Farmed salmon's threat to wild cousins

Farmed salmon pose a major health risk to their wild cousins, according to a Canadian study that found migrating salmon were 73 times likelier to become infected by a deadly parasite when they passed by a fish pen.

Biologists at the University of Alberta in Edmonton studied infections of sea lice among 5,500 juvenile pink salmon, sampled along a 60-kilometre route in British Columbia as the young fish migrated out to sea.

The team netted out a pink salmon or a chum salmon every one to four kilometres (0.5 to 2.5 miles) and counted the lice on its body.

The fish carried almost no lice until they approached an isolated salmon farm along route, and then became heavily infested.

Close examination of the farm, anchored in a long, narrow fiord, showed that lice levels there were four times higher than in the open water, and had spread out for a distance of 30 kilometres (18 miles) along the migration route.

Passing fish faced a statistically 73-times higher risk of infection by having to swim past the farm.

The study is published in Proceedings of the Royal Society B, a journal of British's de-facto national academy of science, the Royal Society.

Environmental groups in Canada, Norway and Scotland have been fierce critics of salmon farms, an industry that brings in hundreds of millions of dollars per year.

They say that uneaten food thrown into the pens, can decay on the sea bed, promoting deoxygenation of confined waters. In addition, farmed salmon which escape and interbreed with wild fish can destroy the quality of the species' gene pool.

Wild salmon are already under threat in many rivers of the north Pacific and north Atlantic because of overfishing at sea.

Sea lice are small parasitic crustaceans that cling to the sides of the fish and feed on it. They cause open lesions that prevent the salmon from maintaining its salt-water balance and, in great numbers, they can literally eat the fish alive.

Juvenile fish are more vulnerable than adults because of their small size, say the authors, led by Martin Krkosek.