When the Ecosystem Is Struggling, Every Fish Matters More

A new study examining the severe decline of Yukon River Chinook salmon offers an important lesson for fisheries management across Alaska: when changing ecosystem conditions become a major driver of population decline, managers must be more rigorous in controlling the losses they can influence.

Yukon River Chinook salmon have declined for decades, but adult runs saw an especially severe downturn from 2019 through 2023, reaching record lows. In the study, researchers combined juvenile marine surveys, adult monitoring data, harvest records, and bycatch estimates to assess mortality across the Chinook life cycle.

Researchers found that the latest collapse could not be explained by weak juvenile recruitment alone. Beginning in 2016, estimated natural mortality among post-juvenile Chinook increased significantly. Interestingly, that rise coincided with prolonged marine heatwaves in the Bering Sea, which was followed by the steepest decline in adult Yukon Chinook abundance on record. In other words, when the Bering Sea warmed substantially, the adult Yukon Chinook population dropped substantially. 

The heatwave period also coincided with major changes in the food web. Populations of prey, such as capelin, also declined sharply, potentially reducing the food available to Chinook salmon. The National Oceanic and Atmospheric Administration (NOAA) reported that natural mortality remained above pre-heatwave levels through 2023—even after the most severe heatwave conditions had subsided.

The study improves our understanding of where recent losses occurred and how rapidly changing marine conditions can affect a salmon population. It also raises an essential management question: what should managers do when one of the largest sources of mortality is beyond their immediate control?

The answer cannot be to treat the remaining human-caused sources of mortality as unimportant. Researchers did not identify directed harvest or estimated bycatch as leading drivers of the population-level collapse. But a source of mortality is still meaningful even if it is not the largest source.

Population declines stem from multiple pressures acting across a fish’s life cycle, including marine conditions, prey availability, predation, disease, harvest, bycatch, and habitat conditions. The influence of these forces may shift over time, but together they shape the burden on the population. That complexity should lead to more comprehensive management, not less.

For instance, managers aren’t going to expand directed harvest opportunities right now, simply because research has shown that harvest isn’t a leading source of increased mortality. Similarly, and importantly, our commitment to reducing Chinook bycatch should remain as strong as ever in light of this new information. 

A Chinook salmon killed as bycatch was not killed by the heatwave. But that fish can no longer return to the river to spawn or contribute to the next generation. When natural mortality rises and fewer fish survive in the marine environment, avoidable losses become more important, not less, and require greater scrutiny. A smaller source of mortality may not explain the collapse, but it can still slow recovery, compound other pressures, or remove fish that would otherwise reach the spawning grounds.

The central question is not whether bycatch, harvest, or another human activity caused the collapse on its own. The better question is:

When ecosystem conditions are already driving high mortality, putting the stock at greater risk, are managers and the fishing sectors doing everything possible to reduce human-caused losses within their control? 

Managers cannot reverse a marine heatwave, rebuild a food web, or guarantee that capelin and other prey will recover. They can ensure the full use of avoidance tools, strengthen monitoring and accountability, reduce incidental mortality where practicable, and base decisions on current abundance and productivity.

The study did not comprehensively assess every gear interaction, habitat effect, indirect food-web consequence, or unobserved source of fishing mortality. These issues should not be presented as proven causes of the collapse without supporting evidence, nor should they be ignored simply because they fell outside the scope of this study.

Science can quantify some pieces of the puzzle with greater confidence than others, but management must consider how those pieces interact. Fish experience the combined effects of ecosystem change and human activity, not pressures isolated in separate reports, agencies, and management processes.

Uncertainty does not prove that an activity is causing harm, nor does it prove that it is harmless. When abundance is historically low, natural mortality has increased, and the consequences of further decline are severe, uncertainty should prompt managers to examine avoidable losses more carefully.

Climate-driven decline must not be used to ignore the human impact, which can be controlled and which can help prevent future mortality. Even if our mortality impact is not the primary cause, it is still part of the problem.

For years, Yukon River communities have faced severe restrictions and closures of subsistence and commercial salmon fisheries. They have lost access to food, income, cultural practices, and opportunities to pass fishing knowledge to younger generations.

That burden raises a question about how conservation responsibility is distributed. When in-river users have lost nearly all directed harvest, every sector that encounters Yukon Chinook should be expected to demonstrate meaningful conservation responsibility.

The new Yukon Chinook research shows that the ecological conditions supporting Alaska’s fisheries are changing. Historical assumptions about survival, productivity, and recovery may no longer hold. Management standards must adapt accordingly.

A mortality limit set during a more productive period may not offer the same protection when abundance is at record lows and natural mortality has increased. That does not automatically mean every existing limit is inadequate. It means managers should assess whether current bycatch limits, avoidance requirements, harvest controls, monitoring systems, and conservation measures remain appropriate for the ecosystem we have now.

We cannot control the temperature of the Bering Sea from one season to the next. We can manage avoidable fishing mortality, strengthen monitoring and accountability, improve conservation measures, and distribute responsibility.

When the ecosystem is taking more, we must be even more disciplined in preventing losses.

Complexity requires us to address every meaningful aspect of the problem, especially those within our control.

Photo courtesy of G. Winston