L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Specziár, A.: Effects of the diversion of the Danube on two ruffe species in the Szigetköz area

MISCELLANEA ZOOLOGICA HUNGARICA Tomus 10. 1995 p. 99-102 Effects of the diversion of the Danube on two ruffe species in the Szigetköz area by A. Specziár (Received May 26, 1995) Abstract: After the diversion of the Danube, the resulting habitat loss and alteration negatively affected the condition of the surviving populations of two ruffe species. The oldest individuals appeared to be most sensitive to these changes. Because of these alterations, the future existence of Gymnoceplialus baloni in the Szigetköz area seems doubtful. Key words: habitat alteration, Gabcikovo Hydropower Dam, Gymnocephalus cernuus, Gymnocephalus baloni, condition, age structure. Introduction The diversion of the River Danube (on October 25, 1992 at Cunovo) resulted in the diminution of fish habitats in the Szigetköz area. The total water surface of the side branches shrunk to 30% of the original value, and the water level sank so strongly that considerable depth remained at very few sites only (e.g. in deep pools behind cross dams). As a consequence, the surviving fish population crammed together in these areas. The inundation that arrived in November 1992 filled up the branch system again, displacing the fish from their réfugia, which, after the inundation had passed, were crammed together in the deeper parts again (also this time greatly reduced in numbers). During winter (1992/93), the population suffered again (specimens of both Gymnocephalus cernuus and G. baloni died in considerable numbers at this occasion). As a result of the inundation in July 1993, and the advancing artificial water recharge, the aquatic habitat expanded again (even though the original parameters [e.g. discharge] could not be restored). Studies dealing with the expected ichthyofaunistic changes were conducted even in the earliest stages of the construction of the Gabcikovo-Nagymaros river barrage system (Holcik et al. 1981, Jancsó & Tóth 1987, Tóth 1983, Vida 1990a, b, 1993a). As the Gabcikovo hydropower dam came into force, the original fish fauna of the Szigetköz area suffered a considerable reduction, which is believed to intensify in the future (Guti 1993, Vida 1993b). We investigated to what extent the surviving populations of two species of ruffe Gymnocephalus cernuus (limnophilous) and G. baloni (rheophilous) - were affected by the above mentioned processes. Material and method Adult specimens collected by electrofishing, lift nets and angling device in the Ásvány branch system of the Szigetköz area (1823 to 1816 rkm) between 1989 and 1994 were used in the study (in total, 151 specimens of G. baloni and 157 specimens of G. cernuus). The sporadical data used for comparison