UC Davis

AbstractCalifornia’s Chinook salmon populations (Oncorhynchus tshawytscha) are undergoing severe and widespread population declines and extirpations, and examples of recovered or restored wild runs of salmon in California are rare. Most salmon returning to California streams are of hatchery origin and have largely replaced salmon of natural origin. I provide a case history for the dynamics of a Chinook salmon population currently recovering in a dam-controlled stream. Following implementation of a functional flow regime and other restoration actions, spawning Chinook salmon returned to Lower Putah Creek, a tributary of the Sacramento River, for the first time since the waterway was dammed in 1958. The return of Chinook salmon to Putah Creek following restoration efforts presents a novel opportunity to understand how salmon may become established in freshwater systems in response to direct restoration actions that enhance habitat suitability, providing insight on population dynamics and recolonization processes in systems of restoration and conservation concern. It remains uncertain whether spawning salmon in Putah Creek produce progeny that return to Putah Creek to spawn in subsequent years, and to what degree the yearly spawning cohort is inflated by straying hatchery-origin fish. In this thesis, I used otolith microchemistry and microstructure methods to evaluate natal origin and population age structure across five spawning cohorts of adult Chinook salmon recovered at Putah Creek from 2016 through 2021. Demonstrating the presence of natal-origin fish in Putah Creek spawning cohorts is the first step in the potential development of a self-sustaining and locally-adapted Chinook salmon run in Putah Creek. I found evidence of successful salmon reproduction in Putah Creek and of the presence of natal individuals in spawning cohorts, although hatchery-origin fish were the most abundant origin class in every cohort studied. The proportion of wild-origin and Putah Creek-origin fish increased with successive cohorts, and the population structure of the spawning population varied among study years.