In the year 1993, at the beginning of the construction of the hydroelectric power plant Wien-Freudenau, the University of Natural Resources and Life Sciences Vienna was assigned to do a limnological observation of areas which are most likely to be affected by the impacts of the water storage area. The assignment should contain data collections before building (1993) and after completion (1998) of the power plant. The main focus of these investigations should be given to the impacts on the benthic biocenosis, the ecology of fishes and common questions of limnology.This thesis covers the investigation of the area at the edge of the water storage space, the riverine floodplains of Klosterneuburg, in autumn 2005. The riverine floodplains of Klosterneuburg also enclose the fish bypass channel, which connects the mainstream of the Danube and the floodplains of Klosterneuburg. The exact date of investigation was 1st of August until 31st of October 2005. The thesis treats the migration of fishes between the mainstream of the Danube and the floodplains of Klosterneuburg. Finally it gives a comparison to the collected data of the same investigation area and period in the year 2000. During the period of the investigation the total amount of collected fishes was 1322. The evidence of 32 species from 8 families was provided. The most common fish during the whole period was White bream (Blicca björka), followed by Roach (Rutilus rutilus) and Bleak (Alburnus alburnus). The absolute dominating fish guilds were the euryoecious one. As it came up in this research, a lot of juvenile fishes were moving and passing through the fish bypass channel. Hence we can assume that the fish bypass channel is a very useful and necessary instrument to support the interactive movement of fishes between a river and his connected waterbodies and ecosystems. Moreover we can point out the importance of integration of floodplains in riversystems to provide retention- and reproduction areas for fish.