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Thousands of pink salmon swim upstream to spawn in Valdez, Alaska, in August 2008.Lucas Jackson/Reuters

Salmon farmers in British Columbia have effectively broken a "transmission cycle" in which sea lice were being spread from farmed to wild fish, according to a new scientific paper.

The transmission of sea lice from farms, where millions of fish are raised in tightly packed ocean pens, has been blamed for the collapse of wild salmon stocks, particularly in the Broughton Archipelago off the northeast coast of Vancouver Island, where some runs of pink salmon have declined by as much as 90 per cent in some years.

But a paper published by the Ecological Society of America states that when farmers in that region treat their stock with a chemical known as Slice, they can knock back the sea lice population so effectively that few of the parasites transfer to wild fish.

The paper also reports, however, that farms should use the treatment earlier in the year to maximize the impact.

"It's a very effective drug for now in British Columbia, as long as it's used at the right time," said Stan Proboszcz, one of the authors of the paper and a biologist with Watershed Watch Salmon Society.

Mr. Proboszcz said the research, which was led by Stephanie Peacock of the University of Alberta and Martin Krkosek of the University of Otago, New Zealand, looked at sea lice infestations in farmed and wild salmon stocks over a nine-year period.

The researchers found that outbreaks in the wild populations were significantly lower when farmed fish were treated with Slice late in winter, before wild juvenile pink salmon had begun to migrate past the net pens.

Sea lice outbreaks in the Broughton Archipelago were blamed after some runs of wild pink salmon were virtually wiped out in 2002. In response, some farms along the salmon migration route were left fallow and regulations were set requiring farmed salmon to be treated with pesticides when there were signs of sea lice outbreaks.

Mr. Proboszcz said that, as farms have adopted the use of Slice over the past decade, the mortality rate for wild salmon has fallen.

"Our results indicate positive conservation outcomes due to adaptive changes in management of parasites in salmon aquaculture facilities," the paper states. "These results provide an example of how management of sea lice on farm salmon can be improved, with relevance to management of sea lice on farm salmon in Canada, Europe and other areas of the world."

However, Mr. Proboszcz said he fears Slice could lose its effectiveness, as it has in some places in the Atlantic.

"In every other jurisdiction, resistance has developed. We haven't seen it yet in B.C., but that remains a concern," he said.

Currently, fish farmers are required to treat salmon with Slice between March and June, when sea lice outbreaks occur. But Ms. Peacock said treatments earlier in the year are advisable.

"Government regulators should take note of this independent research and consider adjusting sea lice management regulations accordingly," she said.

Mary Ellen Walling, executive director of the B.C. Salmon Farmers Association, said her industry welcomes the research findings, which confirm what it has long believed.

"The paper really does show the farms' management practices are effective," she said.

Ms. Walling said salmon farmers have been shifting to treatment earlier in the year and will continue that trend.

She said farmers are also aware of the risk that sea lice could develop a resistance to Slice, so they are using the chemical only in small amounts.

"We don't want to overuse that therapeutic," she said.

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