Summer chum salmon die-off likely caused by heat stress, scientists say

Aug 23, 2019

Scientists suspect heat stress killed a large number of summer chum salmon migrating through the Koyukuk River, a tributary of the Yukon. The carcasses held underdeveloped eggs and sperm, indicating that the salmon were far from their spawning grounds, where salmon usually die. (Photo by Stephanie Quinn-Davidson/Yukon River Inter-Tribal Fish Commission)

Salmon have been found dead in rivers across Western Alaska this summer. The largest die-off reported comes from the Koyukuk River, a tributary of the Yukon. Scientists suspect that the summer’s record heat is the cause.

In mid-July, Lisa Bifelt needed to catch fish to eat over the winter, so she and her boyfriend boated to an eddy near her home in Huslia on the Koyukuk River. 

“And then that's when we started noticing the dead salmon," Bifelt remembered. "Like, every place we would stop they would be floating.”

They counted hundreds of dead summer chum. Stephanie Quinn-Davidson directs the Yukon River Inter-Tribal Fish Commission and flew a team of scientists to the Koyukuk to check out what was happening.

In a video from the trip, Quinn-Davidson walks along a shoreline strewn with chum carcasses in various states of decay, buzzing with flies. “Dead salmon washed up, just piles of them,” she narrates.

Cutting open the fish, the team found underdeveloped eggs and sperm in the bellies, indicating the salmon had a long way to go to reach their spawning grounds, where the fish usually die.

“A river that is usually teeming with life felt like a tomb," described UAF Assistant Fisheries Professor Peter Westley, who was also on the trip.

The salmon showed no signs of parasites, lesions, tumors, or other infections. And because other fish species were not dead, pollution and low oxygen seemed unlikely.

“Most of the fish that we came across, other than being dead, looked fairly healthy," Westly observed. "The key indicator that something was wrong was that the fish were dead."

Vanessa von Biela studies the effects of temperature on Yukon salmon, and she said that all signs pointed to one cause of death. 

“I think there's strong evidence to suggest that fish are dying because of heat stress,” she concluded.

Von Biela’s research with the U.S. Geological Survey shows temperatures approaching 70 degrees begin demanding enormous energy from the fish.

“So it's like if you start a long road trip, and you actually end up running out of gas before you make it to your destination,” she explained.

The Yukon River spent more than a month at or above this threshold, clocking the highest temperatures that had ever been recorded on the river.

Alaska Department of Fish and Game Manager Holly Carroll was also on that trip. It’s her job to manage where and when people can fish for summer chum, and how much they can catch. She also suspects that the warm water was responsible for the die-off, saying “It just was probably too much for them.”

She finds the deaths disconcerting, but she’s not rattled by the event.

“There is 1.4 million (summer chum) salmon that came in this river,” Carroll said. “And we might have seen thousands die off, but I'm not worried about the future of this species.”

She says that salmon are resilient, but it’s their resilience that alarmed another scientist on the trip.

“They’re really tough. So if salmon are dying, it points to something fairly serious,” said UAF's Westley.

Despite the die-off, subsistence families who rely on summer chum have not reported difficulty meeting their harvest goals. And king salmon, which are larger, have not been reported dead.