Succession of fish diversity after reconnecting a large floodplain to the upper Danube River

River floodplains are important for their biodiversity and productivity, yet are considered one of the most threatened ecosystem types. Due to increasing awareness of the ecosystem services provided by floodplains, their restoration and reconnection with the main channels have become core topics in policy, management and science. In one of the last remaining alluvial forests at the upper Danube River, a new secondary floodplain channel was artificially created in 2010 using a nature-orientated construction scheme. The objective of this study was to analyze the colonization and succession of the fish community in the new floodplain river and the adjacent temporarily connected floodplain habitats over a period of four years. Following the restoration-induced changes in habitat morphology and availability of habitat space, a very fast initial colonization was observed, with 46% of the source species pool of the Danube present in the study area after only two months. Colonization by fishes was explained by habitat features such as habitat morphology as well as biological features such as the species inventory of the source habitat and life-history traits. The time span required for developing functional demographic structures could be explained by the degree of specialization in different life stages, with faster population development in species with high ecological plasticity. The high representation of small individuals in multiple fish species indicates the importance of the artificial floodplain channel as habitat for early life stages. Consequently, the new floodplain channel seems to at least partly mitigate the habitat loss in the main river.