(CNN) Alaska has been in the throes of an unprecedented heat wave this summer, and the heat stress is killing salmon in large numbers.

Scientists have observed die-offs of several varieties of Alaskan salmon, including sockeye, chum and pink salmon.

Stephanie Quinn-Davidson, director of the Yukon Inter-Tribal Fish Commission, told CNN she took a group of scientists on an expedition along Alaska's Koyokuk River at the end of July, after locals alerted her to salmon die-offs on the stream.

She and the other scientists counted 850 dead unspawned salmon on that expedition, although they estimated the total was likely four to 10 times larger.

They looked for signs of lesions, parasites and infections, but came up empty. Nearly all the salmon they found had "beautiful eggs still inside them," she said. Because the die-off coincided with the heat wave, they concluded that heat stress was the cause of the mass deaths.

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