Notch J.J., McHuron A.S., Michel C.J., Cordoleani F., Johnson M., Henderson M.J., and Ammann A.J. (in press) Outmigration survival of wild Chinook salmon smolts through the Sacramento River during historic drought and high water conditions. environmental biology of fishes.
Populations of wild spring-run Chinook salmon (Oncorhynchus tshawytscha) in California’s Central Valley, once numbering in the millions, have dramatically declined to record low numbers in recent years. Dam construction, habitat degradation, and altered flow regimes have all contributed to depress populations of spring-run salmon, which currently persist in only a few tributaries to the Sacramento River. Mill Creek (Tehama County) continues to support these threatened fish, and contains some of the most pristine spawning and rearing habitat available in the Central Valley. Despite this pristine habitat, the number of spring-run salmon returning to spawn has declined to record low numbers, and is likely attributed to poor outmigration survival rates. Using miniature acoustic tags, it is now possible to track out-migrating juveniles throughout the freshwater migration corridor. From 2013-2017 we captured and acoustic tagged 334 smolts out-migrating from Mill Creek, tracking their movement and survival rates over 250 kilometers through the Sacramento River. During this study California experienced both an unprecedented drought and record rainfall, resulting in dramatic fluctuations in year-to-year river flows and water temperature. Cumulative survival of tagged smolts from Mill Creek through the Sacramento River was 9.5% (±1.6) during the study, with relatively low survival during historic drought conditions in 2015 (4.9% ± 1.6) followed by increased survival during high flows in 2017 (42.3% ± 9.1). From these data, we modeled survival in Mill Creek and the Sacramento River over a range of flow values, which indicated that higher flows in each region result in increased survival rates. Data gathered in this study can help focus management and restoration actions over a relatively long migration corridor to specific regions of low survival, and provide guidance for management actions in the Sacramento River aimed at restoring populations of threatened Central Valley spring-run Chinook salmon.