Farm-raised salmon are prone to infestations of fish-killing parasites called sea lice, but until now, proof that they spread the problem to wild salmon has been lacking.
Now, findings, by researchers at the University of Alberta and University of Victoria, British Columbia, cast more doubt on the wisdom of turning to aquaculture to replace stocks of wild fish.
In fact, the researchers said their findings indicate that the parasite problem occurs wherever wild fish shared the ocean with fish farms.
Young wild salmon encounter sea lice when they swim down river to the sea. The parasites bite the fish, creating open wounds and killing many of the juvenile fish.
The researchers trapped about 5,500 pink and chum salmon swimming in a fjord in British Columbia containing two salmon farms.
Each fish was examined by hand. It was clear that the fish were free of parasites until they neared the farms, but that by the time they passed them and headed out to sea they were so infested they ended up spreading the parasitic lice as they went.
USA Today quoted John Volpe, a co-author of the study, who said: "The take-home message here is that when the consumer expects salmon to be available year-round for $2.50 a pound, you run into trouble."
Volpe went on to note that raising salmon in closed systems in the ocean would take care of the problem and raise the per-fish costs only slightly.