Wild salmon have it tough these days, with dams blocking the routes to their spawning grounds and invasive predators congregating around their streams. But their worst enemy might be a familiar face. Contact with farmed salmon raised for supermarket shoppers seems to be killing off the wild fish, according to the first large-scale study of salmon farming's impact.
Farmed-raised salmon live in large, open pens that are often placed near the mouths of rivers used by migrating wild salmon. Last year, Jennifer Ford, a fisheries scientist at the Ecology Action Centre in Halifax, Canada, showed that these penned-up salmon have been infecting their wild counterparts in British Columbia with sea lice, a lethal flesh-eating parasite. Her research, published in (Science, 14 December 2007, p. 1772), was one of hundreds of small studies across the globe that for decades have warned about the ecological impacts of aquaculture. These downsides include diseases transmitted by farmed fish in the rivers of Norway and genetic problems in Ireland caused by interbreeding between wild salmon and fish that have escaped from their pens. But because the problems vary by region, the aquaculture industry has resisted the idea that its practices are having a global impact on wild salmon. The issues, it says, are local.
Ford looked for a wider perspective. She and fellow Ecology Action Centre ichthyologist Ransom Myers, who died last year, started collecting data about the wild salmon populations in three countries: Ireland, Scotland, and Canada. Then they compared populations of wild salmon living in waterways used for aquaculture to those that have never seen a fish pen. The results varied from country to country--the biggest impact was in Canada and the smallest in Ireland--but, on average, the presence of a fish pen cuts a local wild salmon population in half, the researchers determined. "We showed a pattern of significant declines," says Ford, who reports the findings online this week in PLoS Biology.
Ford's research brings a powerful quantitative rigor to something that scientists have qualitatively known for years, says John Volpe, a marine biologist at the University of Victoria in British Columbia, Canada. "The problem," says Volpe, "is that salmon farming has become a global industry; consumers expect a cheap, fresh supply of the fish." To keep prices down, he says, salmon farmers cut corners and offload the costs to the local environment. But Ford says that there are many ways to reduce the impact--moving pens away from the mouths of rivers, for instance, or raising fish in closed farms away from open bodies of water. She says that she's not trying to get rid of the aquaculture industry, only to recognize the costs of doing business as usual.