Authors: Lente, Vera; Staszny, Ádám;Weiperth, András;Hegedűs, Anna;Keszte, Szilvia;Urbányi, Béla;Ferincz, Árpád;

The outflow of the natural thermal Lake of Hévíz (Hungary) is habitat of several fish species, with nature conservation relevance. In the past few years, numerous thermophile (tropical originated) fishes were reported from this waterbody, from which two taxa (Parachromis hybrid, Vieja hybrid), characterized with strong, self-sustaining population have been studied. Our aims were (1.) to explore their seasonal dispersal dynamics and (2.) to perform basic population biology (feeding and growth) studies. Our results show that distribution of these species are restricted to the thermal water outflow area during the winter, however they spread rapidly in the spring as the water warms. The Bertalanffy growth equation, compared to other habitats, shows slower growth for both species. The Fulton condition factor values were K=2.23±0.24 for Vieja and K=1.69 ±0.20 for Parachromis. In the dietary analysis, we found that detritus (58.1%) and macrophytes (18.9%) constituted a major proportion of the stomach contents of Viejas hybrides. Regarding Parachromis hybrides, the main food items were macroinvertebrates and fish (31.5%, 30.2% respectively). From our results, it can be assumed that these species have a considerable seasonal impact on the native fauna in the whole watercourse section due to their competitive feeding and predation pressure.