Migration and bioenergetics of juvenile Snake River fall Chinook salmon

Abstract

While general patterns in the juvenile outmigration of Pacific salmon are well known, the proximate mechanisms informing migration in individuals are still poorly understood. This thesis describes a complex of individually-based bioenergetic and migration initiation models and their application to fall Chinook salmon. We used the Wisconsin bioenergetics model combined with PIT tag and temperature data to model the growth of individual fish within the rearing habitat. We then created and tested a series of mechanistic models of migration initiation using individual fish mass and growth efficiency as proximate triggers of migration. We examined the performance of these models using CPUE data and maximum likelihood optimization methods and found that the model with both fish mass and growth efficiency as possible trigger methods performed the best; we refer to this model as the Mass-Growth model. To further test this model, we then created a correlative model of migration initiation which we refer to as the Age-Growth model. We found the predictions of the Mass-Growth and Age-Growth models to be comparable. We then applied the Mass-Growth and bioenergetic models to PIT data from ocean-type and reservoir-type fall Chinook to examine possible triggers that result in the two different life history strategies when fish enter Lower Granite Reservoir. Our models predicted that reservoir-type fish were more likely to initiate migration later and at smaller sizes than ocean-type fish, and that the proximate triggers for migration were more likely to be reversed for reservoir-type fish after they entered Lower Granite Reservoir. We determined that the stratified temperature regime in Lower Granite Reservoir was the primary cause of the reversal of modeled migration triggers; for this reason, we then assert that the cool-water pools in Snake and Columbia River reservoirs provide temperature refuges with favorable growing conditions that result in slower-growing fall Chinook salmon following a reservoir-type life history.