In Canada’s Fraser River a mysterious illness has killed millions of Pacific salmon, and scientists have a new hypothesis about why: the wild salmon are suffering from viral infections similar to those linked to some forms of leukaemia and lymphoma.
For 60 years before the early 1990s, an average of nearly 8 million wild salmon returned from the Pacific Ocean to the Fraser River each year to spawn.
Now the salmon industry is in a state of collapse, with fish mortality rates ranging from 40 per cent to 95 per cent.
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The salmon run has been highly variable. The worst year was 2009 when there were 1.5 million salmon, yet it was followed by the best year, 2010, when there were 30 million salmon. But the overall trend is downward.
Losses were particularly high in elevated river temperatures; warmer water makes it more difficult to deliver oxygen to the tissues of salmon.
Seven of the last 10 summers have been the hottest on record for Fraser River. But experts say it is too soon to pin the blame on global warming.