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

of the life history, reproduction and habits of the rare fish species (Eudontomyzon mariae, Abramis ballerus, A. sapa, Gobio albipinnatus, G kessleri, Rutilus pigus virgo, Cobitis aurata, C. taenia, Misgurnus fossilis, Umbra krameri, Gymnocephalus baloni, G. cernuus, G. schraetzer, Zingel zingel, Z. stréber, etc.) is poor. 9. From the fishes of the flood-plain, those that started to hibernate probably became stuck to the isolated, deeper parts of the side-branch systems. For this reason, they could not survive the winter. The level of the remaining water bodies in the flood-plain sank to the level of the ground water, in a period when the species were inactive. Most of them froze or dried. Those that successfully hibernated are in such a bad condition that they can not reproduce in spring, and due to the loss of their habitat they can regenerate only with difficulties. Most of the species of the flood-plain that found their way to the main channel could not hibernate there, their condition deteriorated, the specimens died or were consumed by carnivores. Summarizing, the selected time of the divertion fell into a period critical from the ichthyofaunistical point of view. 10. When the seasonal changes in water level will no more occur in these areas, spawning, fry development and hibernating will be problematic, all these being based on cyclic hydrological conditions. The hibernating, spawning, feeding and fry development sites do not overlap, so the mosaic-like water bodies can no more function. Migratory species (e.g. Vimba vimba), which live in other reaches of the river Danube, regularly spawned in the side-branch systems of the Szigetköz area. So the loss of spawning sites will affect the fish fauna of the lower reaches of the Danube as well. 11. The disconnected backwaters have practically disappeared, together with their fish populations. From the whole of the Szigetköz, the nationally protected Umbra krameri, which is endemic to the Carpathian Basin is only recorded with certainity from this area. 12. Of all the water bodies of the Szigetköz, the Mosoni-Duna is the only one that has slightly been affected because of its connections with the Lajta, Rábca, Rába and Marcal rivers. References Balon, E. (1964): Verzeichnis und ökologische Charakteristik der Fische der Donau. - Hydrobiologia 24: 441-451. Bankovics, A, (1989): Halak [Fishes] - In: Rakonczay, Z. (ed.): Vörös Könyv [Red Data Book]. Akadémiai Kiadó, Budapest, pp. 166-170. Banarescu, P. (1964): Pisces-Osteichthyes -In: Fauna Republici populäre Romine, vol. 13, Bucuresti, A.R.P.R., 959 pp. Berinkey, L. (1966): Halak - Pisces. - In: Magyarország Állatvilága (Fauna Hungáriáé), 79, Akadémiai Kiadó, Budapest, 136 pp. Holc'ik, J. (1982): Review and evolution of Hucho (Salmonidae). -Acta Sei Nat Brno 16(3): 1-29. Holíik, J., Basti, I., Ertl, M. & Vranovsky, M. (1981): Hydrobiology and ichthyology of the Czechoslovak Danube in relation to predicted changes after the construction of the Gabeikovo-Nagymaros river barrage system. - Works Lab. Fish Res. Hydrobiol. 3: 19-158. Lelek, A. (1987): Threatened fishes of Europe. - In: The freshwater fishes of Europe, Vol. 9, Aula Verlag, Wiesbaden, 343 pp. MN Elnöki Tanács (1982): A Magyar Népköztársaság Elnöki Tanácsának 1982. évi 4. számú törvényerejű rendelete a természetvédelemről [Law 1982/4 on nature protection and conservation]. Magyar Közlöny, 1982 (14): 165-200.