As part of the LIFE+ project Traisen, the lower reaches of the river Traisen will be completely restored until 2019. Besides of the structural measures, the ecological effects of the project must be assessed. For this purpose, a fishecological assessment before, during and after the construction measures will be implemented. The topics of this master thesis are the evaluation of the ecological status of the river Traisen, as well as the recolonization potential of the rebuilt sections through connected waterbodies. Therefore a fishecological assessment via electrofishing took place in late summer 2014. Additionally the results were compared with older assessments. According to the Fish Index Austria, the fishecological condition of the existing river Traisen could only be evaluated with a “moderate”, “poor” and in one section even a “bad” -status. Therefore the condition misses the objectives of the EU Water Framework Directive in every section. According to the Water Framework Directive, a “good” ecological status in every river should be reached. Despite of the numerous morphological deficits, the reproduction potential of some fish species is comparably high, especially in indifferent species. Rheophile species occur, compared to the guiding principle, only in lower abundances. Some species, like Sperlin, Asp, Ide, Dace, Zingel or Danube streber, which reproduction potentials are estimated very low, can migrate from the Danube into the Traisen-system again after the construction works are finished. Other species, which are rarer, like the European mud-minnow, Danubian gudgeon, Balkanian barbel or Riffle dace will probably not settle in the system or it will at least take a very long time. The future situation of the fishcoenosis in the revitalized section of the river Traisen is depending on the structural adjustments as well as on the future residual flow.