Salmon farming operations have reduced wild salmon populations by up to 70 per cent in several areas around the world and are threatening the future of the endangered stocks, says a new scientific study.

The research by two Canadian marine biologists showed dramatic declines in the abundance of wild salmon populations whose migration takes them past salmon farms in Canada, Ireland and Scotland.

A young pink salmon infected with sea lice.
(Courtesy of Alexandra Morton/Science)

"Our estimates are that they [the farms] reduced the survival of wild populations by more than half," Jennifer Ford, lead author of the study published Monday in the Public Library of Science journal, said in Halifax.

"Less than half of the juvenile salmon from those populations that would have survived to come back and reproduce actually come back because they're killed by some mechanism that has to do with salmon farming."

The authors, including the late Halifax biologist Ransom Myers, claim the study is the first of its kind to take an international view of stock sizes in countries that have significant salmon aquaculture industries.

Wild salmon populations in Atlantic Canada have been hit the hardest, Ford said, with rivers in New Brunswick and Newfoundland that have stocks that swim past farms dropping steeply over the years.

The scientists compared the survival of wild salmon that travel near farms to those that don't, finding that upward of 50 per cent of the salmon that do pass by farms don't survive.

Study builds on previous research

"There's really strong evidence that this can have impacts on wild salmon and in particular in places like Atlantic Canada, where Atlantic salmon populations are doing so badly," Ford said. "It's worrying."

The paper didn't look at the causes of the declines, which have been discussed in a series of studies over the last decade that have linked the reductions to disease, interbreeding of escaped salmon and lice from farmed fish.

An article last December asserted that Canadian fish farming is destroying wild salmon stocks and could completely wipe them out within four years in one area of British Columbia. The study, published in the journal Science, contends that aquaculture damages wild populations by infecting juveniles with fatal parasites.

Trevor Swerdfager, director general of aquaculture management for the federal fisheries department, said he will take a close look at the new research. But he added he has so far not seen any proof that salmon farms harm wild populations.

"We look at the impact of salmon farming on wild salmon — if there is one — and we just haven't seen those sort of impacts," he said from Ottawa.

Stock declines are a mystery, Fisheries official says

Stock declines, particularly in the Bay of Fundy, are still a bit of a mystery, Swerdfager said, but there are other pressures at play that could be linked to the reductions.

Ecosystem changes, fishing and other stresses linked to climate change are likely having an effect on the health of the wild populations, he said.

"Atlantic salmon populations are not what they were historically, but can you tie that to the absence or presence of salmon farms? I don't think so," he said, adding that researchers looking at that stock have never linked the decline to farms.

The latest research by Ford, which covered a period from 2003 to 2006, also looked at a large region off British Columbia, which has a substantial salmon aquaculture industry.

Ford said only pink salmon that passed by salmon farms in that region showed sharp declines.

She said some salmon populations in the Bay of Fundy are endangered while one has become extinct. She and Myers, who died last year after the research was complete, found that the number of juvenile salmon that return to the bay to spawn is less than 10 fish a year whereas there were hundreds of them in the 1980s.