A River Never Sleeps

Germinating spore of Ichthyophonus

A deadly parasite has invaded the Yukon River, where it may be killing thousands of chinook salmon before they have a chance to spawn.

Studies by fish pathologists at the University of Washington have found that nearly 25% of the fish in the river last year were infected with a parasite known as ichthyophonus.

Fish in the lower river seem to be able to live with the parasite, but as the chinook go farther up river, passing through Alaska and entering Canadian tributaries in the Yukon, they grow increasingly sick, and many die.

Fishermen first spotted signs of the problem several years ago, but they reported it was rare, showing up in just a few fish each season.

Concerned about the poor shape some of the fish were in, and the smell some of them gave off, fishermen drew the matter to the attention of the U.S. Fish and Wildlife service in Alaska. In 1997 lab tests identified the problem as ichthyophonus.

Since then the problem has grown increasingly worse.

Almost one third of the chinooks taken near the U.S.-Canada border last year showed signs of the parasite. Curiously, farther inland, at Whitehorse, the number of infected spawners drops dramatically to about 10%.

A Yukon River Chinnok Salmon

Biologists aren’t sure whether this change indicates the infected fish - which would amount to tens of thousands - are dying off in the river, before they reach Whitehorse, or if the long running Canadian fish are for some reason able to resist the infection.

Studies will be undertaken this summer to try and solve that mystery.

The Canadian chinook stocks are already depressed, and if ichthyophonus are killing fish in the river, it could have devastating impact on the run.

Biologists are also worried the parasite could show up in other rivers.

Ichthyophonus is a common pathogen that is found around the world in salmon, herring, haddock and other fish.

Over time, it’s likely the Yukon River salmon will build up a resistance to the parasite. But the fear is that in the meantime, it might wipe out some stocks that are already weak.