Flow-mediated effects on travel time, routing, and survival of juvenile Chinook salmon in a spatially complex, tidally forced river delta

We evaluated the interacting influences of river flows and tides on travel time, routing, and survival of juvenile late-fall Chinook salmon (Oncorhynchus tshawytscha) migrating through the Sacramento–San Joaquin River Delta. To quantify these effects, we jointly modeled the travel time, survival, and migration routing in relation to individual time-varying covariates of acoustic-tagged salmon within a Bayesian framework. We used observed arrival times for detected individuals and imputed arrival times for undetected individuals to assign covariate values in each reach. We found travel time was inversely related to river inflow in all reaches, yet survival was positively related to inflow only in reaches that transitioned from bidirectional tidal flows to unidirectional flow with increasing inflows. We also found that the probability of fish entering the interior Delta, a low-survival reach, declined as inflow increased. Our study illustrates how river inflows interact with tides to influence fish survival during the critical transition between freshwater and ocean environments. Furthermore, our analytical framework introduces new techniques to integrate formally over missing covariate values to quantify effects of time-varying covariates.