Abstract
The introduction and spread of alien fish species pose a major threat to native communities and ecosystem functioning in freshwaters. Black bullhead is one of the most successful invaders in European waters with several detrimental effects on native biota and ecosystems. In this study, we used stable isotope analysis to compare the body size and season-dependent diet, trophic position, isotopic niche size, and niche overlap of the invasive black bullhead with two native fish species (roach and European perch) in Lake Balaton, Hungary. We found that black bullhead could be characterized by invertivore-piscivorous feeding habit with a high rate of fish consumption. The rate of fish predation by invasive black bullhead increased with body size, while no seasonal differences were observed in fish consumption. Contrary to our hypothesis, little evidence of actual feeding competition was found between black bullhead and native fishes. Our results suggest that the studied species assimilate distinct energy resources in different proportions leading to a substantial amount of niche partitioning among them. We conclude that black bullhead may represent a threat for native, small-sized fishes primarily through predation and recommend urgent management actions (e.g. selective removal of the species) to minimize its adverse impacts on native communities.