The poor 2009 sockeye salmon return in the Fraser River, mentioned previously on the bayblab, prompted the Cohen Commission in order to better understand what happened. The final report of the Commission is due this year but in the meantime there is a large amount of information about the ongoing findings of the Commission available here. Many potential causes are being examined, however much of the media coverage of the commission has focused on the possible role of infectious salmon anemia (ISA). The possibility of the presence of this virus in wild pacific salmon has become very political and has resulted in accusations of attacks on scientific credibility. It would certainly have implications for the controversial salmon farming operations in the area.
As the name implies the virus causes anemia by infecting red blood cells of infected salmon and has an extremely high mortality, as high as 90%. This virus is commonly associated with atlantic salmon farms and has affected farming operations all over the world in recurrent epidemic outbreaks. Spread of the virus between farms can be examined by correlations between seaway and genetic distances between viral isolates.
ISA is an Orthomyxovirus, like influenza, and therefore has a small segmented negative sense single stranded RNA genome. This genome arrangement allows for reassortment when a cell is superinfected, and confers the ability of the virus to rapidly change in a population, much like influenza. Hopefully this virus is unlike influenza and will not successfully cross the species barrier and maintain a high mortality in pacific salmon.
On a positive note, it is possible that a local ISA strain has been in pacific salmon species for years. It would be interesting to know to what degree it is genetically related to the ISA found in atlantic salmon farms. Fortunately it has also been demonstrated that pacific salmon are highly resistant to previously characterized ISA (pdf).