The importance of functioning habitats for the lifecycles of River Danube fish species becomes clearly visible in times when degraded-, impounded-, or channelized stretches prevail, and when current speed reduced sections of the river dominate the fluvial ecosystem. Major segments of today’s rivers are not supporting essential requirements of riverine fish.
We investigated the near-natural fish by-pass system of Vienna/Freudenau for its function as a compensatory fish habitat. The study was conducted continuously over three years - 15 years after construction. The present data show, that the chosen natural like construction of the by-pass system provides similar functioning habitats as natural tributaries do. More than 17,000 fish and a surprisingly high number of 43 species, including several protected- and endangered species, in all life stages - eggs, larvae, juveniles and adults - were captured. Furthermore, the indicator species of the free-flowing Danube, nase and barbel, migrated into the fish pass in very large quantities and successfully spawned before returning. Therefore, the system serves as an important key ecosystem for the conservation of the remaining native fish fauna. The heterogenic habitat configuration provides conditions for all ecological guilds and consequently increases significantly the spotted biodiversity. Finally, approved management tools are discussed