Farmed Salmon Decimating Wild Salmon Worldwide

James Owen
for National Geographic News
February 12, 2008
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.

Dramatic Declines

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.

Meanwhile Atlantic salmon suffered greater declines than wild Pacific salmon, according to the research.

This result may have been influenced by the fact that Atlantic salmon is also farmed in Pacific regions where the species isn't naturally found, Ford said.

"If those salmon escape, they won't breed with wild populations, whereas in the Atlantic we know that escaped farm salmon do interbreed," she added.

Though previous studies have found similar declines on a regional level due to disease spread and interbreeding, this is the first worldwide assessment of such impacts, Ford said.

Norway was the only major salmon producer with native salmon populations that wasn't included in the study, she added.

Salmon aquaculture is so widespread in Norway that the team couldn't find a large enough area without salmon farms to make a comparison between farmed and unfarmed areas, she explained.

"It's not uncommon for 50 percent of the salmon in [Norwegian] rivers to be escaped farm salmon," she said.

The findings appear in this week's issue of the journal Public Library of Science Biology.

Sea Lice

Scientists say that interbreeding between wild and farmed salmon may reduce the fitness of native populations and their long-term ability to survive.

Past research also suggests that crowded farm salmon develop and spread lethal diseases such as furunculosis and infectious salmon anaemia.

Studies likewise show that salmon farms act as breeding grounds for parasitic sea lice, which spread out and cause deadly infestations in juvenile salmon and sea trout when the fish migrate to sea.

"To stop impacts on wild salmon, we need to reduce diseases such as salmon lice and others and to stop farmed salmon from escaping and entering rivers with wild salmon," Ford said.

"The clearest solution would be to move [salmon farms] to land or to some place where there aren't wild salmon populations that they can impact."

Other Explanations?

But Oslo, Norway-based Marine Harvest, the world's largest salmon farming company, questioned Ford's findings.

Factors other than those linked to aquaculture may have had a more significant impact on the survival of wild populations in the study, company spokesperson Arne Hjeltnes said.

In the case of Canadian Pacific salmon, widespread declines have been observed since the 1970s, Hjeltnes said.

"Salmon farming was a response to these declines," he added.

And compared to commercially fished Pacific salmon, "farmed salmon is a controlled and closely monitored food item," Hjeltnes said.

The United Nations' Food and Agriculture Organization predicts that global demand for seafood will increase by 40 percent by 2030, he noted.

"We believe that the farming of salmon is an environmentally friendly answer to the increasing need for seafood in the world."

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