Salmon survival: Betting on the right fish

Salmon used to be savvy investors. For millennia, they spread their risk: returning upstream to spawn in California rivers at ages 3, 4, 5, even 6. If one year brought drought or brutal ocean conditions, another generation could ensure populations wouldn’t tank. It was a biological hedge fund, diversified across time.

“Just like you don’t want all your stocks in one company … you also don’t want all your individuals to be represented by exactly the same genetic type” or the same age, said Eric Palkovacs, UC Santa Cruz Fisheries Collaborative Program director. 

Palkovacs and scientists from across the state and as far away as Norway published a study tracking the long-term collapse of salmon age diversity, finding that today’s Chinook salmon populations in the Central Valley are all-in on a single bet. Three-year-olds dominate the group, while 5-year-olds are rare and 6-year-olds are mostly absent. The study was focused on the Feather River and its tributary, the Yuba River, but Palkovacs said salmon age structure is a backbone of resilience everywhere, including for the salmon runs in Santa Cruz County waters such as Scott Creek and the San Lorenzo River. The loss of age diversity helps explain why modern salmon runs swing so wildly from abundance to collapse. 

Counting what we’ve lost

Using fish ear bones that grow in rings like trees, researchers can count those rings and age fish harvested in recent decades. Then they compare them with salmon bones recovered from Indigenous archaeological sites on the two rivers, located in the Sierra Nevada and eastern Sacramento Valley. Some of the bones date back as far as 7,000 years, thousands of years before the pyramids of Egypt were built.

“To my knowledge, this is the only dataset that goes back this far,” said Palkovacs. “We know we’ve lost [salmon] age structure in the past 100 years. The question that we were asking with this study is, what about more than a hundred years ago? … What about a thousand years ago, 5,000 years ago, or more?”

Throughout history, Chinook salmon swam back to the streams of their birth at various ages, maintaining a remarkably stable age structure across generations. Even with sustained Indigenous harvest, fish continued to mature and return at different ages — a pattern now confirmed by archaeological evidence. “Indigenous people have a long history of relying on salmon,” said Palkovacs, describing weirs, spears and nets carefully adapted to the rhythms of each river and season.

European colonization began to unravel that balance. Dams blocked rivers, hydraulic mining choked streams with sediment and widespread habitat destruction drove up mortality, especially for big, older fish. For salmon, which spawn once and die, timing is everything. Fish that race back to freshwater at 3 years old have a better shot at slipping past nets, turbines and drought. Over time, those conditions favor early returns and steadily erode the older age classes. 

It leads to a punishing boom-and-bust cycle. A single brutal year of drought, unusually warm ocean waters, poor water quality or other harsh environmental events can wipe out an entire cohort; three years later, almost no fish will return to streams. That instability helped trigger three consecutive years of commercial salmon fishery closures and sharply restricted recreational fishing seasons. It has also shadowed decades of decline in populations that spawn in Santa Cruz County rivers and streams.

Rebuilding the run

The archaeological bones — excavated in partnership with the federally recognized Estom Yumeka Maidu Tribe of the Enterprise Rancheria, based in Oroville — offer a rare deep-time benchmark that is reshaping restoration goals. With a clearer view of what healthy salmon populations once looked like, scientists can now ask, “Have we really restored age structure to what it was before major human impacts … or do we still have a long way to go?” said Palkovacs.

For Jessie Ruiz, a member of the Maidu Tribe, and a coauthor on the study, the findings affirm lived experience and traditional knowledge. Salmon, he said, carry a deep traditional weight for many of California’s indigenous communities. “Salmon is not only a resource of food, but it’s also in our creation stories.”

Ruiz helped oversee the sampling along the Feather and Yuba rivers, building the trust needed for tribal monitors to allow scientists to study fish remains from culturally sensitive sites. These archeological excavations uncovered not only the salmon bones but also artifacts like hearths, house floors, and tools, offering a window into the lives of people who once lived there. He acknowledged a long history of tribal distrust toward government agencies and researchers but said collaboration is improving. “It’s good that tribes team up with archaeologists and scientists,” he said. 

That spirit of cooperation stands in stark contrast to many hatchery models built simply to churn out fish, said Ruiz. But at the Monterey Bay Salmon and Trout Project in Santa Cruz County, Executive Director Ben Harris is taking a different approach. At the Kingfisher Flat Conservation Hatchery, every spawning fish is recorded into a detailed “parentage matrix” — a carefully mapped breeding plan that tracks a fish’s genes and the number of times it is used for spawning. By monitoring each fish’s lineage, the program ensures no single family dominates the population, preserving a wide mix of ages, sizes and life-history strategies. 

Palkovacs advocates combining hatcheries with habitat restoration to protect wild stocks while preserving natural diversity. In collaboration with tribal fisheries, researchers are focused on restoring historical upstream spawning habitats. Palkavocs is putting this approach into practice on the Klamath River, with a UCSC Center for Coastal Climate Resilience project that works to restore habitat after the largest dam removal in U.S. history. This method serves as “a natural way of favoring larger, later-maturing fish,” he said. Reopening access to upstream areas gives these individuals the best chance to reach the small, cold streams at the river’s source, where the conditions are ideal for safe spawning and early growth. 

This careful, deliberate approach highlights a key lesson from the new research: rebuilding salmon isn’t just about boosting numbers. It’s about restoring the full spread of ages that once steadied the runs. Salmon ferry marine nutrients upstream and tie mountain creeks to the Pacific. As Palkovacs put it, “They connect our local place to the broader world.” Without older fish in the mix, populations are effectively betting everything on a single year — and in a climate-whiplash era, that’s a dangerously risky gamble.

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