Understanding the fate of Yukon River Chinook salmon in their 3,000 kilometre migration journey

The majestic Yukon River carves a path through over 3,000 kilometers of rugged wilderness from the icy headwaters located in British Columbia and Yukon, to the remote expanse of the Bering Sea in Alaska. Along its journey, this great river nurtures a remarkable diversity of life, including the Chinook salmon. These salmon undertake one of the longest freshwater migrations in the world travelling on the Yukon River, returning year after year to spawn in the waters where they were born. In recent decades, Chinook salmon have been returning to the Yukon River and its tributaries in fewer and fewer numbers, and in the past five years, they have been disappearing on the way to their spawning grounds.

The Chinook crisis

Once an abundant population, Yukon River Chinook salmon have plummeted to their lowest levels ever recorded. Between 2019 and 2023, Canadian-origin Chinook salmon experienced five consecutive years of substantial in-river, pre-spawn mortality, and in 2022 and 2023, returns to Canada were the lowest seen forcing governments to take drastic action. In 2024, a seven-year (i.e., one full lifecycle) Chinook fishing moratorium in the Yukon River was implemented, signaling just how dire the situation has become. For Indigenous communities, subsistence fishers, and local economies that rely on Chinook, the stakes couldn’t be higher.

Behind the scenes, scientists, Indigenous leaders, and governments are trying to understand why these fish are vanishing. Recent studies suggest that up to half of the Yukon River Chinook that spawn in Canada may be dying enroute to their spawning grounds. But the reasons remain unclear, with suspected causes ranging from disease to rising water temperatures due to climate change.

To uncover the mystery and inform recovery efforts, we launched a three-year study in 2023. We leveraged Alaska Department of Fish and Game’s (Alaska state government) Chinook tagging efforts at the mouth of the Yukon River to monitor their upstream migration within the over 300,000 km² Canadian Yukon River drainage. As we partner with Yukon First Nations and the Alaska State government, this project is providing insight into the fate of Chinook salmon as they make their epic journey upriver.

Working together with technology to track Chinook salmon

Radio telemetry helps us to track tagged salmon when signals are detected by receivers strategically located on the ground in towers installed along water courses or attached on an aircraft during aerial surveys. In our case, we are tracking the movements of Chinook salmon, specifically those destined for Canadian waters, that are tagged at the Yukon River’s mouth in Alaska as they begin travelling thousands of kilometres inland to their spawning grounds. Chinook that spawn in Canada enter the Yukon River in Alaska starting in mid-May and reach the Canadian border about two months later, in early July until mid-August. Each tagged fish also undergoes tissue sampling, which, in conjunction with the tagging provides us with critical data about its speed, timing, origin, demographics, and, most importantly, survival.

In collaboration with Yukon First Nations and the Alaska state government, a total of 26 stationary radio towers have been installed throughout the Yukon River drainage to detect the tagged fish. Twenty towers were set up in Canada this year, including 7 by Yukon First Nations and the Yukon First Nation Salmon Stewardship Alliance with support from individual Nations and the Aboriginal Aquatic Resource and Oceans Management program. To complete one of the more remote tower installations, we embarked on a multi-day river journey with the Vuntut Gwitchin community of Old Crow to a historic Vuntut Gwitchin village in the headwaters of the Porcupine River drainage on the Whitestone River. Collaboration with Yukon First Nations extended to aerial surveys, where we completed 13 days of flying to expand our geographical range to detect Chinook salmon near their spawning habitat in some of the most remote areas of Canada. This collaboration highlighted the critical role of Indigenous Knowledge and local expertise in the study's success.

Detecting Chinook salmon and preliminary findings

The results from the second year of this project are already shedding light on Chinook migration. During the 2024 field season, we detected 96 of the 119 tagged Chinook salmon destined for Canada. Each detection brings scientists closer to understanding why some of the salmon are failing to complete their 2000 km journey upstream into Canada. Currently, leading theories point to heat stress and disease, particularly infections caused by the parasite Ichthyophonus, which is made worse by warmer waters. Understanding these factors is essential to improving management strategies for Chinook populations in the Yukon River.

The significance of the work

For millennia, wild Pacific salmon are a cornerstone of life for Yukon First Nations, providing sustenance, cultural identity, and a connection to the land. Chinook salmon are also vital to the Yukon River ecosystem, feeding predators like bears and eagles and delivering nutrients to freshwater habitats. The Yukon River also supports all other Pacific salmon species in addition to Chinook, including large runs of chum salmon which also migrate vast distances into Canada to spawn.

Declining Chinook returns have sent ripples through the river’s ecosystem and human communities alike. They have also impacted the Yukon River Salmon Agreement under the Pacific Salmon Treaty, which governs Pacific salmon management efforts between Canada and the United States.

Our work through the Pacific Salmon Strategy Initiative is playing a significant role in supporting projects like this one that helps us understand the various factors that are contributing to declines in vulnerable Pacific salmon populations. We have also directed funding to over 20 other studies that are examining the impacts of climate change on Pacific salmon populations. Building our understanding of Pacific salmon and their ecosystems will help to inform the management and recovery of Pacific salmon in the years to come.