The crash of the Yukon River's king salmon has been blamed on all sorts of culprits over the years, from predators along the banks to the pollock trawlers working the Bering Sea. New federal research shifts the focus offshore — to a marine heatwave that gripped that sea from 2016 to 2020.
Published in Ecological Applications and written up by NOAA Fisheries in July, the study rebuilds the entire life cycle of Yukon Chinook from decades of juvenile surveys, spawning counts and bycatch data gathered by NOAA, the Alaska Department of Fish and Game and Fisheries and Oceans Canada. Four possible causes went under the microscope: poor juvenile production, natural mortality later in life, directed harvest, and pollock-fishery bycatch.
It was the ocean that stood out. From about 2016, mortality among older juveniles and homeward-bound adults spiked — precisely as the Bering Sea heatwave peaked. A run that had once counted in the hundreds of thousands fell away, bottoming out at a record low return in 2022.
"Not surprisingly, we found evidence to suggest that impacts operating in the early life stages have likely contributed to declines in run sizes over the past two decades, which is consistent with previous research," said Dr. Lukas DeFilippo, the NOAA Alaska Fisheries Science Center scientist who led the work.
The twist was the timing. The team wrote of "an apparent shift in the critical life history stages and processes," meaning the point at which the population is most vulnerable had slid from a fish's first year in freshwater to its final years at sea. Warm water forces a salmon to burn more energy even as prey like capelin thins, and the heat has been connected to flare-ups of the parasite Ichthyophonus. By 2023, well after the heatwave had passed, deaths still had not returned to normal.
That collides with one of the state's fiercest disputes. For years, subsistence and tribal leaders across Western Alaska have accused the Bering Sea pollock fleet of catching salmon as bycatch, and in February a bloc of tribal groups asked the North Pacific Fishery Management Council to impose a hard cap. "The pollock industry is incapable of self-managing this issue on its own," wrote leaders including Vivian Korthuis, chief executive of the Association of Village Council Presidents, in an appeal focused on chum. The council declined a fleet-wide hard cap and settled on a limited "corridor" closure tied to a new bycatch limit.
What no one contests is the toll on people. King fishing on the Yukon has been off-limits to subsistence households for years — Washington and Ottawa agreed a seven-year moratorium in 2024 — in communities where the fish is dinner, a paycheck and a marker of who they are. "Over the last six years, closures have cost our region an estimated $27 million in direct income," Krystal Lapp, a policy analyst for the Tanana Chiefs Conference, told the council. "Salmon is our identity," the conference's chief, Brian Ridley, told state lawmakers considering a ban on bottom trawling in Alaska waters.
The awkward takeaway for managers is that closing fisheries may not restore the kings if the sea is what is killing them. Co-author Dr. Kathrine Howard described the model as a starting point rather than the last word: "A model like this is a big step towards promoting research on the factors affecting salmon productivity across their life cycles, which was recommended by the recently convened Alaska Salmon Research Task Force."
The paper ends on a caution instead of a cure. "Given that marine heatwaves are expected to become more frequent and severe with continued warming," its authors wrote, "similar rises in mortality — and concomitant limitation of productivity and recovery potential — as described here could become increasingly common in the future."
