An extraordinary heat wave has proved fatal for thousands of Alaska salmon this summer, CNN reports. Alerted by locals, scientists traveled along the Koyukuk River in July and spotted 850 unspawned dead salmon. "We were boating, going about 35 or 40 miles per hour, and we know we missed a lot," Stephanie Quinn-Davidson, director of the Yukon Inter-Tribal Fish Commission, tells the Juneau Empire. "On a boat going by relatively fast, we were probably getting at most half the fish and at the least about ten percent of the fish." Unable to find infections, parasites, or lesions, they pointed a finger at the coinciding heat wave.
After all, scientists have never recorded higher than 76 degrees around Cook Inlet, south of Anchorage, since they began logging stream temperatures there in 2002—but on July 7 they measured 81.7 degrees. "2019 exceeded the value we expected for the worst-case scenario in 2069," says Sue Mauger, who co-authored a 2016 study on climate change in Alaska. Experts say the salmon likely die by heat-induced heart attack or suffocation when heat lowers oxygen in the water. One hope is that salmon who make it to spawning grounds will give life to equally resilient fish: "Hopefully they'll pass those genes on that allowed them to persist," says Davidson. (Read more salmon stories.)