The Central Valley fall-run Chinook salmon (Oncorhynchus tshawytscha) is the dominant population complex supporting the California and Southern Oregon commercial salmon fishery. The stock is largely dominated by hatchery productionand has shown high variability in adult returns, suggesting that hatchery practices are critical to thelong-term sustainability of the fishery. We compiled information from numerous sources to synthesizetrends in the number, location, size, and timingof fall-run Chinook salmon released from the five Central Valley hatcheries between 1946 and 2012. Approximately 2 billion fish were released duringthis period, nearly half of which were released from the single federally operated hatchery. Juveniles have been planted off-site in the estuary with increasing frequency since the early 1980s, particularly by state-operated hatcheries. Approximately 78% of all releases occurred between January and June, including ~25% in April and ~20% in May. Release timing and size trends differed among hatcheries,and were correlated. For example, the Coleman and Nimbus hatcheries tended to release small fish (<5 g, on average) early in the year, while the Feather, Mokelumne, and Merced hatcheries tendedto release larger fish (>10 g, on average) later in the year. Moreover, sizes-at-release (by month) haveincreased since the 1980s, leading to the emergence of a new life-history type that now comprises nearly all of the estuary releases: springtime releases of large ocean-ready “advanced smolts.” We collapsed release timing and size data into a single index of life-history diversity and our results indicate a reduction in juvenile life-history diversity, with decreased variability in release number, timing, and size through time. Together, these results indicate a reduction in the diversity of life-history types represented in the fall-run Chinook salmon hatchery releases, which may be a factor that contributes to the decreased stability of the Central Valley fall-run Chinook salmon stock complex.

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.