L. Forró szerk.: Miscellanea Zoologica Hungarica 13. 2000 (Budapest, 2000)

Spolwind, R., Schludermann, C. , Schuster, A.; Waidbacher, H.: Comparison of fish and amphibian communities in a floodplain system of the rivers Traisen and Danube westwards of Vienna

culture as well as human influence in aquatic systems. All Austrian amphibian species are in the "Red List of Endangered Species" (Grillitsch et al. 1983). Many Austrian fish species are endangered, the present stock development is rather deplorable. About 70% of the Austrian autochthonous fish fauna is on the "Red List of Endangered Species" (Spindler et al. 1997). The major threats to the survival of the indigenous fish fauna are human-caused changes in their habitats, i.e. the water bodies. The main factor for the deplorable situation seems to be the large number of measures of hydraulic engineering and diverse use of water for economical and recreational purposes. Many fish species are extremely sensitive to environmental impacts and are increasingly used as bioindicators (Spindler et al. 1997). Their presence and absence in water bodies as well as community and population parameters of selected species provide information on the ecological status of aquatic systems. Co-existence of amphibian and fish coenoses depends to a high degree on the availability of numerous habitat types and possible habitat segregation (e.g. flooded, shallow areas for amphibian spawning) to avoid interspecific predator-prey interactions (Clausnitzer 1983a, b, Pintar & Spolwind 1998, Spolwind & Pintar 1997a, b). This fact implicates that a simultaneous screening of both groups and an according analysis (mostly multivariate) is a sound way for evaluating the status of an aquatic or semiaquatic system. In this investigation area two significantly different alluvial subsystems are characterised: the upstream Traisen region is a heavily regulated system with degraded forests; water bodies are strictly limited. The downstream region, the Danube-Traisen system, is also regulated, but shows high ground water level with large riverine forests. Both systems differ in density, structure and biocoenotic characterisation of their backwaters (Waidbacher et «/.1998, Schludermann et al. 1998). Materials and method Investigation area The studied region is situated in the inundation area of a potential centennial spate occurrence (HQ 100) at the Traisen from Wilhelmsburg to the mouth into the Danube. In the area downstream of the town of Traismauer inundation areas of the Traisen and the Danube overlap, and characteristics of backwater types differ in stretches of the Traisen. 85 different backwaters were studied, 44 of them are situated in the heavily human-influenced area upstream of Traismauer to Wilhelmsburg, 41 backwaters were found in the more undisturbed region downstream of Traismauer. Characterisation of different types of waterbodies followed Gepp (1985) and Pintar & Spolwind (1998). The Traisen can be characterised as follows: a middle stretch river with stream order 6 (Horton 1945, Strahler 1957) with pluvio-nivale discharge in the transit of hyporhitral to epipotamal. Historically various temporary and permanent backwaters were found (Schludermann 1998); today the Traisen is a heavily modified river, characterised by regulation and residual flow problems. Due to a historical flood event (HQ 100) many backwater systems showed impacts as altered morphology and inputs of drifted fish species. The following types were distinguished: Temporary backwaters: different types of systems that are only temporarily filled with water, backwaters differ in size, choriotope, vegetation and shade. Permanent backwaters: due to their different hydrology we distinguish: — backwaters of first order: backwaters that are permanently connected to a flowing water system, in this case the river Traisen. — backwaters of second order: more isolated backwaters that are in connection with a flow water system at a higher mean water level.