Lice threaten Canada’s salmon

Lice harboured by farmed fish are killing wild salmon on Canada’s west coast, new work has confirmed. The study shows serious declines in fish populations, which could lead to the total collapse of runs in those rivers in less than a decade. Salmon lice (Lepeophtheirus salmonis ) are natural parasites of fish in Canadian waters. They usually infect adult salmon, which can tolerate mild infestations well. The trouble begins when naturally infected wild pink salmon return to their home rivers to spawn, swimming past nets full of farmed Atlantic salmon on their way. Their lice shed larvae, which settle and thrive on the farmed fish. The wild juveniles then swim out to sea, passing by these shoals of infected farmed fish and dramatically increasing their chance of picking up a louse. The infection can have disastrous consequences on the young fish. Fisheries ecologist Martin Krkošek at the University of Alberta in Edmonton and his colleagues looked at estimates of the number of salmon returning to each of 71 rivers along the central coast of British Columbia from 1970 to 2006, and found that before the louse infestations began in 2001 the populations were stable.

Fish from seven of those rivers had to swim by at least one fish farm. Once the infestation took hold, those populations began to decline, even though they had been closed to commercial fishing. In 2002, most of the young pink salmon that had swum past farm pens on their seaward journey failed to return: fisheries managers expected 3.6 million fish, but counted only 147,000. Runs of pink salmon in the rest of British Columbia were healthy that year. In 2003, fisheries managers had emptied their farm nets before the smolts left their natal streams, to prevent lice transmission. This seemed to work. Fewer juveniles picked up lice and Krkošek calculated that deaths from lice fell to 1/3 the rate seen the year before. But the farm fish are back in their pens now; there are at present no plans to remove the fish again. At the current rate of decline, the runs in these rivers will drop to less than 1% of their natural levels in four generations, or eight years, the team reports in Science. “Right now, we’re already half way along that curve,” Krkošek says. “This is a cautionary tale,” says Ray Hilborn, a fisheries biologist at the University of Washington in Seattle. Although farming currently threatens “only a handful” of the salmon runs in this region, regulators should respond cautiously to the growing demand for fish farming, he says. “It raises the concern that you could have some other more contagious pathogen that could be incubated in the farms and then spread around from there.”

January 8, 2008

Original web page at Nature