Fish Assemblages of the ‘Alte Donau’ System: Communities Under Various Pressures

Abstract

'Alte Donau' was formerly a major arm of the braided Danube in Vienna and served as habitat for a cold-water river fish assemblage. After river regulation, the habitat types for fish changed towards a warm-water fish environment with high temperature, high nutrient content and reasonable macrophyte stands. Accordingly, the fish species and biomasses changed in the direction of backwater fish compositions. Currently, 22 fish species are recorded. In the early 1990ies, the macrophyte stands collapsed and heavy blooms of algae and cyanobacteria produced very instable habitat conditions, particularly in summer, close to oxygen depletion, and fish kills. A main nutrient source for algal growth, phosphorus, was removed from the water column by an Iron-III-Chloride treatment, and the fish community responded by consuming food particles from benthic and surface areas instead of the missing plankton. Despite these ecosystem changes, no increased mortality in the fish community was observed during this period. With the intention of keeping high transparency in the water column, a bio-manipulation experiment was introduced by increasing predators via stocking to reduce plankton feeding Bleak. The species Asp was the most effective predator (consuming up to 5.8 individuals of bleak/day), followed by Pike and Pike-Perch. The water transparency -expressed as Secchi depth -increased by up to 15 centimeters in the experimental area 'Kaiserwasser', compared to the control sections. Most of the fish species occurring in the Alte Donau are connected to macrophyte stands, at least in some stages of their life span. Denser water plant communities (expressed in CKI -Cumulative Kohler Index) are not only preferred as habitats by young of the year-class fishes, fish larvae and foragers, but are also used as spawning grounds. There is a clear correlation between the density of plants and fish in the horizontal distribution across the water body of the Alte Donau. Furthermore, the vertical distribution of fish was also associated with plant stock density. Management of macrophytes is done regularly through mowing activities and plant harvesting. It is important to note that young fish should have the opportunity to escape from a dense macrophyte canopy before the plant biomass is removed from the water column. Small boats with mowing bars and separated macrophyte harvesting devices exert clearly less pressure on accidentally removed individuals out of the living habitat (31.3 kg of harvested macrophytes with 1 removed fish - immediate convey belt harvesting after mowing remove 1 fish every 2.1 kg).