SPATIAL AND SEASONAL CHARACTERISTICS OF O-NASUS IN THE RIVER DANUBE, AUSTRIA( FISH NURSERY HABITATS OF NASE, CHONDROSTOMA)

The influence of the hydrological regime (water level fluctuations, cu rrent conditions), temperature and structural components (substratum, flooded vegetation, woody debris) on the distribution of 0+ nase, Chon drostoma nasus, in three nursery habitats (two gravel banks and a shel tered bay: Gb 1, Gb 2 and Bay, respectively) were analysed applying a systematic sampling design. Sampling was carried out from May to Augus t in weekly (May - July) to monthly (July - August) intervalls in 1994 . Detailed contour maps of each habitat were made by professional surv eying. This data set formed the basis for further analysis with a Geog raphical Information System (GIS). At each site, water current, depth, temperature and fish density were measured at three points perpendicu lar to the bank in 5 m intervalls along 100 m of shoreline. With 1508 points collected in total, substratum, type and density of flooded veg etation, algae, woody debris, canopy were estimated at each sampling p oint. Main channel discharge influenced velocity, depth and temperatur e conditions in each habitat. Fish density, species number and species diversity were distinctly higher at the sheltered Bay compared to the two gravel banks. Within each habitat, nase selected sites with low w ater current, which was the overwhelming controlling factor for the di stribution and occurrence of this species throughout the investigation period: 70% of the total catch was in the velocity range between 1 to 10 cm s(-1).