Annual dynamics of the fish stock in a backwater of the River Dyje

Canalization of the lower reaches of the River Dyje included the isolation of its meanders. One of these meander lakes remained connected at its downstream end with the River Dyje at river kilometre (r. km) 4.44. The meander lake was 500 m long, with a width varying from 30 to 50 m, and a surface area of 1.2-2.0 ha depending on the water level in the river. In 1997-1999 we studied the temporal changes in the species composition of the fish stock, using gill nets and electrofishing. We found 38 fish species in the backwater under study. Cyprinid fishes were most common, especially Rutilus rutilus, Alburnus alburnus, Abramis brama, A. ballerus, Blicca bjoerkna, Carassius auratus. We also caught rare species such as Abramis sapa, Pelecus cultratus and Gymnocephalus schraetser, or species new to the Czech Republic, Sander volgense, Gymnocephalus baloni and Proterorhinus marmoratus. In addition to the cyprinid species mentioned above, we also found Esox lucius, Sander lucioperca, Perca fluviatilis and Gymnocephalus cernuus spawning in the backwater. There was also a permanent population of Cobitis elongatoides. The fish stock in the backwater was highly variable. Species diversity, size structure and abundance varied over the course of the year. It was markedly affected by the water discharge in the River Dyje. The results obtained document the importance of river backwaters for the fish communities inhabiting the river channel.