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A new study proposes that sea lice from salmon farms aren't behind the decline in wild salmon stocks.John Lehmann/The Globe and Mail

Sea lice from farmed salmon are not to blame for a dramatic collapse of wild stocks on the West Coast, a trio of Canadian and U.S. researchers has concluded, using industry data previously not available to scientists.

The findings, published on Monday in the Proceedings of the National Academy of Sciences, are a major departure from a growing body of research that has pointed to sea lice from fish farms as key suspects in the decline of wild Pacific salmon in British Columbia.

Industry representatives say they hope the paper will change the tenor of the increasingly truculent debate in B.C., in which the aquaculture industry has been under attack for its environmental impact.

The report, by Gary Marty of the University of California, Davis, Sonja Saksida of the B.C. Centre for Aquatic Health Sciences and Terrance Quinn of the University of Alaska, is unequivocal: There is no direct correlation between the number of lice in salmon farms and the decline of wild stocks.

The lead author, Dr. Marty, said the researchers got unprecedented access to detailed fish health and farm productivity records from 26 fish farms in the Broughton Archipelago, off the northeast shoulder of Vancouver Island.

The data, which the industry has previously refused to release to researchers, covered 10 years for health information and 20 years for productivity, and were compared with 60 years of records showing the returns of wild pink salmon to nearby rivers.

"[Wild]salmon populations normally go up and down and our analysis shows that that is not affected by sea lice numbers or farmed fish harvest numbers," Dr. Marty said.

"The data from Broughton Archipelago pink salmon populations and sea lice experiments best fit the conclusion that the majority of pink salmon deaths are caused by something other than sea lice, and our farm data supports the conclusion that farm lice did not significantly decrease pink salmon productivity over the past decade," states the paper, which was edited by Carl Walters, a fisheries researcher at the University of B.C.

Dr. Marty, who has worked for the fish farming industry in the United States, said the finding means environmentalists' demands that fish farms be moved away from the migratory routes of wild salmon are not justified.

"Based on the lack of evidence for a significant negative relationship between farm fish and pink salmon productivity, the data do not support the hypothesis that separating farm fish from wild fish will increase pink salmon marine survival," the paper states.

The research did find, however, that fish farms grow a lot of sea lice, with two farms having an estimated 18.7 million lice in one monthly checkup.

And Dr. Marty said there is no doubt sea lice move from farms to wild fish in the spring, when juvenile salmon migrate past on their way to the open ocean.

But he said the reverse is also true, with adult wild salmon being the source of lice that infest farmed salmon in the fall.

Dr. Marty, whose colleague, Dr. Saksida, does contract work for fish farms in the study area, said the researchers did not determine what is causing wild stocks to collapse.

"Sea lice is one component to consider, but we need to have a broader look at what is going on," he said, suggesting that more effort be put into examining diseases.

Alexandra Morton, a researcher and environmental activist who has published papers in science journals about the negative impact of sea lice on wild salmon, said she isn't convinced by the research, which runs dramatically contrary to her findings.

"I did an extensive study on the impact of sea lice on juvenile wild salmon and I watched these fish die. You can't convince me otherwise. It's something I have observed," she said.

"I think any fish pathologist would look at these fish [infested with sea lice]and say they are heavily compromised. And so this paper really should have given us some clue as to what killed these fish. To just put out that it was something different I think is, well, not credible," Ms. Morton said.

Clare Backman, director of environmental compliance for Marine Harvest Canada, B.C.'s largest aquaculture company, said the study is welcome good news for the fish-farming industry.

"This paper does turn the argument around quite significantly," said Mr. Backman, whose company is responsible for more than half the 80,000 tonnes of fresh salmon produced in B.C. each year.

"This report provides a counterargument to the simplistic statement that sea lice from salmon farms are the cause of the decline of some of the Pacific stocks," he said.

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