Winter OBAN: A Bayesian Stage-Structured Life-Cycle Model of Winter-Run Chinook in the Sacramento River, California

Salmonid research in the Bay – Delta has tended to be focused on the controllable freshwater factors that affect salmon run variability, such as flows and diversions, but there has been less emphasis on other sources of variability such as the ocean.  We constructed a life-cycle model of winter-run Chinook that accounted for mortality during all phases of the life-history, that estimated model coefficients in a statistical framework, evaluated covariates that may explain dynamic vital rates, and incorporated uncertainty in the model structure and model coefficients (through Bayesian estimation).  The model is entitled Oncohynchus Bayesian Analysis (OBAN).  Evaluation of multiple anthropogenic and environmental driver variables indicated that temperature in the spawning reaches, minimum flows in the fry rearing reaches, access to Yolo bypass, water exports in the Delta, upwelling dynamics in the Gulf of Farallones, and ocean harvest were able to explain variability in the winter-run Chinook population dynamics.  Evaluation of the impact of the effects indicated that the winter-run abundance was most sensitive to temperatures in the spawning reaches and flows in the fry rearing stage.  The sensitivity of the model was dependent upon the abundance data to which the model was statistically fit.  Adult escapement and juvenile counts at Red Bluff Diversion Dam (RBDD) were used to evaluate the upstream portion of the life-history, whereas the impact of factors occurring after RBDD are less easily identified due to correlation of survival rates in the Delta, Gulf of Farallones, and ocean stages.  Such life history models are critical for understanding the factors that have been associated with changes in population abundance historically; furthermore, they provide an integral linkage to the decision-making process that must recommend actions with a high probability of recovering populations in the future.