Due to the increasing globalization and ongoing introduction of exotic species, native species are confronted with competitors with unknown outcomes. Catfish Ictalurus nebulosus Le Sueur, 1819 is native to eastern regions of North America. The species was introduced to some European countries, including Poland, in the 1880s for hobby and farming purposes, and it is currently present in freshwater bodies of almost the whole Europe. In this study, stomach content analyses were used to study food and habitat partitioning between catfish and native fish coexisting in a nutrient-poor lake. The results suggest strong competition between catfish and native perch, resulting in the dominance structure of fish untypical for nutrient-poor lakes. Moreover, catfish, which are opportunistic food generalists, displayed a very narrow food niche and selectively foraged on the most nutritional food items. This may suggest that the foraging opportunity of catfish undoubtedly supports the spreading of this species among various habitats, but catfish may become specialists when colonizing some ecosystems. In the context of the current expansion of catfish towards the eastern periphery of Europe, monitoring of its population sizes, trophic niches and effects on lower trophic levels are essential for the management and mitigation of its potential impacts on invaded freshwater ecosystems in Europe.