Yukon River King Salmon Crisis
Photo Credit: Dr. John Cloud | NOAA
Scientists have struggled to explain why Yukon River chinook salmon have crashed so dramatically in recent decades. A study published last spring sheds new light on what experts say could be the main culprit: marine heat waves in the Bering Sea.
Scientists at NOAA Fisheries and UAF conducted the study and published it in the journal Ecological Applications in April. They considered four major reasons why Yukon River chinook, or king salmon, have suffered steep declines since the late ‘90s, affecting fishing and jeopardizing the food security, health, and a multigenerational way of life for residents along the massive transboundary river. The four major factors thought to be driving the decline include natural mortality, salmon harvests, accidental bycatch intercepted in other fisheries, and ocean survival rates, meaning the number of juvenile salmon that live through their freshwater stage and make it to the sea to grow and rear before returning to natal waters to spawn.
Other contributing factors include habitat degradation and competition from hatchery fish.
To figure out what’s caused the 90 percent population decline between 1981 and 2023, the scientists devised a new model from several data sources to reconstruct the salmon life cycle. Understanding the population dynamics of Yukon River salmon is tough given the river’s enormous size and the salmon’s vast range.
Pieces of the Puzzle
Stretching from its headwaters in British Columbia, the river flows through the Yukon and across the border into Alaska starting at the village of Eagle. It flows nearly 2,000 miles altogether to its mouth at the Bering Sea.
Historically, the Yukon River has supported one of the largest populations of chinook salmon in the world, with annual averages of nearly 400,000 fish river-wide. The large, wild fish are prized for their oily meat and delicate taste. But run sizes of adult kings reached their lowest levels ever following a downturn from 2019 to 2023, according to the research.
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Given the biological, dietary, and cultural emergency on their hands, Canada and the United States signed a seven-year agreement in 2024 to close the Yukon River to commercial, sport, subsistence, and all other types of king salmon fishing in hopes of rebuilding the stocks. Limited harvest opportunities for cultural and ceremonial purposes were built into the agreement, but villagers along the river have said it’s not enough.
Without the ability to catch king salmon, a critical food source, Tanana Chiefs Conference said in October 2024 that tribal members along the Yukon have seen a massive uptick in diabetes, prediabetes, and even malnutrition.
To put the pieces of the puzzle together of why Yukon kings are in trouble, the NOAA Fisheries team relied on bycatch information and juvenile marine survey data collected by the Alaska Department of Fish and Game (ADF&G). They also used adult king salmon monitoring data gathered by ADF&G and Fisheries and Oceans Canada.
After creating the model, the scientists ran the numbers to understand which part of the salmon lifecycle was most vulnerable.
Lead researcher Lukas DeFilippo said in a published statement that his team concluded that impacts in the salmon’s early life likely contributed to declines later, a finding consistent with previous research.
The research team also found that, beginning in 2016, natural mortality in older king salmon began sharply increasing. That year is when the Bering Sea started experiencing some of its most prolonged and acute marine heatwaves.
Another intense marine heatwave occurred in 2019. These extreme events caused wide-ranging shifts in Bering Sea ecosystem conditions and the food web, according to the study. During this period, for example, a mass seabird die-off and an unusual gray whale mortality event occurred, likely due to the poor availability of prey species.
Scientists suspect that during these periods, which coincided with record low sea ice, salmon also found less forage fish available in their usual Bering Sea feeding grounds, such as capelin and herring. Even after the marine heatwaves subsided, prey species continued to decline, possibly explaining why natural mortality of salmon remained elevated in years after the water returned to near-average temperatures.
The salmon may have also been affected by the parasite Ichthyophonus, increased metabolic rates due to the warmer waters they encountered, and disease.
According to the study, “Our findings suggest that extreme environmental events, such as marine heatwaves, have the potential to induce novel stage-specific survival bottlenecks, reshaping the key life history periods and processes therein acting to constrain population productivity.”
Beginning in 2016, natural mortality in older king salmon began sharply increasing. That year is when the Bering Sea started experiencing some of its most prolonged and acute marine heatwaves.
Limitations of the Model
Importantly, the study did not find that bycatch or harvest levels contributed significantly to the population declines. “Our findings indicate comparatively limited impacts of harvest and bycatch removals,” the study said.
That’s noteworthy because many fish advocates, Indigenous leaders, and environmentalists have loudly blamed industrial trawlers for Yukon River salmon decline, saying the vessels and their massive nets scoop up tens of thousands of king salmon even though they’re targeting other species like pollock.
Given the political dimension of the research topic, some critics on social media have blasted the NOAA Fisheries and UAF study as flawed.
The scientists acknowledge that the study has some significant limitations. The model they developed, for example, doesn’t include data from the ‘80s and early to mid-’90s when Yukon River king salmon numbers were at higher abundance levels.
Even so, researchers warn that marine heatwaves are expected to become more frequent and severe, so they recommend that fishery regulators explore management tools that can “adaptively respond” to changing conditions.
While the outlook for the future of Yukon River chinook salmon is grim, some good news emerged towards the end of the summer. The ADF&G reported that 34,289 chinook salmon had passed a sonar counter in Eagle as of August 17, when the run was just about over. While that’s below the historical average of 46,959 fish, it’s still better than expected and larger than what fishery observers have counted since 2020.
The sonar counter in Eagle is often used to gauge how many Yukon River salmon are returning to Canada to spawn.
The four major factors thought to be driving the decline [of king salmon] include natural mortality, salmon harvests, accidental bycatch intercepted in other fisheries, and ocean survival rates, meaning the number of juvenile salmon that live through their freshwater stage and make it to the sea to grow and rear before returning to natal waters to spawn.
Traditional Foods Crisis
It’s not just king salmon that are in crisis on the Yukon River. Heat stress is also blamed for a mass die-off of chum salmon in 2019 in the Koyukuk River, a tributary of the Yukon. And in 2021, Yukon River summer and fall chum salmon runs hit their lowest point since 1981, according to an ADF&G status report.
The Yukon River salmon crisis is widely seen as an environmental justice matter since it disproportionately affects the ability of Alaska Natives and First Nations to access a key traditional food.
“The food security crisis posed by salmon management processes, and the current warming environment in Alaska more broadly, disproportionately affect Indigenous people and subsistence fishers,” said Edward Alexander, with the Gwich’in Council International.
Alexander believes that fisheries management rules should change to include marine protected areas, additional scientific review by the National Academy of Sciences, and include more Indigenous people on decision-making bodies.
His comments came with last year’s release of a peer-reviewed policy brief in the journal Arctic, Antarctic, and Alpine Research. It’s titled “Food security crisis in the Yukon River Basin: where have the salmon gone and what can be done?”
Among various specific policy recommendations, the brief states that the integration of social sciences, such as Indigenous knowledge, into salmon research would broaden understanding of the causes and consequences of the Yukon River salmon crisis, thus leading to potential solutions.
