L. Forró szerk.: Miscellanea Zoologica Hungarica 8. 1993 (Budapest, 1993)
Vida, A.: Expected effects of the Gabčikovo river barrage system on the ichthyofauna of the Szigetköz and its values
A general description of the Szigetköz is to be found in Vida (1990a, 1990b, 1993). Material and methods Taking into account the changes observed in the months following the diverting of the Danube (from October 1992 to March 1993), I attempt to predict the expected ichthyofaunistical tendencies. As a basis, the ecological requirements of the individual species were considered (Balon 1964, Bánarescu 1964, Berinkey 1966, Hotók 1982, Lelek 1987, Pintér 1989, Sokolov & Vasilev 1989). Besides, the results of complete ichthyofaunistic surveys of the past ten years (Vida 1990a, 1990b, 1993) and the expected habitat changes were used to make assessments. Lelek's (1989) work was followed in the categorizing of the individual fish species, which is was based on their status on European level. The three time categories used throughout this study (before October 1992, a few months later, and a few years later) refer respectively to the conditions prior the diverting of the Danube, to the few months past since then (from October to March) and to the period after conditions will have become stabilized after these rapid changes. The third category (prognosis) is thought to be valid only when the started processes continue without human interactions intended to reduce their effects. Regarding the Gabiikovo river barrage system, two works deal with the expected ichthyofaunistical changes. In the papers by Tóth (1983) and Holcik et al (1981) the predicted negative effects on the ichthyofauna are also considered. For a qualification of their species lists see Vida (1993). Schiemer & Spindler (1989), like the authors mentioned before, qualify the effects of the Austrian barrages as negative on the fish fauna of the upper Danube-reach, eventhough the ichthyofauna appears to be, based on their investigations, much more poor than that of the Szigetköz. Collectings were made monthly in the period following the diverting of the Danube, the material and methods being similar to those described in Vida (1993). This study deals with all water types found in the Szigetköz, except the Bagamér side arm system, where no changes are expected to occur due to the diverting. Results and discussion The individual water types found in the Szigetköz area have already differently sustained or will sustain injury due to the diverting of the Danube, and are thus discussed separately in this study. The main channel (Danube River) The main channel in the Szigetköz area is of a sub-montane riverine character, with fish species bound to this water type (Barbus barbus, Chondrostoma nasus, Zingel zingel, etc.). Besides these characteristic species, préalpine elements are to be found in great numbers. As this area is part of the migratory route of pontocaspian species, its assemblage contains some rare elements, i.e. Caspialosa kessleri pontica, Coregonus lavaretus and C albula. Thymallus thymallus possibly occurs here as well. Other species forming this assemblage have also a primarily pontocaspian distribution. The elevation is reduced by 19 m in 56 km (40 cm/km) and the drift speed is as high as 2.5 m/s, being unique within Hungary, so numerous rare, rheophilous fish species (Pararutilus frisii meidingeri, Gobio kessleri, Gymnocephalus schraetzer, Zingel stréber) occur here. Taking these facts into consideration, it is easy to understand, that on this 60 km long reach the number of fish species (n=57) was extremely high before the Danube divertion. After the divertion, the main channel ennarrowed and lost its contacts with the tidal zone, the littoral zone of which was especially important from the fish biological point of view. In addition, it lost most of its contacts with
