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 (λ). We estimated λ 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 °C. We found bull trout λ were generally stable at sites with mean August temperature below 10 °C but significantly decreasing, rare, or extirpated at 58% of the sites with temperatures exceeding 10 °C. Brown trout λ were highest in SR and sites with temperatures exceeding 12 °C. Declining bull trout λ at sites where brown trout were absent suggest brown trout are likely replacing bull trout in a warming climate.

Introduced species can alter the dynamics of food webs, species interactions, populations, and communities and can reduce a system's recreational value. Several reports have documented the establishment of Redside Shiners Richardsonius balteatus in mountain lakes, resulting in declines in sizes and abundance of salmonid species. An illegal bait bucket release introduced Redside Shiners into Green Canyon Lake, Montana, USA, in the 1980s. Recently, local anglers began complaining of decreases in size and catch rates of Westslope Cutthroat Trout (WCT) Oncorhynchus clarkii lewisi. Our first objective was to understand the diet overlap between Redside Shiners and WCT in Green Canyon Lake. Based on δ 15 N and δ 13 C isotopic signatures and diet contents, we found little evidence that WCT were substantially preying upon Redside Shiners. We found moderate diet overlap between WCT and Redside Shiners, with both species predominately feeding on Diptera. For our second objective, we developed a population model to assess potential options for eradicating Redside Shiners. We observed Redside Shiners up to 7 years of age and with a maximum TL of 159 mm, high annual survival (50%), and mature females at age 2. We used an age-structured model to evaluate two hypothetical suppression actions: piscicide application and stocking of Rainbow Trout O. mykiss. Four consecutive piscicide treatments had a 20% probability of eradicating Redside Shiners, increasing to a 100% eradication probability after seven treatments. Bioenergetic simulations indicated that an individual Rainbow Trout (500 g) would consume 1,759 g of Redside Shiners annually. Incorporating this predation mortality and a compensatory response (50% increase in age-1 survival), we estimated that maintained abundances of up to 700 Rainbow Trout would not cause a decline in the Redside Shiner population. Our results suggest that eradication of an introduced prey fish like the Redside Shiner would be difficult and that effective removal will require dedicated effort and resources by managers.The anthropogenic movement of species into novel habitats is a major driver of environmental change across the globe (Vitousek et al. 1997;Kolar and Lodge 2000;Gallardo et al. 2016). In freshwater systems, nonnative fish are often introduced through either authorized or unauthorized release (Rahel 2004;Rahel and Smith 2018;