Fish farms are a growing part of the economy in Canada's coastal communities, but they've long been the subject of debate over their impact on the environment. A new report this week suggests they may be more harmful to wild fish stocks than previously believed—in some cases, driving native fish populations to the verge of extinction.

A study published in the journal Science makes the strongest link yet between the spread of sea lice from fish farms and declining populations of wild salmon, its authors say; and the report warns that the situation in British Columbia’s Broughton archipelago may be so extreme that the area’s pink salmon population may be almost completely obliterated in four years.

“This is the first time that scientists have had enough detailed data to actually measure the impact of sea lice on the wild salmon population,” says the study’s lead author, University of Alberta researcher Martin Krkosek. “The impact on those populations is severe.”

The Broughton Archipelago, located a few hundred kilometres north of Vancouver on B.C.’s coast, is home to 27 salmon farms. A few are massive operations, their pens containing over a million fish for an 18-month period. Most fish farms use an open net system, where salmon are raised in cages in a body of water.

The density of the fish population in these farms makes them hot spots for sea louse infestation, and the open net system they use means that the lice from the domestic population spread out into the natural environment. For an adult salmon, sea lice are little more than a nuisance. Though the tiny parasites often attach themselves to mature fish in the waters off British Columbia, they don’t do much harm. When they encounter young salmon smolt, however, it's a different matter.

Juvenile salmon—smaller than your little finger, and still without scales—make their way down the rivers and along inlets to the ocean, where they mature. Because the Broughton fish farms are located near the salmon's spawning grounds, the smolt have to run what researcher Alexandra Morton calls an “80-kilometre gauntlet” past the farms and the estimated one billion sea louse larvae that the they produce each generation. “This is a fatal collision between the parasite and the host,” she says. “It’s very common to see sea lice eating holes, causing open wounds in the little fish.” Morton and her colleagues say the effect on the region's salmon stocks has been dramatic.

They analyzed data gathered by the federal government that estimated salmon populations within the Broughton, and in an area to its north. Between 1970 and 2001, both populations remained strong, despite the presence of commercial fishing activity. But in 2001, after the first major sea lice infestation, the report says that the number of wild pink salmon in the Broughton declined dramatically, dipping by 80 per cent, while the more northerly population remained stable. "Based on that rate of decline," Krkosek says, "it will take only another four years for the Broughton populations to disappear if the sea lice infestations continue."

Not everybody is as convinced that the Broughton’s pink salmon stocks are on the brink of disaster. Brian Riddell, a scientist with the federal department of fisheries and oceans, sounds a note of caution. “I think that the conclusions offer too extreme a prediction in terms of extinction of populations,” he says. “It’s inconsistent with the observations that we have.”

Riddell, who manages the department’s pacific salmon research, says that while there’s no question that the archipelago’s pink salmon population is depressed relative to historical levels—and that action has been taken against sea lice infestation—he doesn’t believe that there is sufficient evidence in the data to suggest that the population will dwindle to nothing.

“We’re not saying that we’re not concerned about the status of pink salmon in the Broughton,” he says, “but we definitely think it’s a major step to imply to people that there’s imminent extinction.”

While no spokesperson was available from the British Columbia Salmon Farmers Association, a media release in response to the Science article took issue with several of the study’s points, noting that the numbers of pink salmon dipped all along the Pacific coast in 2006, and that the population is subject to biannual variation. It also claimed that infestations of sea lice in B.C.'s salmon farms were being carefully controlled.

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