By Helen Briggs
Science and nature reporter, BBC News
Wild salmon on Canada's west coast are being driven to extinction by parasites from nearby fish farms, a study claims.
Animals like bears rely on pink salmon
Wild pink salmon around the Broughton Archipelago are declining rapidly and will die out within 10 years if no action is taken, say researchers.
They say the data, published in Science, raises serious concerns about the global expansion of aquaculture.
Sea lice from farms are known to infect wild salmon, but until now the impact on wild populations has been uncertain.
"The impact is so severe that the viability of the wild salmon populations is threatened," said lead researcher Martin Krkosek from the University of Alberta in Edmonton, Canada.
Dr Krkosek and colleagues compiled data on the numbers of pink salmon in rivers around the central coast of British Columbia.
They compared populations of salmon that had come into contact with salmon farms with those that had not been exposed, from 1970 to the present day.
Using a mathematical model of population growth rates, they show that sea lice from industrial fish farms are reducing the numbers of wild pink salmon - a Pacific salmon species - to the extent that the fish could be locally extinct in eight years or less.
Dr Krkosek said the population growth rate was "severely depressed".
"It means that the probability of extinction is 100% and the only question is how long it is going to take," he told BBC News.
Scientists say commercial open-net salmon farms are a "haven" for sea lice - naturally occurring parasites that attach to the skin and muscle of salmon.
Mature fish can survive being infested by a few lice but tiny juvenile salmon are particularly vulnerable to attack.
They come into contact with sea lice when they swim past fish farms on their migratory routes from rivers to the sea.
"Salmon farming breaks a natural law," explained study co-author Alexandra Morton, director of the Salmon Coast Field Station, located in the Broughton Archipelago.
"In the natural system, the youngest salmon are not exposed to sea lice because the adult salmon that carry the parasite are offshore. But fish farms cause a deadly collision between the vulnerable young salmon and sea lice. They are not equipped to survive this, and they don't."
Scientists say there are a number of solutions to the problem, including moving farms away from rivers used by wild salmon or putting farmed salmon in pens that are completely sealed off from the surrounding environment.
"The most obvious thing to do is to move the farm out of the way of the wild fish," Dr Krkosek told BBC News.
"Don't put them on the migration route, and don't put them near the spawning rivers. Another option is to move to closed containment technology where the net pen is replaced with a physical barrier that prevents the exchange of parasites - that would solve the problem too."
Dr Krkosek said the impact of fish farms on wild salmon has been "an emotionally, politically and economically charged debate" in Canada.
"Salmon are considered a natural treasure to Canadians, but salmon farming has a lot of economic opportunity - we really need economic activity to supplement coastal economies where fisheries and other resource centres are not doing as well," he explained.
"So there are economic benefits to having salmon farms, but the way that it is currently being done is very damaging to the environment and there are better ways of doing it."
The report in Science has implications for other parts of the world where salmon is farmed, such as Norway and Scotland.
Other species of salmon are known to become infected with sea lice, but they vary in their ability to withstand this.
Sid Patten, chief executive of the Scottish Salmon Producers' Organisation, said the Canadian research bore "little resemblance to the situation in Scotland".
He said fish farmers, wild fish interests and the Scottish government had been working together for many years around the north-west coast and islands to develop local area management plans "for the benefit of both wild and farmed salmon".
"I am delighted to report that there are very positive results coming from this process such as increased numbers of wild salmon returning to some rivers," he said.
"This summer, the Scottish government presented our work to the Canadians who were very interested in exploring a similar model for Canada."