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23The organization of fish assemblages in offshore, deep channel habitats is poorly known in 24 very large rivers compared with shoreline, littoral areas. We report on the parameters and 25 testing of an electrified benthic frame trawl (EBFT), developed for monitoring the distribution Zingel streber in the river, which proved to be extremely rare in SE catches. The EBFT 35 caught larger/older individuals of many species than SE, and indicated diverse size/age 36 structure for many species offshore. Our survey revealed that offshore areas are intensively 37 used by a variety of species, which occur relatively evenly, but with variable abundance in the 38 Danube River. We suggest that even a relatively small (i.e. 2 m wide 1 m high) EBFT can be

Matching habitat typology and ecological assemblages can be useful in environmental management. We examined whether a priori defined riverine sections correspond with distinct fish assemblage types along the >2000 km long course of the Danube River, Europe.We also tested whether different sampling methods (i.e. day and night inshore electric fishing and offshore benthic trawling) provide consistent typological results. Analysis of assemblage similarities, indicator species analysis, non-metric multidimensional scaling (NMDS) and kmeans analyses indicated that fish assemblages of the a priori defined Upper-, Middle and Lower-Danubian sections differed slightly, within class variability was high. Although indicator species analysis showed that the Upper-Danube belongs to the barbel (Barbus barbus) zone and the Middle-and Lower Danube belong to the bream (Abramis spp) zone, indicator values of the character species were generally low. The NMDS analyses suggested a weak gradient in assemblage structure along the course of the river with relatively high variability between neighbouring sites. K-means analyses revealed that many sampling sites were in a different class than the a priori defined sections, and classifications at other group numbers did not lead to better classification outcome. Overall, the results do not suggest clearly distinguishable assemblage types with distinct boundaries in the potamal section of a great river. Nevertheless, the division of the potamon to smaller sections may explain some variability in fish assemblage structure, and could be used for bioassessment purposes. The study also shows the importance of multihabitat and multigear surveys in the typological assessment of great rivers.

Hydromorphological features of rivers, such as flow depth, flow velocity, and the composition of bed material play a crucial role in the habitat selection of fish. Although these basic hydromorphological parameters can be determined with high spatial and temporal resolution using state‐of‐the‐art investigation methods, only few studies deal with the connection of habitat parameters and abundance of fishes (i.e., habitat modelling) in large rivers. The aim of this study is to fill this gap by introducing the so‐called habitat maps connecting the results of 3D hydrodynamic simulations and results of offshore fish ecological surveys. The steps of habitat modelling are introduced through the case study of the Danube River focusing on evaluating the habitat use and suitability of two fish species representative to this section of the river (i.e., the round goby Neogobius melanostomus and the Danube streber Zingel streber). A novel method for mapping the bed material composition, playing a crucial role in the habitat assessment, is also presented along the numerical modelling.

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