Using Flow Events to Predict Chinook Salmon Emigration Patterns

Several fish monitoring sites are established in the Sacramento-San Joaquin Delta to help understand population dynamics of emigrating Chinook salmon juveniles.  Emigration patterns of endangered winter-run Chinook and of fall-run Chinook were examined over several years to determine flow based relationships.  Sampling from a key site on the Sacramento River located upstream of the Delta show that winter-run sampling peaks occur during flow pulses in the fall/winter and may be useful to predict when a large segment of the population is migrating into the Delta.  This information is valuable in balancing water operations with fish protection in the highly managed water ways.  Ten years of data were examined to determine what triggers large and abrupt population movements that vary within a 4 month window in any given year.  Base, peak, magnitude and flow ratio were compared against cumulative population movement recorded during each winter-run emigration season.  Three recent years were analyzed to determine if the flow based population trends corroborate the finding of the previous ten years.

Fall-run Chinook have been known to use the estuaries for rearing as fry (<70mm) which is not common in the other Chinook salmon runs of the Central Valley of California.  It has been observed that in the wetter winters when there is increased Delta outflow during January and March the fry can be sampled by beach seines in San Pablo Bay.  Fry estuary rearing is beneficial to the fall run population as a whole since it increases the life history diversification and recent studies have shown that fry migrants contribute significantly to adult escapement.   Several years were examined to determine the potential effects on this life history strategy by analyzing Delta outflow and Delta Export to Inflow ratios during January through March.  It became evident that the fry estuary strategy has declined in recent years mainly due to drier hydrology likely exacerbated by constant or increasing export demand which influences emigration route selection and survival.  Delta outflow has an effect on the salinity gradient in brackish water of the estuaries and may provide insight onto what flow regime enhances suitable habitat conditions for fry migrants.  Comparing the most recent Delta outflow to the previous 13 years should provide more information on what flow thresholds are important for predicting fry survival to and use of the San Francisco Bay estuary.