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Study: Sea Lice Killing Off Wild Salmon

December 13, 2007

Researchers have new evidence that as the density of salmon farms increases, they can drive nearby wild salmon runs to extinction. The problem is sea lice, a natural parasite that normally attaches to adult salmon with little ill effect and has little contact with vulnerable juvenile salmon. All that changes, however, when fish farms move in.

A study in the journal Science to be published Friday shows that sea lice infestations around salmon farms in British Columbia’s Broughton Archipelago have reached a density so high they are killing juvenile wild pink salmon at a rate fast enough to drive local runs to extinction in eight years if nothing is done – and four years have already passed.

“We’ve seen sea lice infestations on juvenile salmon in Norway, Ireland, Scotland and Canada, but it’s been unclear and very contentious what the impact of the sea lice is on the wild salmon population,” said Martin Krkosek, lead author of the study and a doctoral candidate at the Center for Mathematical Biology at the University of Alberta.

“What’s really new and exciting about this paper is this is the first time scientists have had enough detailed data to actually measure the impact of sea lice on wild salmon populations,” he said.

Principally funded by the Canadian Natural Sciences and Engineering Research Council, the peer-reviewed study is the latest in a series by a group of scientists trying to push the Canadian government to place more strict regulations on salmon farms to control sea lice.

Based on government stream surveys, the study used a computer model to analyze pink salmon returns in 64 rivers without exposure to salmon farms and seven rivers where young fish must migrate past at least one salmon farm. The study considered returns before and after sea lice infestations were noticed in wild fish in 2001.

The Canadian Department of Fisheries and Oceans, which regulates salmon farms and is responsible for protecting wild salmon, said the study overstates the risks, which are not consistent with figures for pink salmon returns since 2002, when populations collapsed.

“They are asking us to believe 80 percent mortality is from one source,” said Brian Riddell, head of the salmon science branch of the department’s Pacific Biological Station in Nanaimo, B.C. “That’s simply unrealistic.”

The authors suggested that the simple solution is to move fish farms out of salmon migration corridors, but Riddell said that was unrealistic given the prevalence of wild salmon.

When West Coast salmon catches in the United States crashed in the 1990s, farmed salmon filled the gap in supermarket coolers. Global production has been growing ever since.

Canada trails Chile, Norway and Scotland in farmed salmon production. British Columbia reports 120 salmon farms in the province produced 78,000 tons in 2006 worth $407.4 million in Canadian dollars, the bulk of it going to nearby U.S. markets. About 20 farms operate in Broughton Archipelago, north of Vancouver Island.

Wild pink salmon are not a commercially important species, but they are an important food source for orcas and other salmon in the ocean. They also provide food for bears and other wildlife and nutrients for trees.

In natural conditions, the adult salmon that carry the sea lice aren’t in the migration channels and rivers at the same time as young pink and chum salmon, so the little fish are rarely exposed.

When fish farms move in, hundreds of thousands of adults are raised in floating net pens anchored year-round in the channels where the young fish migrate. The study suggested that the density of fish farms reached a tipping point in 2001 that triggered a killer sea lice infestation.

Alexandra Morton, a co-author and director of the Salmon Coast Field Station in the archipelago, said wild salmon are surviving commercial fishing but not sea lice.

“The trajectory is much steeper than we expected,” she said. “That data says it takes eight years to take a salmon population from historic abundance levels down to 1 percent of that. Pink salmon are on a two-year cycle. That’s four generations. And we have two left.”

Ray Hilborn, a professor of fisheries at the University of Washington who was not associated with the study, said he replicated the analysis and agreed with the conclusions. But the data are “noisy,” with a lot of variability, because stream surveys are far from exact, he said.

“In terms of anything like statistical significance, it’s dead obvious that counts of fish from those streams in years juveniles swam by farms that were active is much lower,” he said. “It is a very localized effect,” he said, not likely to threaten pink salmon populations in general.




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