The article eludes to a viral infection of some sort and even brings warmer waters into the picture. This cold be the first piece in a very complicated puzzle of salmon existence.
Five years ago, the researchers noticed that some Fraser sockeye appeared to show unusual signs of physiological stress while at sea. In the new study, they take that work to the genomic level. Salmon were caught, biopsied and tagged with radio transmitters in the ocean, about 120 miles from the Fraser; at the Fraser’s mouth; and again on their spawning grounds. For each stage, the researchers could look for patterns in gene expression, then see if they tracked with differences in fate.
A pattern stood out. Many of the fish displayed high activity in a set of genes typically activated in response to viral infection. When this genomic signature was found in ocean fish, they were 13.5 times more likely to die before reaching the Fraser. When the signature was found in fish tagged in the river, they were 50 percent more likely to die before reaching their spawning grounds. In fish tagged on their spawning grounds, those with the signature were 3.7 times more likely to die without mating.