The Importance of Phenology and Thermal Exposure to Early Life History Success of Nonnative Smallmouth Bass in the Yellowstone River

Knowledge of potential spread by introduced species is critical to effective management and conservation. The Smallmouth Bass Micropterus dolomieu an example a fish that has been globally, often spreads after introduction, substantial predatory impacts on assemblages. Nonnative in the free-flowing Yellowstone River, Montana, have expanded from warmer, downstream sections river into colder, upstream containing socio-economically valuable trout fisheries. We sought insight mechanisms controlling evaluating whether progressively colder climates physiologically constrained successful recruitment limiting age-0 growth preventing overwinter survival (i.e., population establishment). documented phenology, growth, across temperature gradient leading their extent River. establishment did not appear limited water alone. Age-0 body size at onset winter differ significantly between reaches reaches. Instead, earlier hatch timing exhibited some individuals allowed them experience longer growing seasons than many sections. This counter-intuitive hatching phenology mediated much expected decreases climates. Furthermore, evidence simulations starvation mortality indicated distribution successfully recruited age-1 year-class during four consecutive years. However, were rare or absent throughout uppermost adults, suggesting something other limits discourages reproduction farther upstream. Taken together, our results suggest yet reached thermal limit River future may challenge fisheries managers tasked with coldwater this river.