Climate change can have significant impacts on native ecosystems and therefore many studies have attempted to project future population dynamics. However, most studies have considered only a single species, although population dynamics will be affected by environment-dependent species interactions. Here, I analyzed two-decadal population dynamics data of a stream salmonid, in relation to global warming and a competitive species. Results indicate that temperature warming increases the competitive species, which could potentially decreases the population size of the focal species. At the same time, temperature warming also has a positive effect, probably via increased growth rate. This study demonstrated the complex outcome of climate change and also the importance of considering competitive species when projecting future population dynamics.