The drought in the West could be creating conditions in the Klamath River straddling Oregon and California for a repeat of a 2002 fish kill that claimed tens of thousands of adult salmon, biologists said.
Low water and warm temperatures have slowed the upriver migration of spring chinook, allowing infections by parasites as the salmon crowd together in cool water pools.
A similar fate is expected for fall chinook that will start arriving in coming weeks.
"The risk factors this year are piling up," Mike Belchick, biologist for the Yurok Tribe, said Wednesday. The tribe depends on Klamath River salmon for food and ceremonies.
The deadly parasite has been detected at high levels in salmon earlier in the year than usual. The parasite thrives in warm water, infesting the gills of fish and suffocating them.
Warm water has left fish too lethargic to swim upstream, so they congregate in pools fed by cool springs, where the crowding contributes to the spread of the disease.
The river is running slightly higher than it was in 2002 but this year's projected return of 120,000 chinook is lower than average.
The Yurok and Hoopa Valley tribes have called on the U.S. Bureau of Reclamation to send extra water down the Klamath and Trinity rivers for the fish.
But there is little available in the drought and any releases are being saved for the most effective times.
"At this point, we are just really watching the situation," said bureau spokeswoman Erin Curtis. "It is the fourth year of severe drought. Water supplies are very limited and the situation is not good."
The water in the rivers is tightly split between fish and irrigation projects.
Conditions were similar last year, but the bureau was able to release extra water down both rivers. While some fish died of disease, the numbers were not great, said Wade Sinnen, senior environmental scientist for the California Department of Fish and Wildlife.
However, no one really knows how much water is needed to avert a major fish kill, given the variables of how many fish return and how hot the weather gets.
"Every time we've done flow augmentations, there has been litigation associated with it," Sinnen said.
In 2001, water was shut off to the Klamath Reclamation Project to leave water in the Klamath River for protected salmon, setting off bitter confrontations between farmers and federal marshals.
The next year, the Bush administration restored irrigation to the Klamath project, creating conditions in the river for disease that killed as many as 62,000 salmon.
Farmers, tribes, salmon fishermen and states later hammered out agreements to remove three dams on the Klamath River, restore the river and give farmers more predictability about water, But the plan remains stalled in Congress, opposed by House Republicans.
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