Natural and Hatchery Juvenile River Outmigration and the Influence of Biological and Environmental Factors

The California Central Valley Chinook fall run exhibits high life history diversity with some juveniles rearing in river habitats until smoltification. Others leave the rivers earlier, at a smaller size and unable to tolerate salt water, and continue to rear in alternative habitats like floodplains, deltas, and bays. In contrast, hatcheries release juveniles with minimum variability; juveniles are typically large, have completed smoltification, and are released during a narrow window of time. A better understanding is needed of how natural and hatchery juveniles contribute to the population and if their contributions are related to their outmigration strategies and the associated biological and environmental conditions during that time. Our objective was to determine the important factors influencing natural juvenile outmigration timing and hatchery ocean entry timing and condition using statistical (GLMM) and mechanistic (DEB) models. Natural outmigration was influenced by local temperature and flow conditions, which were not considered in selecting where and when to release hatchery juveniles, though they impacted condition at ocean entry. Early river outmigrants can contribute a large fraction of the returning spawners. Thus it is important to include these strategies in life cycle models, which can easily be done with the statistical models developed here.