Study links salmon farms to lice outbreak

by Valerie Shore

Young salmon with sea lice attached

The transfer of parasitic sea lice from salmon farms to wild salmon in B.C. is much larger and more extensive than previously believed, says a study co-authored by a UVic researcher.

The study was published in a recent edition of the journal of the British Royal Society. One of the authors is Dr. John Volpe, a faculty member in UVic’s school of environmental studies. The two other authors are University of Alberta researchers Mark Lewis and Marty Krkosek (a graduate student co-supervised by Volpe).

The study combined advanced models of disease transfer with extensive field work in the Broughton Archipelago off northeastern Vancouver Island, where many commercial salmon farms are located.

The farms are open cages of closely packed Atlantic salmon which provide ideal conditions for the spread of sea lice—external parasites that eat the slime, skin and blood of their fish host. In large numbers, the lice weaken and eventually kill the fish. Adult salmon can tolerate light infections, but young fish are especially vulnerable.

Because sea lice occur naturally, a bitter debate rages in B.C. over whether fish farms contribute to sea lice infections in wild salmon. This study shows indisputably that they do, says Volpe. “There’s no ambiguity in our data whatsoever.”

Using new, non-lethal sampling techniques, the team studied lice infection levels of 5,500 young pink and chum salmon as they approached and passed a farm anchored in a long, thin fjord.

The team found that sea lice production from the farm was 30,000 times higher than natural levels. “Infection of the salmon was 73 times higher than ambient levels near the farm and exceeded ambient levels for 30 km of the wild migration route,” says Volpe.

The study also revealed another cause for concern. As the migrating school of lice-infected young salmon moves out to sea, new generations of lice may be infecting salmon from other streams—and possibly other species such as herring. “What we have is a moving, growing mass of contagion that has the potential to infect fish stocks all along the coast,” warns Volpe.

Volpe says he and Krkosek are now turning their attention to the impact of lice infestations at the population and ecosystem levels. “The debate over the role of farms as a breeding ground for sea lice is over,” says Volpe. “Now we have to find out what it means and how we can deal with it.”

The study was funded by the Natural Sciences and Engineering Research Council, the David Suzuki Foundation and the Raincoast Conservation Society.