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Aquatic scientists divided on role of sea lice from salmon farms in decline of native salmon in B.C.


Salmon farms in British Columbia may pose a threat to wild salmon stocks, a paper published today in the Canadian Journal of Fisheries and Aquatic Sciences claims. The paper presents evidence that native fish sampled near the farms are more heavily infected with parasitic sea lice. Lead author Alexandra Morton, a registered professional biologist and private researcher, believes the parasites multiply on the farms and are then transmitted to juvenile native salmon, causing recent drastic declines in wild fish populations. "If we don’t do anything, we’re definitely going to lose the wild salmon," said Morton.

Morton monitored the levels of infection of sea lice (unrelated to human lice), naturally occurring parasites that infect salmon only, on juvenile pink and chum salmon in British Columbia’s Broughton Archipelago, a chain of islands between the mainland coast and the northern end of Vancouver Island. She then compared infection rates on salmon from sites near to and far from the farms.

"We found 3 cases of sea lice in a sample of 1,018 juvenile salmon outside of the Broughton Archipelago. Within the Broughton Archipelago," where there are 28 Atlantic salmon farms, "we found 4,338 of this species of sea louse on 1,138 salmon," -- a 1,000-fold difference, said Morton. Her study showed potentially lethal levels of infection in 90 percent of wild juvenile salmon. Morton believes the young native salmon become infected when they swim near the farms during their migration from freshwater streams to the open ocean.

Morton said that to preserve native salmon stocks, "the farm fish have to be separated from the wild fish. There are alternative technologies that allow farmers to grow fish in facilities that provide a barrier to the marine environment." A barrier would prevent transfer of disease and parasites between the farmed and wild fish.

Because of concerns about possible effects of sea lice on native fish, 11 of 27 Atlantic salmon farms in the Broughton Archipelago were closed during the migration of the pink salmon in 2003 (a practice called "fallowing"). "It was a big economic loss to the farmers," said Morton. And it didn’t entirely solve the problem. "We still had over 20 percent of the fish infected, and the farmers can not repeat this measure this year."

Scott McKinley, Professor and Senior Canada Research Chair of Animal Sciences at the University of British Columbia and Executive Scientific Director of AquaNET, a National Network of Centres of Excellence in aquaculture and environmental research whose mandate is to foster a sustainable aquaculture sector in Canada, disagrees with Morton’s conclusions. He suggests that there is no evidence that native fish are declining due to farming.

"With any fish population, one or two years of surveys does not make a trend. . .. There have been drastic declines in pink salmon before, and that was before there were farms here," said McKinley. "There is no study published showing a cause-and-effect relationship between sea lice on wild and farmed fish. . . All the work that’s out there is based on correlations."

McKinley suggests that other explanations for the population fluctuations in wild fish are also likely. For example, population crashes could result from limited resource availability or fishing pressure. Fluctuations in water temperature on a global scale, such as those caused by El Niño, could make the salmon sick and stressed. "If you happen to be weak or stressed in terms of general health, you tend to be more susceptible to parasite infection."

Pressures from environmentalist groups about sea lice are forcing the aquaculture sector to make sacrifices based on inadequate information, McKinley said. He said that in the Broughton Archipelago "the farms were fallowed because of pressure from environmentalists who believed that there was a problem with sea lice on the farms. Although this wasn’t backed by scientific evidence, farms cooperated and likely lost a lot of money."

Morton argues that similar outbreaks of sea lice paired with declines in native salmon in Norway, Scotland, and Ireland corroborate her findings. "In Norway, there are very strict regulations about how many lice you’re allowed on your fish."

However, McKinley stressed that environmental conditions in Europe are different from those in British Columbia, and he warns against global extrapolations. He said that AquaNET, in collaboration with other national and international scientists, plans to study how native and farmed fish are affected by sea lice and conduct risk analyses of lice treatments.

Morton insists that if regulatory action is delayed, the consequences to wild fish could be serious. "The Norwegian scientists have said to me that they expected this problem to arise on the Pacific coast and that we will have good years for sea lice and bad years, but in the end we will lose our wild stocks. That seems unnecessary. Wild salmon are ecologically critical, and we have other options."

The Canadian Journal of Fisheries and Aquatic Sciences is a scientific peer-reviewed journal published by the NRC Research Press.

Alexandra Morton | EurekAlert!
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