Would hydrologic climate changes in Sierra Nevada streams influence trout persistence? Trans

Abstract.—We predicted the consequences of climate change for sympatric populations of brown trout Salmo trutta and rainbow trout Oncorhynchus mykiss in an upstream and a downstream reach of a Sierra Nevada stream with the help of an individual-based trout population model. The model evaluated the ecological effects of two anticipated responses to climate change: (1) a shift in peak flows from spring to winter and (2) an increase in stream temperature. Changes in temperature and flow regime both influenced simulated persistence of the two trout species. We hypothesized a decrease in the fall-spawning brown trout population as a result of winter floods that scour brown trout redds. Although scouring mortality showed the expected pattern, effects of seasonal shifts in flow on simulated dewatering of redds was equally important and tended to compensate for scouring. Because trout are coldwater fishes, we hypothesized that a rise in mean stream temperature would be harmful to both species, particularly in downstream reaches. We found that a climate change scenario with a 28C increase in average stream temperature benefited both species in the cooler upstream reach but was harmful in the warmer downstream reach. Overall, our results supported the hypothesis that climate change will restrict trout to higher elevations in the Sierra Nevada. Finally, the combined effects of elevated temperature and shifted flow differed from th