More than a quarter million sockeye salmon returning from the ocean to spawn are either dead or dying in the Columbia River and its tributaries due to warming water temperatures.
Federal and state fisheries biologists say water that is 5 to 6 degrees warmer is wiping out at least half of this year's returning population of the cold-water species.
Ritchie Graves of the National Oceanic and Atmospheric Administration says up to 80 percent of the population could ultimately perish.
Officials are trying to cool flows by releasing cold water from selected reservoirs.
Experts say drought and record heat this summer are behind the high water temperatures.
Thirteen species of salmon and steelhead are listed as endangered or threatened in the Columbia River basin.