California’s Central Valley is home to four salmon runs, including fall, late-fall, spring, and the endangered winter-run Chinook Salmon Oncorhynchus tshawytscha. The development of California’s water delivery infrastructure affected all Central Valley salmonids, particularly the Sacramento River Winter-run Chinook Salmon (winter-run). Winter-run now exist as a single population, which is constrained to spawn outside of their historic range in the mainstem Sacramento River below Shasta and Keswick Dams. While the historic distribution of winter-run included Battle Creek, a tributary to the Sacramento River downstream of Shasta Dam, they were locally extirpated from this tributary due to migration barriers and habitat degradation associated with hydropower development. An ongoing, multifaceted recovery effort, which includes habitat restoration, fish passage, and reintroduction through captive propagation, has resulted in multiple years of winter-run Chinook Salmon returning to Battle Creek beginning in 2019.
In the current study, we use an Ensemble Random Forest approach to evaluate the relative impacts of several factors on return success and precocious maturation in adult winter-run returning to Battle Creek using genetic tagging and associated hatchery pedigree data. Among the covariates in the model, number of eyed eggs per mm female body length, spawn year and relative fecundity were the most predictive factors (Figure 1). While sperm storage method (fresh versus cryogenic storage) was less predictive, cryogenic storage of sperm had the highest nominal impact and was negatively correlated with return success and positively correlated with precocious maturation. Collectively, the results of this study highlight the demographic success of the Battle Creek reintroduction effort, as well as illuminate critical factors to consider in the adaptive management of winter-run in the Central Valley. More broadly, these conclusions will serve to inform the responsible use of conservation aquaculture as a tool for the reintroduction of extirpated fish populations across the globe.