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.
Quinn-Davidson said she’d been working as a scientist for eight years and had “never heard of anything to this extent before.”
“I’m not sure people expected how large a die-off we’d see on these rivers,” she said.
The heat wave is higher than climate change models predicted
Scientists have been tracking stream temperatures around the Cook Inlet, located south of Anchorage, since 2002. They’ve never recorded a temperature above 76 degrees Fahrenheit. Until now.
On July 7, a major salmon stream on the west side of the Cook Inlet registered 81.7 degrees.
“2019 exceeded the value we expected for the worst-case scenario in 2069,” she said.
Mauger said that the warm temperatures are affecting salmon in various ways, depending on the stream.
“Physiologically, the fish can’t get oxygen moving through their bellies,” Mauger said. In other places in the state, the salmon “didn’t have the energy to spawn and died with healthy eggs in their bellies.”
Salmon under threat
Salmon populations are under stress from other angles as well.
With fewer salmon to eat, populations of orca whales have steadily declined over the past decades.
“That’s very good,” she said. “Salmon have sustained the way of life of the people of Alaska for thousands of years.”
CNN’s Alisha Ebrahimji contributed to this story.