Landscape Drivers of Life History Diversity within and Among North American Steelhead Populations

Smolting in anadromous salmonids involves a suite of behavioral and physiological metamorphoses that prepare fish for the transition from freshwater to marine life. Variation in the age at which fish smolt is an important component of life history diversity that promotes population resilience by reducing variation in abundance over time. Age and size at smolting are also critical determinants of individual fitness, and may reflect tradeoffs between competing selection pressures in freshwater and marine ecosystems. In this study, we synthesize data from North American populations of steelhead trout Oncorhynchus mykiss to examine landscape drivers of variation in age and size at smolting at geographic, regional, and catchment scales. We develop a predictive model for smolt age and size based on freshwater growth trajectories. This model shows promise for incorporation into individual-based modeling approaches that can evaluate how land management influences habitat structure, individual growth performance, and variation in life history expression.