State-Dependent Life History Models for Steelhead in Multiple Watersheds

Steelhead (Oncorhynchus mykiss) display a wide array of life history variation (including the purely resident form, rainbow trout). I describe combining proximate (physiological) and ultimate (expected reproductive success) considerations to generalize the notion of a threshold size for emigration or maturity through the development of a state-dependent life history model implemented through stochastic dynamic programming.  This modeling framework has been tailored and extended for application to multiple habitats: its first implementation in small coastal creeks of California, an extension to predict the effects of much higher growth rates in large Central Valley rivers, and further modification to account for movement between upstream and downstream habitats within the freshwater phase of the lifecycle.  The first application to coastal streams captured the distribution of life histories among females but did not fully explain individual fish movements. The second application captured regional differences in life histories. The third application indicated that a mortality–growth rate tradeoff can explain much of the life history variation, but this tradeoff alone cannot drive a mixture of habitat use strategies by fish of a similar state (i.e., length), suggesting a consideration of density-dependent effects on the habitat risk-reward tradeoff.