Long-term changes in the fish assemblage structure of a shallow eutrophic reservoir (Lake Hídvégi, Hungary), with special reference to the exotic Carassius gibelio.

European reservoirs are considered to be well characterized from many aspects. However, the description of long-term sequence of events in the fish fauna is restricted and mostly limited to deep, oligomictic reservoirs. Lake Hidvegi is a shallow hypertrophic reservoir (average depth = 1.14 m), which started to operate in 1985. The main function of this reservoir has been to retain nutrients carried by the River Zala in order to protect the water quality of Lake Balaton. The changes in the fish fauna of Lake Hidvegi was monitored in selected localities using standard method (electrofishing). Species composition and assemblage structure changed significantly during the study period, between 1986 and 2011. Following the inundation, the rapid expansion of gibel carp (Carassius gibelio BLOCH, 1782) occurred, but after a peak this tendency turned. Surprisingly, based on the among-years variance of the relative abundances, most of the species were not affected seriously by the invasion. By the end of the studied period, the native generalist roach (Rutilus rutilus L. 1758) became the most abundant species, amounting to more than 50% of the stock. By analyzing functional guilds, we identified an increasing trend in omnivore abundance, in parallel with the disappearance of planktivores. Our results suggest that the fish assemblage structure can be considered as metastable, disturbance tolerant and is mostly composed of generalist cyprinid species still after 29 years of operation.