Diversity of fish communities in the waters of the Podyji National Park

The River Dyje, and its 43 km long valley, forms the axis of the Podyji National Park, which was proclaimed in 1991. Previously, the course of the Dyje in that part was populated by a fish community of the barbel type, with Barbus barbus, Chondrostoma nasus and Leuciscus cephalus being the dominant species. The construction of the Vranov Dam in 1934, at the upper boundary of the National Park and, later, of the Znojmo Dam (1966) at its lower boundary, resulted in a gradual transformation of the native barbel-type fish community into one of the salmonid type all along the active course of the River Dyje (river km 137.0 to 175.5). Nowadays, the dominant species include Salmo trutta m. fario, Cottus gobio, and numerous Thymallus thymallus. This situation is also apparent in fishermen's catches, which tend to be dominated by salmonids (over 95%). In the Znojmo Dam reservoir, which has flooded the lower section of the Dyje within the Podyji National Park, the dominant species include Rutilus rutilus, Abramis brama, Perca fluviatilis and Gymnocephalus cernuus. In all, 16 fish species have been noted. The fundamental transformation of the native fish community, from the barbel type to a salmonid type, is the consequence of a changed water temperature and discharge regime and fragmentation of the river continuum by the two dams.