Yukon Inter-Tribal Fish Commission director Stephanie Quinn-Davidson told CNN she and other scientists counted 850 dead unspawned salmon on an expedition along Alaska's Koyokuk River in late July.
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The timing of the die-off with the heat wave, combined with no signs of infection or parasites, led the scientists to conclude heat stress was the cause of the deaths. They estimated the total deaths was four to 10 times the 850 counted. Most of the salmon, Quinn-Davidson said, were carrying healthy eggs.
"The die-off coincided with record-breaking temperatures in Alaska," Quinn-Davisson wrote in an Aug. 2 Facebook post. "Communities on the Koyukuk reached 90 degrees July 7-11... 25 degrees above average!! July 12 is when locals started seeing dead chum salmon floating downriver. We are fairly confident these salmon... after surviving years in the ocean and migrating 700 miles upriver to finally spawn... died of heat stress before reaching their spawning grounds."
"We don't know the exact extent of mortality that this warm water event had on the salmon, but we know that the escapements to the Koyukuk river were almost 100,000 chum salmon less than we expected," Holly Carroll, with the Alaska Department of Fish and Game, told The Independent.