Shedding New Light on Yukon River Chinook Salmon Declines

Yukon River Chinook salmon have been declining for decades. These declines have limited salmon harvest and posed hardships for local communities that depend on these fish for their food sovereignty, culture, and livelihoods.

Adult run sizes recently reached their lowest point ever following a downturn from 2019 to 2023. The causes were unclear. But a new study identified a heatwave-associated increase in natural mortality as an important driver of this most recent and particularly severe decline.

Addressing the Challenge

Like most salmon, Yukon River Chinook are anadromous. They hatch in freshwater, migrate to the ocean to grow and mature, and return to freshwater to reproduce. Adult salmon spawn in the same river in which they hatched, and each of these reproductive units is called a "run."

One of the main challenges for understanding Yukon River Chinook salmon runs is their vast range. Their life cycle spans the interior of Alaska and Canada to the Bering Sea and beyond. This makes monitoring and studying them-and identifying the causes of their declines-extremely difficult. New research led by NOAA's Alaska Fisheries Science Center and its partners aimed to overcome these challenges. Using cutting-edge analyses, they pulled together multiple data sources from across the life cycle into a unified model framework.

Integrative Research Leads to Comprehensive Model

To reconstruct the full life cycle, this new modeling approach combined data from multiple sources:

One of the first questions that the research team set out to answer with this new tool was: Where in the life cycle are Chinook salmon experiencing significant impacts that could be driving declines? The team examined four factors across different stages of their life cycle:

1. Juvenile recruitment to the ocean: the number of juveniles produced per spawner that survive in freshwater and outmigrate to the ocean
2. Mortality due to natural causes in the marine environment during the juvenile to the adult life stages (natural mortality)
3. Harvest on adult fish in directed fisheries
4. Bycatch of non-targeted marine fisheries

NOAA researcher and the study's lead author, Dr. Lukas DeFilippo, noted, "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."

However, the research team found something else that stood out. Beginning in 2016, natural mortality in the post-juvenile life stages began increasing substantially.

Shortly after this period, adult Yukon River Chinook salmon runs suffered their steepest drop in abundance to date. Interestingly, this collapse could not be explained by weak juvenile recruitment to the ocean alone. Rather, it reflected the effects of increased mortality occurring after the juvenile stage, such as later in the marine life cycle or during adult spawning migrations. This suggests that the bottleneck limiting adult production has shifted to later life stages.

The increase in natural mortality aligns with a period of acute and prolonged marine heatwaves in the Bering Sea. Some of the dramatic changes to the marine ecosystem during this time may have contributed to the increased mortality. For instance, steep declines in prey species- such as capelin -during and following marine heatwaves limit energy available to predators like Chinook salmon. The study also indicated that-despite fewer severe heatwave events in the Bering Sea since 2020-natural mortality rates had not yet returned to their lower, pre-heatwave levels as of 2023. They continued to limit population productivity.

Finally, the study found no evidence that harvest of adult salmon or bycatch removals prior to spawning have been significant factors in population level declines of Yukon River Chinook salmon.

The integrative approach developed in this study has a number of broad applications for future research as well. "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 ," added Dr. Kathrine Howard, co-author of the study and Task Force member. The research team has plans to leverage this new tool for future work aimed at supporting Yukon River Chinook salmon research and recovery.