Wild salmon are being driven into extinction by fish farms that were supposed to be saving them from overfishing, Canadian scientists believe.

Farmed fish often have sea lice, which have caused havoc among salmon in the natural environment. Up to 80 per cent of wild salmon have been wiped out in areas where there are high numbers of fish farms and research suggests that 99 per cent will be gone by 2011.

The findings were limited to an area of western Canada where wild salmon have a high exposure rate to fish farms, but the researchers believe that the findings reflect what is happening in other parts of the world.

They found that wild pink salmon, Oncorhynchus gorbuscha, are exposed to sea lice at a much earlier and vulnerable age than they would be in the absence of fish farms. Under natural conditions wild salmon generally only suffer from sea lice,Lepeophtheirus salmonis, when they are adults living in open oceans. As adults they can cope with lice but the juveniles are much more easily harmed.

Infestations at fish farms allow the lice in their free-swimming larval stage to transfer from fish farm nets into the rivers where wild, juvenile salmon travel to reach the sea. Sea lice latch on to the fish and feed on their skin and muscle tissue.

“Salmon farming breaks a natural law,” Alexandra Morton, one of the authors of the study and the director of the Salmon Coast Field Station in Broughton, Canada, said.

“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.”

Martin Krkosek, a fisheries ecologist from the University of Alberta, said: “The impact is so severe that the viability of the wild salmon populations is threatened. If nothing changes we are going to lose these fish.”

The researchers, reporting in the journalScience, said that their findings called into question plans to expand the fish farm industry to keep other species. “Rather than benefiting wild fish, industrial aquaculture may contribute to declines in ocean fisheries and ecosystems,” they wrote. “These results suggest that salmon farms can cause parasite outbreaks that erode the capacity of a coastal ecosystem to support wild salmon populations.”

They found that when farms on the main salmon migration route were shut down temporarily, sea lice numbers dropped and salmon populations increased.

Ray Hilborn, a fisheries biologist from the University of Washington, said the study illustrated the dangers of fish farming to wild populations.

“This paper is really about a lot more than salmon. It is about the impacts of net pen aquaculture on wild fish. This is the first study where we can evaluate these interactions and it certainly raises serious concerns about proposed aquaculture for other species such as cod, halibut and sablefish.”

Off the scale

— Farms have been found to have 30,000 times more lice than the natural level

— 5,000 eggs can be laid by a single female

— A third of fish on the market are farmed

— The industry has grown 8.8 per cent each year since 1970s

— 15,000 fish can be found in a single pen

— It can take five pounds of fish caught in the wild to produce one pound from a fish farm

Sources: UN, Greenpeace

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