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

Vida, A.: Threatened fishes of the Szigetköz

- SAS of Ásvány; - SAS of Bagamér. These channels may function as natural water purification areas for the river in case of active water exchange between the main channel and the side arms. The system is filled up with allochtonous organic and mineral matter during high flood condition. As the flood peak passes and the direct communication disappears the flow velocity and the turbulence of the water decreases. Then, a series of physical, chemical and biological processes will result in a considerable eutrophication process. Because of the difference in the available water supply (i.e. bank filtration) the quality of the individual water bodies can vary in a broad scale. Extreme conditions might occur that only certain fish species can usually tolerate (e.g. Misgurnus fossilis, Carassius carassius). The low-water of winter months is usually followed by medium or high levels due to the snow melting in the Alps (the so-called "icy-flood"). This is the main spawning-season for the species of this area. Most of the fishes of the main channel also spawn in the branch system, except the stenotopic rheophilous salmonids, the Zingel species and Cottus gobio. The growing fry populations can reach the main channel gradually still in juvenile stage. In June or July the so-called "green flood" takes place. This is usually the highest one in Szigetköz. Many isolated water bodies become connected with the branch system only at this time. Certain species, usually characteristic to the main channel (e.g. Barbus barbus, Chondrostoma nasus, Vimba vimba), have been found in the branch system in this period. From September onwards, the water level becomes low or very low, creating separated marginal water bodies again. Rheophilous species may return to the main channel, where the water is clear by now and rich in oxygen. The discrete sections isolated by transverse rock fills in the given side arm apparently look like an "ichthyological paradise". Although the species number is very high (often 30-35) along the short (some 10 meters long) bank, this is not due to the specific habitat. Only a few species (Leuciscus cephalus, Rutilus rutilus, Lota lota, Gymnocephalus baloni, Perca fluviatilis, Proterorhinus marmoratus) are constant fauna-elements of such places. The majority is made up of specimens isolated from the main channel. They just try to manage to survive until the next high water (sometimes half a year), many become weak and often fall prey to predators. Several of these perishing specimens are actually members of protected and/or endangered species, characteristic to the "sub-montane zone" (Leuciscus leuciscus, Vimba vimba, Barbus barbus, Chondrostoma nasus, Alburnoides bipunctatus, Gobio kessleri, Gymnocephalus schraetzer). Special water body types of the branch system are the permanent "inner lakes" of some islands. These communities are similar to the flood free area, although during high water conditions they are connected to the branch system. These water bodies serve as very good spawning sites for many species. The total number of fish species in the branch system is 52. Disconnected backwaters and marshlands of the flood free area Before the great river regulation scheme for flood control and navigation has started in the last century, the side arms of the Danube were margined by extended marshlands and isolated water bodies. Nowadays there remains only a small fraction of this once extended world, and even this will probably be lost rather