It is no secret that many of the salmon that make their way from oceans upriver to spawn don't make it, but a new study may have found the reason why.
Canadian scientists have found that that many of the Sockeye salmon (Oncorhynchus nerka) that die in the river share a key genetic signature that indicates they are suffering from metabolic and immune-related stress.
Kristina Miller and colleagues tagged wild salmon in the ocean and lower stretches of the Fraser river in British Columbia with tracking devices, took genetic biopsies from the gills of the fish, and then monitored them as they migrated upriver to spawn. The researchers then compared gene expression in the fish that survived the trip and in those that didn’t.
Although no single physiological reason could be found to cause the deaths, it was found that in most of the fish that had died there was a lot of gene activity for genes that are typically associated with viral activity suggesting that the cause of the deaths may be due to these fish carrying a virus.
The genetic signature is present before fish enter the river which suggests that a virus must infect the fish before they enter the river and persists to the spawning areas. Fish tagged at sea found to carry the viral genetic signature had a 13.5 times greater chance of dying before spawning compared to those thought to not carry the virus.
This study may prove highly significant as the long-term population viability of Fraser River Sockeye salmon (Oncorhynchus nerka) is threatened by unusually high levels of mortality as they swim to their spawning areas before they spawn.
Since the 1990s Sockeye salmon productivity has declined significantly to the point that returns in 2009 were less than the replacement rate. In 2009 the Canadian Prime Minister announced an inquiry into the salmon collapse after massive mortalities of 40-95% were recorded en route to spawning grounds.