While no one has any solid numbers, estimates of the number of farmed Atlantic salmon off Europe, eastern Canada, Maine, the Pacific coast of North America and Chile run from 100 million to 200 million each year. The potential threat to wild salmon posed by farmed ones is said to be threefold. First, it is argued, the farm fish are bred for selected characteristics like growth rate and tameness, and this could make them less genetically diverse and so less suited for survival in the wild. The concern is that when they escape and breed with wild fish, they may reduce the ability of the offspring to survive. Surveys on the Magaguadavic River in southern New Brunswick have found that fish escaped from offshore farm pens outnumbered wild ones by up to eight to one, and Dr. Windsor said large numbers had escaped in Europe.
Experts say they have no difficulty distinguishing the two varieties.
Second, farm fish may compete with wild ones for habitat and food. Third, diseases and parasites are said to flourish in salmon farm pens, and these may be transmitted to wild fish. Two parasites in particular, tiny brown sea lice and a microscopic organism called Gyrodactylus salaris, literally cover salmon and kill them by eating their flesh. Norway has had to poison all life in some of its salmon rivers to get rid of sea lice. In North America, aquaculture interests say that while some farm fish may escape their pens, there is no evidence of any effect on wild salmon.