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November 1, 2024

Adult Chinook salmon in Alaska and Canada are in trouble, and USGS WFRC scientists are in a race against the clock to find the cause behind their disappearance and a viable solution. A staple in many diets, this salmon species is considered a lifeblood of the region.

A mysterious phenomenon has scientists searching for an explanation. Adult Chinook, or King, salmon returning to the Yukon River are dying before they reach their spawning grounds in Alaska and Canada, causing an international fisheries management issue. This adult “en-route” mortality is of particular concern in the Yukon River system, where salmon provide many cultural and commercial needs and account for over 75% of diets among many indigenous groups. 

Given how vital they are, Yukon River Chinook are counted with sonar technology in the lower portion of the river and the data are used to divide the total adult Chinook salmon returns among all resource users. In Alaska, the returning salmon are allocated among commercial and traditional, subsistence fisheries. A proportion is also left in the river to reach Canada and/or spawn and produce the next generation of fish. Unfortunately, in recent years, the numbers of fish reaching the Canadian border have been well below the anticipated numbers projected by the sonar counts. The discrepancies between the sonar-based anticipated returns to the Canada boarder and the actual numbers most likely result from en-route mortality that occurred somewhere between the lower river sonar location and the Canadian border.

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fish wheel used to catch Chinook salmon in the Yukon River
Fish wheel, a primary means of catching Chinook salmon in the middle section of the Yukon River

In the mid-1980s, subsistence fishermen began reporting an unusual condition in some Yukon River Chinook. The fish smelled mildly ‘fruity’, did not dry properly, and had white spots on their heart, liver, and throughout the fillets The Alaska Department of Fish and Game Disease Diagnostic Laboratory determined the cause was Ichthyophonus, a parasite that is common among marine fishes throughout the northern hemisphere. Throughout the past several decades, the parasite and resulting disease have cycled through the Yukon River Chinook salmon population, and the fish are currently experiencing a period of several years with very high Ichthyophonus prevalence. 

During periods of high Ichthyophonus prevalence, the adult Chinook salmon generally enter the mouth of the river with low-level infections.  However, as the fish swim up the river preparing to spawn, their immune systems shut down and the low-level infections can progress to disease.  When accompanied by elevated water temperatures in the Yukon River, the pace of this natural senescence and disease progression becomes accelerated into an ecological arms race, whereby the infected Chinook salmon are trying to reach the spawning grounds before the disease kills the fish.  

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Spawning Yukon River Chinook salmon
A spawning Yukon River Chinook salmon captured near Pilot Station, Alaska, in June 2018 as part of an experimental temperature manipulation study to validate heat stress biomarkers

In some years, fish processors in the middle section of the Yukon River reported that up to 20% of the Yukon Chinook they purchased from fishermen had to be discarded because of unsightly lesions in the muscle tissue that were caused by advanced Ichthyophonus disease.  In the early 2000s, fishing for Yukon Chinook was ordered to come to a halt, including subsistence fishing, in what became known as a federal fishing disaster. The federal government provided disaster relief funding for Native Alaskan and Native American groups along the river, who were unable to meet their subsistence requirements. Because of the low returns and the elevated en-route mortality, an international panel decided again in 2024 to curtail harvest of Chinook salmon in the Yukon River. This agreement, while providing limited harvest opportunities for ceremonial harvest use, has effectively ceased subsistence and commercial harvest for the next seven years.  

While Ichthyophonus infections are believed to be a  contributing factor, its currently thought that a combination of stressors including warming waters, prey conditions, at-sea fisheries, and habitat degradation interact to affect the health and survival of Chinook adults as they migrate up the Yukon River to ultimately spawn and reproduce. The U.S. Geological Survey's (USGS) Western Fisheries Research Center (WFRC) is working, in partnership with the Alaska Department of Fish and Game (ADFG), Alaska Pacific University, U.S. Fish and Wildlife Service, and others  to unravel the mystery. 

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Male scientist training female scientist to inject Yukon River Chinook salmon with Ichthyophonus parasite
USGS scientist Jake Gregg training Alaska Pacific University graduate student Nadia Barcelona how to inject Yukon River Chinook salmon with Ichthyophonus at the Marrowstone Marine Field Station (MMFS)

Using resources that are unique to the WFRC, we are attempting to develop tools that can be used to forecast whether Chinook salmon in the lower Yukon River will be healthy enough to survive the 1,700-mile freshwater migration and successfully spawn. Our team of WFRC scientists engaged in this effort are among the global authorities on Ichthyophonus, are experts in understanding changes in infection severity as the fish swim up the Yukon river, and are mathematical modelers who will integrate and forecast Ichthyophonus disease change under current and forecasted climate scenarios. Further, laboratory experiments are being performed at WFRC’s Marrowstone Marine Field Station to determine the cause-and-effect relationships between Ichthyophonus infection prevalence and severity, water temperatures, and the swimming performance of Yukon River Chinook salmon. 

USGS staff from other Science Centers--including the Alaska Science Center and the Western Ecological Research Center--are simultaneously evaluating the relationships between temperature stress and Chinook salmon survival in the Yukon River, searching for physiological biomarkers of infection prevalence and severity, and trying to identify a panel of gene regulation in infected fish  that may provide early insights into infection and disease accompanied with host survival to the spawning grounds. See more information from the Alaska Science Center here.

Understanding the interplay between these stressors is critical information for managers. With this, they will be able to better isolate problems and predict the number of adults that will survive through Alaska and enter into Canada, thereby providing food and cultural resources to the Native Alaskan and Athabaskan communities in the U.S. and First Nations people of Canada, while still facilitating the passage of enough salmon to achieve escapement and spawning goals for the Yukon River watershed.

 

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