for National Geographic News
The growing global appetite for cheap farmed salmon is imperiling wild fish populations across the planet, scientists warn.
The first worldwide assessment of the impact of cultivated salmon on wild stocks found that where native populations encounter salmon farms, the numbers of wild fish crash, on average, by more than 50 percent.
The farmed fish spread diseases and parasites to wild salmon. Some cultivated escapees also interbreed with the native fish, reducing the ability of their offspring to survive, researchers say.
"The overall trend, over and over again around the world, is that salmon farming seems to have a negative impact on wild salmon," said lead researcher Jennifer Ford of Dalhousie University, in Halifax, Nova Scotia.
"The mortality from farming that we find is really large in many cases—more than 50 percent reductions every year," she added. "That is not sustainable for any populations."
A region with an annual farmed salmon harvest of 15,000 tons would suffer an average 73 percent loss in wild populations, the study found.
Many salmon farming regions now produce in excess of 20,000 tons a year, the study added.
The new research used official government data from Canada, Scotland, and Ireland to compare the survival of wild salmon and sea trout in regions with salmon farms to adjacent, farm-free areas.
Researchers found a dramatic fall in salmon catches and abundance since the 1980s in areas of the North Atlantic and northeast Pacific where production of farmed salmon has increased over the same period.
Sea trout, which like salmon breed in rivers and feed at sea, were particularly hard hit.
Sea trout might be expected to experience higher mortalities than salmon, because they spend longer periods in coastal waters where fish farms are sited, the study said.
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