Opponents of open-ocean net-pen aquaculture have long contended that farmed salmon can pass disease to wild salmon populations. Now a study in the Dec. 14, 2007, issue of the journal Science shows compelling evidence that parasitic sea lice infestations caused by salmon farms are driving nearby populations of wild pink salmon toward extinction. The decline has been so rapid that scientists expect a 99 percent collapse of these populations within four years if no action is taken to move salmon farms from the path of the wild populations.
"The speed of the decline was a surprise, but not the basic outcome of the study," said Alexandra Morton, a biologist and director of the Salmon Coast Research Station, who did much of the field work for the study. "In my field work, I've encountered entire schools of juvenile salmon too parasitized to swim."
In the wild, Morton said, juveniles never encounter sea lice. Adult salmon, who can tolerate sea lice, return from the ocean to spawn, where the fresh water kills the sea lice on their bodies. When the juvenile salmon are ready to head out to sea, there are no adult salmon nearby who could pass the parasite. "Now, because the fish farms are on the wild salmon migration route, the juvenile salmon run into a cloud of sea lice," Morton said. "A fish farm can produce a billion sea lice in just four weeks, and because the juveniles don't have scales yet for protection, the lice literally bore right through the fishes' flesh."
"This study confirms what scientists have told us for years—with open net pens, you just can't control the interactions between wild nature and the farmed environment," said Andrea Kavanagh, director of the Pure Salmon Campaign, based in Washington, D.C.
Kavanagh said that while consumer and retailer feedback can put pressure on aquaculture farms to change their methods, those changes may not happen quickly enough to ward off extinction for threatened populations. "The first step that needs to be taken is the relocation of farms out of the path of salmon migration routes," she said. "Beyond that, we need to have a big conversation on whether open farms are the right technology to raise farmed fish." Kavanagh suggested the use of closed tanks, allowing control of all inputs and providing a critical barrier between wild and farmed fish. Though more expensive, closed systems might eventually qualify for organic certification—something the Pure Salmon Campaign believes should not be given to open-ocean net-pen salmon.
"Right now, [aquaculture farms] have a free cleaning service," said Morton. "Ocean currents wash away the waste, and it's typical corporate behavior that they want to externalize these costs to keep down the price. So, if you want farmed fish, pick another species."