The ecological affinity of fish species to different habitat types in the course of their life cycles is investigated in the free-flowing sections of the Austrian Danube. The results point to the great significance of the degree of hydrological integration between the river and its adjoining waters. In the breeding and fry stage, rheophilic species are bound to the river itself, but the preferred zones show great variations according to both fish age and species. A highly structured shoreline is important in providing a gradient of water current and a spectrum of food sources. Regulated embankments are characterized by an extremely reduced fauna with low population densities of eurytopic species. These results give evidence that the conservation of the characteristic species association is incompatible with the planned hydroelectric power dams in the remaining, free flowing zones of the Danube. The importance of the fish fauna as an indicator group for the condition of large river systems is discussed from the conservation point of view.

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Extended abstractThe hydrological regime of the originally, nonregulated Danube was characterised by a continuous swelling of the river due to snow break in the Alpes in spring time, which resulted in a high-water-period for several weeks . At that time, huge flood areas were inundated along the main channel, which play a major role as spawning areas for phytophilic fish species . The subsequently slow droping of the water levels during the summer months allowed the adults, larvae and juveniles to migrate into the main river or backwaters . Since 1870, this course of natural occurring fluctuations in waterlevels has been considerably altered by man-made regulation and canalization, which lead to a distinct increase of the rapidity of the high water events . For example : during the high water from August 1985, the water level increased within one day from 340 cm up to 780 cm in Vienna, and lasted only for four days . The question was addressed, to which extent the changing hydrological conditions are reflected in year-class strength of the fish populations .A one-year-study on the abundance and agestructure of fishes was carried out in the alluvial zone of the free flowing part of the River Danube downstreams of Vienna (Austria) . Fish were collected using gill nets with mesh sizes 40, 45, and 50 mm as well as threewall-nets with mesh sizes 180/28/180 mm . Scales were taken from each individual fish for age determinations, using the methods described by Bagenal (1980) . Standardisation of data were calculated according to Ricker (1975) . The best representative for this investigations seemed to be the silver bream (Blicca bjoerkna) . This eurytopic, phytophilic species is bound to flooded vegetation in sheltered backwaters for spawning substrates and it is rather common in the backwaters as well as in the main river (Schiemer & Spindler, 1989) . Additionally, the silver bream is very catchable by the fishing gears in use, and it is not attractive to the fisherman . It can be assumed, that exclusively natural causes are reflected in the population structure of this species . Blicca bjoerkna inhabits the whole waterbody, so that random sampling could be presumed .The catch curve from the silver bream is shown in Fig . 1 . The steeply ascending left limb of the curve represent age classes, which are incompletely captured by the methods : i .e. they are taken less frequently in relation to their abundance (Ricker, 1975) . Therefore only the right limb is representative for population structure . This part of the figure shows a rather irregular course, indicating distinct differences in survival rates between the successive age classes. Arrows sign years of apparently higher or lower survival .Year-class strength were considerably lower in the years 1981 and 1985 . In this years, the highest waterlevels were exceeded by floods during the summer months . Though it is obvious, that summer high water events have tremendous effects on year class strength of fishes, as a high portion of the juveniles is killed, because of ...