UC Davis

Salmon populations are declining globally, and there are few examples of successful conservation science culminating in partial or fully-recovered runs. We report on the status of a recovering run of anadromous Chinook salmon (Oncorhynchus tshawytscha) in Lower Putah Creek, California. Following environmental litigation in 2000, changes to flow management, in addition to restoration initiatives, habitat conditions became suitable again for Central Valley fall-run Chinook salmon. Suitability was confirmed in this study based on annual return of spawning adults and juvenile production in Lower Putah Creek over a six-year period. Yet despite initial optimism, we identify several factors impeding a more full recovery of salmon in the watershed. Bottlenecks include fish passage barriers that influence timing of adult up-river migrations and juvenile smolt outmigration survival. Additionally, a mass mortality event (fish kill), including pre-spawn adult salmon during fall 2021 demonstrated critical water quality issues that will be increasingly amplified under projected climate change conditions. Thus, while carcass surveys and downstream migrant trapping suggest past restoration actions have generally improved habitat for native fishes including salmon, additional management is still needed. Similar to other long-term studies of restoration, this work also demonstrates that ecological recovery in freshwater ecosystems is frequently slow, non-linear and incomplete. A more robust recovery of salmon in Putah Creek, and similarly reconciled ecosystems, will require science, long-term data collection and broad collaborations. Nevertheless, Putah Creek remains an ecological bright spot, and thus an example of native salmon recovery potential, especially in California and the Pacific Coast.