Recent research indicates that extremely warm temperatures can turn Alaska’s salmon streams into unfriendly, even lethal habitats. While Alaskan scientists are just beginning to study the impact of warmer temperatures on salmon streams, it is already a familiar reality for many Canadian fish biologists.
Warming temperatures in British Columbia's Fraser River have long been known to hurt and even kill sockeye salmon. It’s been studied for decades, and Vanessa Von Biela, a biologist with the federal Alaska Science Center, told those attending the Alaska Marine Science Symposium in January that a small increase in river temperatures on the Fraser can mean a lot less salmon spawning.
“More than half the salmon run can die in the river,” Von Biela said. “That includes en route mortality and pre-spawn mortality, where they die on the spawning ground.”
Until recently, the Canadian experience didn’t seem relevant to Alaska’s colder salmon streams, but that’s changing. Alaska’s ocean and streams are now warming faster than those to the south, and last year’s recordbreaking heat delivered dead, unspawned salmon to our streams.
Von Biela saw it coming. A few years ago, she set up experiments to document the temperature tolerance of Yukon River king salmon. She set up tanks to test some of the returning fish as they entered the drainage, to see what temperatures caused heat stress. She had to look for specific genes and proteins because salmon don’t look like they’re in trouble until it’s too late.
“And those salmon tended to look healthy other than they were dead,” she explained.
Von Biela tested Yukon king salmon for six hours in water that was 14 degrees Celsius, or 57 degrees Fahrenheit, all the way up to 21 degrees Celsius, which is 69.8 degrees Fahrenheit.
“And we certainly felt that the ones in 21 degree C[elsius] treatment were stressed, and in fact that was the only treatment where we had mortality. Half of those fish did die,” she said.
The fish that survived the tanks showed changes in both genetic markers and proteins at the various temperatures. This allowed her to create a way to measure heat stress in Yukon kings.
“And that’s our small set,” she said, “that allows us to classify all the other fish we catch in the drainage.”
Now, all Von Biela needed was to get water temperatures from further upriver, and some fish tissue from the catch to sample. The samples were easy to get; the water temperatures were not. The Alaska Dept. of Fish and Game monitors water temperatures at the mouth of the Yukon, and that water is relatively cool when the fish are starting their run up the river in June. She needed July's temperatures from the middle of the Yukon’s mainstem, when kings are swimming through to spawning grounds. Von Biela found the data she needed at a fish wheel.
“This is data from middle of the Yukon river from Stan Zuray’s fish wheel,” she said. “He is a citizen scientist and subsistence fisher, and he spent several years attaching a Tidbit data logger to his fish wheel and gathering these data.”
The fish wheel showed water temperatures higher than 64 degrees Fahrenheit for extended periods of time, but nothing as high as the 69.8 degree Fahrenheit, or 21 degree Celsius, top range that she had tested fish for.
Working with a host of collaborators, Von Biela gathered data from Yukon River fish in 2016 and 2017. They found plenty of evidence of heat stress in the 477 fish they sampled. Because 2016 was a little cooler than 2017, they could document what that change meant to the king salmon.
“Like here at the Rapid Rivers fish wheel where in 2016, the cooler year, only 18 percent of the fish showed evidence of heat stress, but then in 2017, when it was warmer and the warmth was more prolonged, we had 97 percent of fish be classified with heat stress,” said Von Biela.
That stress was there even though the fish wheel never showed temperatures as high as 21 degrees Celsius during the two-year study. But Von Biela also pointed out that that kind of heat did occur at the Andreafsky River, a tributary of the Lower Yukon.
“And in 2016 we even had temperatures exceed 21 degrees Celsius,” she explained while pointing to the data on a graph. “And the weir crew did see those outward signs that fish were struggling and lethargic. And about a third of the season they did not handle fish for age-sex-length because they did not want to have additional stress. And here at this site at 2016, we estimate that 98 percent of the fish we handled had heat stress."
Von Biela says that her research indicates that fish managers may want to reconsider a fundamental assumption: that letting salmon swim by fishing nets necessarily means that they will make more salmon. In 2019, some salmon failed to spawn, dying with full egg sacks on the spawning grounds.
“We can no longer think that escapement always is a successful spawner,” Von Biela said.
To manage in the new warmer reality, Alaskan biologists could look toward their Canadian colleagues, who use real-time water temperature data to manage the Fraser River sockeye fishery.
Warming river conditions are only part of the situation facing salmon in Alaska. In the second part of this series, we’ll explore the frontiers of what is known about the fish as they return from the high seas, which are also warming.