Short-term effects of inshore restoration measures on early stages,benthic species,and the sublittoral fish assemblage in a large river (Danube,Austria)

The rip-rap along an approx. 3-km-long shore in the main channel of the River Danube in Austria was completely removed to initiate the formation of a river bank through natural erosional and depositional processes. This study aimed to determine the short-term effect of this restoration measure on species composition and abundance of different developmental stages of fish at two spatial scales (micro- and mesohabitat). For this purpose, changes in abundance of early stages, as well as assemblage structure and species diversity of the benthic and the sub-littoral fish community were studied before and after restoration. No significant effects of restoration measures on the benthic fish assemblage were found. Significant and contradictory effects of the measures on the early stages and on the sub-littoral assemblage were observed. The abundance of fish larvae decreased after restoration, and this change could be attributed to the generally higher flow velocities in the new inshore microhabitats. Simultaneously, the species number and the abundance of fishes from the sub-littoral assemblage increased after restoration. This study has found that the main channel of the Danube still contain a high fish species diversity, and potentially plays a crucial role in recruitment of characteristic fluvial fishes.