Method-integrated fish assemblage structure at two spatial scales along a free-flowing stretch of the Austrian Danube

Fish assemblages in large rivers are highly diverse communities. However, a single method to describe these complex systems adequately is not available as of yet. In this paper we propose a combination of normalized catch-per-unit-effort data of three fish sampling methods, thus providing a more inclusive representation of the overall assemblage. We investigate fish species occurrence and abundance in a survey at three consecutive sites in the free-flowing main channel of the Austrian Danube east of Vienna. Using boat and wading electrofishing, as well as longline fishing we collected data in four mesohabitats at each site. We analysed richness, assemblage structure and abundance of single species and tested site and mesohabitat as differentiating forces on the fish assemblage structure, applying PERMANOVA and NMDS. The investigated sites are populated by a species-rich fish assemblage, of which we collected a total of 36 species. The composition of the assemblage was rather consistent except for the occasional occurrence of rare species. At the smaller mesohabitat scale, however, the assemblages differed considerably and hydromorphological conditions were apparently the major structuring force. Overall, the combination of methods considerably improves the assessment across sites and mesohabitats and contributes to our understanding of large river fish assemblages.