Eight Decades of Hatchery Salmon Releases in the California Central Valley: Factors Influencing Straying and Resilience

The California Central Valley contains the southernmost native populations of Chinook Salmon Oncorhynchus tshawytscha, which inhabit a highly variable, anthropogenically altered environment. To mitigate habitat loss and support fisheries, millions of fall-run hatchery salmon are released each year, often transported downstream to avoid in-river mortality, with consequences not fully understood. We synthesize historical trends in release location and timing (1941-2017), focusing on outcomes influencing stock resilience, adult straying, and ocean arrival timing. Over time, juveniles have been transported increasing distances from the source hatchery, particularly during droughts. Transport distance was strongly associated with straying rate (averaging 0-9% vs. 7-89% for salmon released on site vs. in the bay upstream of Golden Gate Bridge, respectively), increasing the effects of hatchery releases on natural spawners. Decreasing variation in release location and timing could reduce spatiotemporal buffering, narrowing ocean arrival timings and increasing risk of mismatch with peak prey production. Central Valley salmon epitomize the pervasive challenge of balancing short-term (e.g., abundance) against long-term (e.g., stability) goals.