Apex predators play a key role in ecosystem stability across environments but their numbers in general are decreasing. By contrast, European catfsh (Silurus glanis), the European freshwater apex predator, is on the increase. However, studies concerning apex predators in freshwaters are scarce in comparison to those in terrestrial and marine ecosystems. The present study combines stomach content and stable isotope analyses with diet preferences of catfsh to reveal its impact on the ecosystem since stocking. Catfsh niche width is extremely wide in comparison to the typical model predator, Northern pike (Esox lucius). Catfsh and pike have diferent individual dietary specialization that results in diferent functional roles in coupling or compartmentalizing distinct food webs. The role of both species in the ecosystem is irreplaceable due to multiple predator efects. The impact of catfsh is apparent across the entire aquatic ecosystem, but herbivores are the most afected ecological group. The key feature of catfsh, and probably a common feature of apex predators in general, is utilization of several dietary strategies by individuals within a population: long-term generalism or specialization and also shortterm specialization. Catfsh, similar to other large-bodied apex predators, have two typical features: enormous generalism and adaptability to new prey sources.