Lousey Death

Sea Lice on Salmon

Pink salmon infected with sea lice. Alexandra Morton

Friday, December 14th, 2007--

Some pink salmon (Oncorhynchus gorbuscha) populations in British Columbia, Canada could be wiped out in four years--not by fishing, but by fish farming, scientists say. Researchers report in the journal Science that juvenile pink salmon are being infected with salmon lice (Lepeophtheirus salmonis), a naturally-occurring parasite, as they pass fish farms on their way from the river (where they hatch) to the ocean. If current trends continue the researchers predict that these pink salmon populations could go extinct within two salmon generations.

Advocates say aquaculture decreases fishing pressure on wild fish populations by providing another source of fish. "Growth in seafood production almost certainly has to come from aquaculture. The question is what sort of fish do we raise, and where do we raise them?" says Rebecca Goldburg, a senior scientist at Environmental Defense, an environmental advocacy group.

Scientists have long suspected fish farming can harm wild stocks. "This study is the first time we've been able to quantify what the impact of sea lice is on the wild population," says Martin Krkošek, a graduate student at the University of Alberta and an author on the paper. Krkošek says he expected to see some effect, but what he and his colleagues found "was much more severe than we originally anticipated."

Lice are transmitted fish-to-fish. Adult pink salmon can tolerate lice--but juveniles can die from just one or two of the parasites, Krkošek says. In a natural system, the juvenile salmon have time to grow up--get scales, gain weight--before they encounter the parasites. Because lice can't live in freshwater, where the salmon hatch, the juveniles don't encounter lice-carrying salmon until they go out to sea. "By the time they encounter the sea lice several months later, they are large enough to tolerate them," Krkošek says.

But when salmon are farmed in net pens in inlets along salmon migration routes--as they are in the Broughton Archipelago where this study was conducted--the juvenile salmon can catch lice from their farmed cousins early in life. Krkošek says: "When the wild juvenile salmon enter the ocean the first thing they encounter are the salmon farms and the sea lice. They get infected as they pass the salmon farms, and a lot of them are dying. They aren't making it out to the ocean."

Not everyone agrees with the sea lice diagnosis: The Canadian Department of Fisheries and Oceans, which is responsible for managing aquaculture and protecting wild salmon in Canada, told the Associated Press that sea lice can not be solely blamed for the collapse and that other factors may play a role.

Salmon farms have existed in the Broughton Archipelago since the 1980s, but sea lice only became a problem in 2001. Krkošek suggests that the louse lag is related to the growing density of farms. Since the 1980s, the number of salmon farmed in the Broughton Archipelago has increased from 125,000 fish to 100 million fish, he says.

Since the first louse infestation in 2001, the number of pink salmon migrating past the farms has declined sharply. Krkošek and his colleagues predict that over the course of eight years (we're currently in the fourth year of this decline), the pink salmon populations, which supported fishing before the louse infestations, will suffer a 99 percent collapse, the researchers say.

One solution might be to move the fish farms away from the migration routes offshore. In the United States, there is legislation in the House and Senate advancing offshore aquaculture, Goldburg says. But offshore fish farming comes with another set of environmental concerns, she adds. "The current legislation doesn't include strong legal mandates for environmental protection. If one of the results of offshore fish farming could be harm to marine fisheries, it just doesn't make sense to pursue offshore farming."

--Flora Lichtman

Sources

Martin Krkošek
Ph.D. Candidate Centre for Mathematical Biology Department of Biological Sciences University of Alberta

Rebecca Goldburg
Senior Scientist Health, Oceans Programs Environmental Defense New York, NY

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