Salmon farms help stock supermarkets but also breed parasitic sea lice that infect young wild salmon and could endanger other important ocean species such as herring, scientists said Tuesday.
Even a single farm can have far-reaching effects, Canadian researchers Martin Krkosek, Mark Lewis and John Volpe found. The study adds fuel to the clamor over farmed versus wild salmon, a debate that extends along Pacific Northwest coastlines.
"We know that the lice do infect other species," said Krkosek, a University of Alberta mathematical biologist. "The transmission from farmed fish to wild fish is much larger than what was previously believed."
Adult salmon can survive such infections, but the younger salmon are more vulnerable. "Normally, juvenile salmon have time to build resistance and put on body mass before they encounter these parasites," Krkosek said.
The Canadian government has found that salmon farms effectively control sea lice. But citing concerns over declining populations of native juvenile salmon off northern Vancouver Island, the government announced plans last week to do more research on the matter.
The study, being published Wednesday in the London-based Proceedings of the Royal Society B, contradicts the government's conclusion on the danger posed by sea lice from salmon farms.
Mixed reactions
Ransom A. Myers, a marine biology professor at Dalhousie University in Canada, called it "a very thorough analysis" that relies on an enormous data set. But Robert Scott McKinley, a University of British Columbia professor of aquaculture, was unconvinced. "I think it hints of fear-mongering," he said.
In the published study, the researchers looked at 5,514 juvenile pink and chum salmon as they swam up two narrow fjords in British Columbia, past a salmon farm.
The study found that as the fish migrated past the farm — about one-eighth of a mile long — clouds of lice infected the juvenile wild salmon at unnaturally high rates for nearly 19 miles around the farm. Normally, sea lice are rarely found on wild juvenile salmon.
"Conservatively, this means that the parasite footprint of the farm is 150 times larger than the farm itself," said Volpe, a University of Victoria marine ecologist.
The study also found that the lice bred as they infected the migrating juvenile salmon, allowing them to re-infect the fish and potentially endanger other marine species.
Some European countries, where salmon farming is popular, use chemicals to control the parasites and dye to turn the salmon flesh pink. The use of those chemicals has led some environmentalists to hold demonstrations run ads urging consumers to boycott farmed salmon. Some grocery stores carry labels saying farmed fish contain dye. And a major study in the journal Science last year found more cancer-causing PCBs in farmed fish over wild fish.