SOCKEYE salmon are dying in large numbers on the River Fraser in British Columbia, one of Canada’s most important salmon rivers.

It is less than three months since it was reported that the River Fraser had been enjoying a huge salmon run, with an estimated 34 million fish – the second large run last year.

But things have taken a turn for the worse with sockeye salmon dying in their thousands before spawning. A mysterious virus is being blamed. The River Fraser certainly has its highs and lows.

The previous year saw just a million sockeye make the journey upstream, a collapse from an expected 10 million fish a few years earlier. This prompted a  government investigation into the collapsed stock.

A large team of researchers from the federal Department of Fisheries and Oceans and three Canadian universities have now found that most of the fish that die before spawning have a common  genomic signature    or a pattern that shows changes have taken place in an array of genes activated to fight infection.

A report published in the journal Science said: “Our hypothesis is that the genomic signal associated with elevated mortality is in response to a virus infecting fish before river entry and that persists to the spawning areas. Studies on the spawning grounds show more than 70 per cent of the salmon that died before spawning had the genomic signature.”

Professor Tony Farrell, research chair at the University of British Columbia s department of zoology, said the research points towards an unknown virus, but the specific cause hasn t been confirmed yet.

Previous studies had shown that in recent years, 40 to 95 per cent of adult sockeye salmon have been dying before they can spawn, both at their spawning grounds and in the Fraser River. Some scientists had suggested that the decline of several species of Pacific salmon, including sockeye, might be related to sea lice from farmed Atlantic salmon in open-net fish farms along Canada’s Pacific coast.