L. Forró szerk.: Miscellanea Zoologica Hungarica 8. 1993 (Budapest, 1993)

Vida, A.: Threatened fishes of the Szigetköz

soon because of drainage and meliorization. Its place is gradually taken over by intensive agriculture. Backwaters can be found both in the flood plain and the flood free areas. Their water is usually rich in organic matter. Backwaters of the flood free area however, cannot refresh themselves with the flood water. The largest backwaters of the flood free area are the ones near Lipót and Dunaszeg and the so called Zátonyi-Duna. Even these are gradually reduced by the accelerated ecological succession. Exceptions are the water bodies connected to irrigation canals. These sites have been studied thoroughly in the last 2-3 years, resulting in the discovery of Umbra krameri. The dominant accompanying species were Misgurnus fossilis and Carassius carassius. Further members of the fish community are Proterorhinus marmoratus and Tinea tinea. A rare species of such waters, Leucaspius delineatus, was found only twice (1989, by G. Guti, in the Cikola-branch system, and 1991, by A. Vida, in Zátonyi-Duna) Umbra krameri is a strictly protected endemic species of the Danube catchment area. The water-regulations of the last century, the drainage of the marshlands eliminated many populations of this fish, yet it is not as endangered as Hucho h. hucho. It still can be found in many parts of the country, where suitable habitats are available. Therefore, it is the special habitat which should be protected. On the other hand, we do not know yet, how much of its genetic variation has been lost with the decreased population size. Genetic variance is necessary for survival, particularly in a changing environment. Since the European mud-minnow has a very much limited distribution (mainly within the Carpathian Basin), and even here exhibits a disjunct pattern, loss of genetic variance can be fatal for the species. At present this species appears to be not too rare in Szigetköz. After the discovery of the first specimen by Csányi in 1989 (Csányi, pers. comm.) it has turned up almost everywhere in the backwaters of the flood-free area. From the irrigation canals we could find it only once, near Hédervár. It is possible, that the species occurs also in the flood plain waters, although evidence for this is still missing. The total number of fish species in the flood-free area's disconnected backwaters is 23. Irrigation canals and water-supply Regulation of the main channel affected the whole water-system of the Szigetköz. The thick layer of sand and gravel, which is the dominant substratum here, is highly permeable to water. Consequently, during high water in the flood-plain, water will raise in the flood protected area as well. Drainage was therefore constructed in this area resulting in a nearly 300 km long canal-system. Although existing former river beds were also used for the canals, the majority is a regularly cleaned strait-line canal. Therefore fish-faunistically interesting sites can be found only at natural or artificial barriers, such as bridges, trees thrown into the water, sharp turns of the canal, etc. The most threatened species of this area is Umbra krameri. The total number of fish species is 27. Ponds of gravel-pits Gravel (ballast)-digging has long been a tradition in the Szigetköz, because of the good quality of the gravel. The groundwater fills up the abandoned pits sooner