The California Department of Fish and Wildlife (CDFW) and National Oceanic and Atmospheric Administration (NOAA) Fisheries biologists are pursuing urgent measures this fall to save some of the last remaining Central Valley spring-run Chinook salmon after the numbers returning from the ocean this year fell sharply toward extinction.

Biologists call this year’s sharp decline a “cohort collapse” because so few threatened adult spring-run Chinook salmon returned to the small streams still accessible to them. Mill and Deer Creek — two of the three streams that hold the remaining independent spring-run populations — each saw fewer than 25 returning adults this year. Returns to Butte Creek — the third independent population — were the lowest since 1991 and adults further suffered impacts of a canal failure in the watershed.

“We are running out of options,” said Cathy Marcinkevage, assistant regional administrator for NOAA Fisheries West Coast region. “We want this species to thrive in the wild, but right now we are worried about losing them.”

Central Valley spring-run Chinook salmon typically follow a 3- or 4-year life cycle, a strategy that provides some resilience to catastrophic events occurring to an individual year class. While other year-classes (or cohorts) will return in coming years, the 2019-2022 drought impacted multiple cohorts, increasing risks for extirpation.

Biologists will capture juvenile fish from Mill, Deer and Butte creeks to start a conservation hatchery program that will safeguard the genetic heritage of the species. UC Davis will house the captive broodstock at the University’s Center for Aquatic Biology and Aquaculture (CABA) for the next two years until a longer-term facility is identified.

“These drastically low returns come at a time when we’ve already been taking extreme measures to protect salmon strongholds and eliminate existing barriers keeping them from their historic habitat,” said CDFW Director Charlton H. Bonham. “We’ve got to continue to do everything we can to preserve these iconic fish.”

Conservation hatcheries are vital to the protection and recovery of other highly imperiled salmon stocks, including endangered Sacramento River winter-run Chinook and Central California Coast Coho salmon.

“It’s a privilege to work with this species, and I’m glad we have facilities and expertise that can help,” said Nann Fangue, UC Davis professor of fish physiological ecology and director of CABA. “My staff, the students and our partners are all really dedicated to this work and to the goal of conserving native species.”

The remaining populations of spring-run Chinook are declining more than 10% each year and face high risk of extinction, according to an updated viability analysis(opens in new tab) by NOAA Fisheries’ Southwest Fisheries Science Center. One population initially benefited from strong adult returns in 2021, but more than 90% of the fish died prior to spawning when high stream temperatures exacerbated by thiamine deficiency and wildfires fueled a disease outbreak in 2021.

Central Valley spring-run Chinook also face high risk from climate change, since dams have cut off much of the high-elevation habitat where they once spawned in cold mountain rivers. Their survival in lower elevation habitat often depends on releases of cold water from reservoirs that face competing demands for their limited volume of water.

“These cold water fish need cold water and that is going to become more limited in California’s climate future on the Valley floor,” said Dr. Rachel Johnson, research biologist at NOAA Fisheries’ Southwest Fisheries Science Center and lead author of the spring-run viability analysis. Their survival in the high temperatures of lower elevation habitat often depends on releases of cold water from reservoirs that already face competing demands.

Scientists will strive to maintain the genetic diversity of the species through the hatchery broodstock program. As the instream flow requirements and habitat restoration efforts improve the odds of the fishes’ survival in the wild, biologists could use hatchery offspring to restore genetically diverse and locally adapted populations of spring-run Chinook in California’s rivers.