Wednesday, March 16, 2011

New Developments in Fraser Sockeye Disease

Recently a group of scientists led by DFO's Kristina Miller published results from a study of prespawn mortality in Fraser River Sockeye. Over the last 18 years some components of the Fraser Sockeye stock complex have experienced extremely high prespawn mortality, with as much as 95% of the run dying in the Fraser prior to their arrival at the spawning grounds. What has troubled researchers is that these fish have foregone their typical pattern of staging for 3-4 weeks at the mouth of the Fraser and have instead entered the river earlier than normal where in river temperatures have proven lethal. Puzzled by the maladaptive behavior biologists set out looking for answers. What they found was that Fraser Sockeye were arriving at the river mouth with elevated levels of cortisol and other hormones normally related to reproductive maturity. The cause of these physiological problems was thought to be disease but until this most recent publication it was unknown.

Miller and her coauthors describe a genomic signature related to a viral infection suggesting that for almost 20 years Fraser Sockeye have been dying because of a viral epidemic, strangely concurrent with the expansion of the Salmon Farming industry in BC. While the link is merely speculative at this point it raises alot of very important and serious questions and DFO would be negligent to ignore the issue any longer. Symptoms of the virus have been documented in sockeye, coho and chinook, both adults and juveniles. Infected juveniles are thought to experience dramatic reductions in marine survival.

Now Miller has reported that the virus may be salmon leukemia, a disease which devastated early attempts to farm chinook salmon in the early 1990s. Salmon leukemia has been documented in Chinook and Steelhead in the Georgia Basin and is particularly virulent because it is capable of being both horizontally and vertically transmitted, meaning it can be passed between adults and from adults to their offspring. Still, the question remains how does a disease epidemic last for nearly 20 years. Diseased individuals experience high levels of mortality meaning that eventually the virus should work its way out of a population, unless their is a constant source of infection. Salmon Farms in the migration corridor remain among the most likely sources for the disease, and since atlantic salmon are asymptomatic carriers of salmon leukemia they have not been tested.

Canadians and Americans alike should be alarmed by these developments. Many of Washington State's salmon spend a significant portion of the year in BC's coastal waters and if salmon leukemia is widespread it is very likely American fish have been exposed and infected. WDFW does not test for salmon leukemia and the degree to which salmon in Puget Sound are diseased is entirely unknown. Recent poor returns to rivers in the Puget Sound system have been attributed to poor marine survival, yet the source of high early marine mortality remains unknown. Is there a possible link between disease in Fraser Sockeye, salmon farming and declining returns in Puget Sound? At this point it is conjecture, but it certainly warrants investigation.

More information on Alex Morton's blog:

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