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Several runs of wild salmon in western Canada could be driven to extinction in four years, at least that's the conclusion of a new study. It finds that the salmon are being ravaged by parasites linked to nearby fish farms.

NPR's John Nielsen has more on this controversial paper.

JOHN NIELSEN: Every spring, millions of baby salmon known as smolt roll down mountain streams in Western Canada. Many of these tiny fish end up in the Pacific Ocean. But in recent years, one kind of salmon - pink salmon - has had trouble reaching those waters.

Salmon expert Alexandra Morton tracks smolt runs in a place called Broughton Archipelago. She says this tiny fish often look half dead.

Ms. ALEXANDRA MORTON (Salmon Expert): They don't even run from my boat. They just sit there floundering. I know there's something very, very wrong, because these guys, when they're healthy, they're like quicksilver. They're like grease lightning. You barely see them.

NIELSEN: Morton runs the salmon coast field station in British Columbia. She says the listless smolts have been attacked by tiny crustaceans called sea lice. These are parasites that don't harm adult salmon, but when a smolt no bigger than a peanut is involved, it's different.

Ms. MORTON: A sea lice on a little salmon is like a raccoon hanging off of your side. It's huge.

NIELSEN: And often lethal. In a new paper published by the journal "Science," a team lead by Martin Krkosek of the University of Alberta says sea lice appear to be killing eight out of every 10 pink salmon smolts that swims through the Broughton Archipelago.

Mr. MARTIN KRKOSEK (University of Alberta): Based on that rate of decline, in another four years, the pink salmon populations in the Broughton Archipelago are going to reach extinction.

NIELSEN: The researchers came to that conclusion after sifting through decades-worth of wild salmon counts collected by Canadian officials. They think the sea lice infestations are coming from giant, floating salmon farms that raise fish in giant nets. Alexandra Morton, one of the authors of the new study, says these farms started showing up near salmon migration routes in the 1980s.

Ms. MORTON: Now, there's farms in here with over a million fish per farm. And what they've done is they have just, they've put too many fish in this water.

NIELSEN: Morton thinks these fish farms have been pumping clouds of sea lice into local waters for years. She says other studies seem to show that when wild salmon swim by these farms, they pick up lots of lice. She and the other authors of the study say the only way to protect wild salmon is to move the farms away from spawning runs or into big tanks built on dry land. But officials in the farming industry and some government officials say they're not impressed by this new study. Kevin Amos is an aquaculture expert with the National Marine Fishery service in the United States.

Dr. KEVIN AMOS (Aquaculture Expert, National Marine Fishery Service): It's a mathematical model that does not demonstrate cause and effect. And it basically leaves out many important elements.

NIELSEN: Like the possibility that wild fish such as stickleback are the main source of the sea lice infestations or that overfishing is a big part of a problem. Amos says the claim that sea lice could drive local rounds of pink salmon to extinction in just four years is statistically ludicrous in his opinion.

But other experts say this study seems convincing. Ray Hilborn is of fisheries biologist at the University of Washington. He says the wakes him wonder about plans to raise other kinds of wild fish like halibut and sablefish in giant pens.

Professor RAY HILBORN (Fisheries biologist, University of Washington): The cautionary note is, look, expect the same thing to happen with those species that's happening with salmon.

NIELSEN: The irony, says Hilborn, is that fish farms have long been touted as a way of taking fishing pressure off of wild fish like salmons. If the studies like this one are confirmed, it could get much harder to build new fish farms.

John Nielsen, NPR News, Washington.

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