Lice from fish farms threaten Canadian wild salmon

CHICAGO (Reuters) - Infestations of sea lice at salmon farms on Canada's west coast are threatening local wild pink salmon populations and could result in their extinction in another four years, Canadian researchers said on Thursday.

They collected nearly four decades of data on the numbers of pink salmon in rivers along the central coast of British Columbia, comparing wild salmon populations exposed to salmon farms to those not exposed.

"The results are striking. Overall the populations that were not exposed to sea lice disease are stable or increasing," said Martin Krkosek, a fisheries ecologist from the University of Alberta in Edmonton, whose study appears in the journal Science.

"This is true even for populations with commercial fishing," Krkosek told reporters in a telephone briefing.

"Overall, populations that were exposed to sea lice-diseased salmon farms are depressed and are declining quickly. This was true even though commercial fishing was closed on these populations," he said.

The researchers believe their findings have implications beyond the region and fish populations they studied. They said their findings indicate that in certain situations fish farms can threaten wild fish populations by concentrating and spreading infectious diseases.

Sea lice are parasites that attach themselves to the skin of wild salmon in the open ocean, feeding on their skin and muscle tissue. Adult salmon can survive a small infestation, but juvenile salmon headed from the river to the sea are too vulnerable to withstand an infestation.

"While sea lice are natural, there are mechanisms in place to ensure they don't kill their host," Krkosek said.

In nature, juvenile salmon can avoid the problem because the infected adults are offshore. But salmon farms that feed into rivers through open net systems can expose young salmon swimming past them to the disease.

"The sea lice killed over 80 percent of the fish in each salmon run where there was infestation," Krkosek said.

He and colleagues based their findings on data from the Broughton Archipelago, a cluster of islands and channels some 260 miles northwest of Vancouver.

It was collected by the Canadian government's fisheries department to estimate how many adult salmon return from the ocean to the rivers in British Columbia each year.

While once rare, there are now more than 20 salmon farms in the area, creating a habitat for sea lice that will overwhelm the wild salmon if not addressed, the researchers said.

If infestations continue, they predict extinction of local salmon populations within the next two generations, or four years.

"The solution is simple," Alexandra Morton, director of the Salmon Coast Field Station in Broughton, told reporters in the briefing. "Build a better barrier to separate the older salmon from the younger salmon."

A barrier would prevent transmission of lice in infested farms from reaching the juvenile salmon in the wild. This can be costly, Morton said, but so would the loss of wild salmon.

(Editing by Todd Eastham)