Farm salmon spread deadly lice

In the Pacific Northwest, sea lice that spread from cultivated salmon to their wild counterparts have become major parasites affecting the wild population. The lice, which are visible to the naked eye, attach to fish and draw blood and nutrients.

John Volpe of the University of Victoria in British Columbia and his colleagues previously reported that sea lice spread readily between these groups of fish (SN: 4/2/05, p. 212: Breeding Parasites Along with Fish: Do sea lice from salmon farms spread far?). The team proposed that fish farms imperil wild salmon, which must swim past the farms as they migrate to the sea.

Farms, Volpe says, are “point sources of lice, pumping out tremendous quantities of infectious larvae.”

But some researchers questioned that assertion because there was limited evidence that the lice cause substantial harm to wild fish.

In the Oct. 17 Proceedings of the National Academy of Sciences, Volpe’s team confirms its earlier findings and reports further that sea lice that have spread from fish farms kill 9 to 95 percent of migrating wild salmon, depending on the season and local circumstances.

Volpe says, “Salmon farms are far and away the major contributor to lice on outmigrating salmon [and] the most significant driver of mortality” in the wild fish during their migration.

As part of the new study, the researchers captured migrating salmon and recorded how many lice were attached. Subsequent mortality was significantly higher among salmon that had at least one louse attached than among fish with none. Death of the fish, says Volpe, was “effectively assured with two lice or more.”