Are brown trout replacing or displacing bull trout populations in a changing climate

Understanding how climate change may facilitate species turnover is an important step in identifying potential conservation strategies. We used data from 33 sites in western Montana to quantify climate associations with native bull trout (Salvelinus confluentus) and non-native brown trout (Salmo trutta) abundance and population growth rates (lambda). We estimated lambda using exponential growth state-space models and delineated study sites based on bull trout use for either spawning and rearing (SR) or foraging, migrating, and overwintering (FMO) habitat. Bull trout abundance was negatively associated with mean August stream temperatures within SR habitat (r = -0.75). Brown trout abundance was generally highest at temperatures between 12 and 14 degrees C. We found bull trout lambda were generally stable at sites with mean August temperature below 10 degrees C but significantly decreasing, rare, or extirpated at 58% of the sites with temperatures exceeding 10 degrees C. Brown trout lambda were highest in SR and sites with temperatures exceeding 12 degrees C. Declining bull trout lambda at sites where brown trout were absent suggest brown trout are likely replacing bull trout in a warming climate.