Wild salmon catch killer pest from captive stocks

NORTH AMERICA - Wild salmon could be succumbing to a plague of parasites because salmon farms are nullifying an adaptation that usually protects young fish from infestation.

Up to 95% of wild juvenile salmon that swim by fish farms die as a result of sea lice infestation caught from salmon in the farms, researchers say.

Under natural circumstances, juvenile salmon do not encounter adults during the first few months of their migration from rivers into the ocean, because adults live far out at sea. Fish farms, which rear mature salmon in net pens, break down that ecological separation, and this may have contributed to the decline of salmon populations.

To investigate, Martin Krkosek at the University of Alberta in Edmonton, Canada, and colleagues surveyed parasitic fish lice on juvenile salmon migrating along the coast of British Columbia in Canada. They found that the salmon are only rarely infested by sea lice before they reach the first fish farm, after which infestation levels jump sharply.

Source: New Scientist