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Why are British Columbia's salmon dying?

British Columbia Supreme Court Justice Bruce Cohen has been reviewing stacks of evidence, much of it conflicting, in the vast "crime scene" of the Province’s missing millions of salmon, and focusing on five environmental culprits in a complex scenario.
Canadian Prime Minister Stephen Harper tasked Cohen with investigating the steady decline since the 1990s and catastrophic collapse in 2009 of the Fraser River's tens of millions of sockeye salmon, a primary native food source and commercial fishing mainstay, that migrate as one-year-olds every spring out of the river, but then have gone missing by the millions instead of returning to spawn, despite there being no signs of massive die-offs along their migration itinerary, the The Globe and Mail reported.
As evidentiary hearings in the special inquiry wrapped up, witness testimony and documentation, including scientific findings, appeared to point to five main factors that may have interacted in the massive fish disappearance:
Sudden, extreme changes in the salinity, temperature, density, windiness and flora in the marine environment along the migration route may have weakened, then killed many salmon.
Diseases, both identified and mysterious (awaiting further research) and an unprecedented lethal Atlantic salmon virus jump to Pacific fish, appear to have caused significant numbers of in-river and pre-spawn salmon deaths.
Experts disagreed vehemently about the roles of government fish hatcheries and commercial fish farms in originating, spreading or boosting the diseases that affected migrating salmon.
Pollutants that harm fish, such as phosphorous, arsenic, chloride, nitrite, selenium and various hydrocarbons, were found in the Fraser, and probably contributed to the salmon population crash, without being a primary factor.
A gradual warming of the ocean and river may be lowering the overall survival rate and triggering early returns to the freshwater environment of the river where the salmon are more vulnerable to pathogens.
In related news, ScienceDaily reported in March that University of British Columbia scientists found certain populations of Fraser River sockeye salmon with extra-challenging migration routes had developed into hardier, more adaptable "superfish."
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