounds. The team then monitored returning adults, caught in traps, coastal nets and by anglers, using DNA profiling to identify parentage.
They found that farm salmon showed an estimated lifetime success of just two percent compared with native stock. The survival rate of adult hybrid salmon was between 27 and 89 percent compared with wild fish.
Even more startling was the finding that 70 percent of second-generation, wild-farm hybrids died within a few weeks of hatching due to outbreeding depression. Ferguson says this is caused by genetic incompatibilities between parents but does not occur until the second generation when recombination of the parental genes takes place.
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