Disease and a warmer Yukon River are wiping out scores of chinook salmon before they ever reach spawning streams in the Yukon, say researchers.
Tepid water is making salmon more susceptible to ichthyophonus, a parasite that strikes mainly at the heart. It’s also making it harder for them to swim.
“At warmer temperatures, the ichthyophonus disease causes very reduced stamina in these fish,” said Jim Winton, head of the fish health section at the Western Fisheries Research Centre in Seattle.
Up to 20 per cent of returning adult chinook may be infected with the parasite.
“It’s tough enough having to swim 2,000 miles and then having to dig a nest and then defend a nest—and if your heart’s not working well, then it’s especially hard,” said Winton.
Chinook with strange white spots—another symptom of ichthyophonus—have begun to show up in the nets of subsistence fishers across the Yukon and Alaska over the last 20 years.
Impossible to smoke correctly, and carrying a strange odour, infected fish must be discarded by subsistence fishers.
“This only compounds the problem, because they’re taking more fish to meet subsistence
needs because they’re having to discard some fraction of these fish that are infected and aren’t suitable for their needs,”
Fishers are known to discard up to 30 per cent of a catch due to ichthyophonus.
Infected chinook have been found at spawning beds all along the river. Around Whitehorse, however, the rate of ichthyophonus suddenly drops significantly.
Scientists suspect that Whitehorse-bound chinook infected with ichthyophonus died before completing their journey.
Yukon River salmon stocks have dropped by half over the last few years. Dismal returns are also predicted this season.
Ichthyophonus is only one factor hammering Pacific salmon populations.
“A lot of these population declines have multiple causes,” said Winton.
Every year, thousands of chinook are snared in the nets of Alaska pollock fishers as “bycatch.” Later this month, Alaska regulators will meet to implement a bycatch reduction plan.
Contact Tristin Hopper at