Native Cutthroat Trout Oncorhynchus clarkii spp. populations in the western U.S. have declined in part due to displacement by nonnative trout, but mechanisms to explain displacement vary by species and geography. Using a production framework, we demonstrated allopatric populations of Rio Grande Cutthroat Trout O. c. virginalis (RGCT) exhibited consistently higher biomass and secondary production rates (0.19–0.92 g DM·m-2·yr-1) than sympatric populations with Brown Trout Salmo trutta (0.01–0.05 g DM·m-2·yr-1) and that an interactive temperature effect on RGCT biomass and production was overshadowed by the presence of Brown Trout across cold and warm streams. Interestingly, over half of trout production was fueled by a small group of taxa including Trichoptera (caddisfly) nymphs and Hymenoptera (ants, wasps, and bees), which were actively selected by both trout species. Similar prey preferences and niche widths suggest that RGCT are experiencing competitive exclusion by Brown Trout. These results offer fresh insight into the unique environmental conditions of headwater streams in the Southwest and justify management strategies to segregate Cutthroat Trout and nonnative trout species.