Breeding parasites along with fish: do sea lice from salmon farms spread far?

Marine parasites known as sea lice spread readily from farmed salmon to wild fish, according to a study of wild salmon in British Columbia. The researchers, funded by several environmental groups, say their work underscores a possible ecological hazard in aquaculture, but critics of the study question the value of its data.

Various sea lice occasionally latch on to the skin of wild saltwater fishes, reducing swimming efficiency and increasing vulnerability to diseases. These lice are a greater problem among farmed fish concentrated in pens.

Some evidence links salmon farms in Atlantic coastal waters to frequent lice infestations among neighboring wild salmon, and scientists have warned that expanding the aquaculture now present in Pacific water could create new reservoirs of disease.

To assess the impact of a British Columbian salmon farm that harbors sea lice, researchers at the University of Alberta in Edmonton examined juvenile wild pink and chum salmon at points along migration corridors that pass close to the farm.

Twenty kilometers up a fjord from the farm, few wild salmon have lice, Martin Krkosek and his colleagues report in an upcoming Proceedings of the Royal Society B. Immature lice are common on wild fish that are passing the farm, and older lice generally predominate on salmon farther along on their journey to the sea.

From the data, Krkosek and his colleagues constructed a mathematical model. It suggests that lice are 73 times as prevalent on salmon near the farm as in distant waters and that the farm elevates infestation rates in salmon as far as 75 km beyond its pens.

"What the model has shown is that the source of the lice, from a spatial point of view, is in the exact same place as the farms," says coauthor John Volpe, now of the University of Victoria in British Columbia.

"It's a very beautiful and elegant study," says parasite ecologist Andrew Dobson of Princeton University. "No one has been able to quantify the impact of one fish farm until now."

That impact has "a much larger spatial scale ... than we previously thought," says Ransom Myers of Dalhousie University in Halifax, Nova Scotia. He credits the study with providing valuable new data and a mathematical framework for interpreting them.

But Scott McKinley of the University of British Columbia criticizes the study for not addressing whether sea lice significantly harm pink or chum salmon, which differ from Atlantic species, and for inferring movements of wild fish rather than tagging the animals with transmitters. He point, out that the Alberta team gets support from environmental groups but says that he accepts no research funding from the aquaculture industry.

Kevin Butterworth, also of the University of British Columbia, offers a tempered perspective. "It's a good beginning," he says referring to the study's mathematical model "but I don't think you can draw any firm conclusions from it."