Disease Transfer
Update! October 17, 2011: Infectious Salmon Anemia virus found in wild B.C. sockeye salmon. Click here for news stories.
High concentrations of fish in open net-cages provide the perfect conditions for the breeding of disease and the spread of disease to wild stocks. Both wild and farmed salmon can catch naturally occurring diseases such as the highly contagious Infectious Hematopoietic Necrosis (IHN), Infectious Salmon Anemia (ISA), Furunculosis, Bacterial Kidney Disease and various species of Vibrio.[1],[2],[3],[4],[5], [6]
However, the effects of such diseases on farmed and wild stocks are very
different. In the wild, sick fish no longer able to keep up with the rest of
the school become easy prey for predators and do not linger to infect other
fish. The density of fish within a farm, however, and the continuous presence
of sick fish can cause disease to rapidly spread and amplify throughout the
farm.
In 2007 and 2008, ISA found to originate from open net-cage salmon farms, devastated the salmon aquaculture industry in Chile.[7],[8],[9],[10],[11],[12]
The strong potential of disease impacting wild salmon has been further highlighted by research published by Kristina Miller et al in the January 14th, 2011 journal Science. The study suggests that a viral infection may be impacting the survival of Fraser River sockeye salmon both in ocean and when in river to spawn.[13] Futher, the Pacific Salmon Commission's June 2010 Synthesis of Evidence from a Workshop on the Decline of Fraser River Sockeye includes "The Panel members' views on pathogens ranged from a very likely contributor to a possible contributor to the Fraser sockeye situation. . . .Only three other hypothesized mechanisms received ratings as high as likely contributing factors."[14]
Even when being treated with antibiotics and other drugs (see Introduction of "Therapeutants" into the Marine Environment) farmed fish may remain contagious, increasing the risk of amplification and transfer of disease to nearby wild salmon. There is also concern about how and where the carcasses of diseased fish (the "morts") are disposed of.[15],[16]
