Fish-habitat associations are poorly known in offshore areas of very large rivers. We examined physical habitat structure and its effect on habitat use and assemblage formation of benthic fishes in the main channel of the Danube River, Hungary. Principal component analysis of physical variables showed that sample unit (i.e., 500 m long reaches) and cross-channel transect-level data of corresponding reaches were highly correlated. We found clear gradients in physical variables from areas with high velocity and higher proportion of hard substratum (pebble and cobble) to areas with low velocity, high mean depth and finer substratum (mainly sand) composition. Variation in velocity was coupled with variation in both mean depth and substratum composition (i.e., Shannon diversity of sediment composition) and higher proportion of silt material. Differences in physical habitat structure (flow, substrate) also manifested among river segments. Classification and regression tree analyses (CART) and fish abundanceoccupancy patterns in the PCA template revealed that many species showed clear responses to environmental heterogeneity (barbel, Barbus barbus; schraetser, Gymnocephalus schraetser; Danube streber, Zingel streber; whitefinned gudgeon, Romanogobio vladikovy; round goby, Neogobius melanostomus) while others (white bream, Blicca bjoerkna) showed very elusive habitat use patterns. Multivariate regression tree analysis confirmed the results of CART and indicated that transectlevel substratum composition was the most important determinant in the formation of benthic assemblages. These results on habitat use can contribute to the more effective conservation management of offshore fish assemblages, which is important due to increasing inland navigation in the Danube River.

Computational Fluid Dynamics is an effective tool for assessing non-present conditions, thus also in habitat evaluation within ecohydraulics. Deciding whether to apply a one-, two- or three-dimensional numerical approach, is an optimization that needs to be performed by every task, given the capability and the demands of specific approaches. In this paper we compare the utility of two-dimensional (2D) versus three-dimensional (3D) hydrodynamical simulations for ecohydraulic purposes. The basis of the comparation were 1) three simulated abiotic variables: water depth, depth averaged flow velocity and bed material composition, and 2) an overall performance in a meso-scale fish habitat evaluation, based on the simulated three variables. The biotic parameters for the models were the habitat suitability curves of three fish species, the Danube streber (Zingel streber), the round goby (Neogobius melanostomus) and the white bream (Blicca bjoerkna). We found that in terms of ecohydraulic utilization, the 2D approach performed sufficiently to simulate the hydrodynamics of a large river. The errors originating from the 3D-2D simplification yielded negligible differences in habitat evaluation, and the agreement in the habitat suitability indices calculated from the simulated metrics was satisfactory. Henceforth, the theory was turned into an application as we performed habitat mapping on a 100 km long, Hungarian reach of the Danube River, with the abiotic parameters resulting from a 2D hydrodynamical simulation. The possibility of simplifying the approach from 3D to 2D provides a cost-efficient numerical tool at larger scales for ecohydraulic studies, and especially for evaluating habitat suitability of riverine fish.

Direct or indirect effects of nuclear power plants' (NPPs) warmwater effluents on the structure of biotic assemblages are poorly known in very large rivers. We examined changes in physical habitat structure, temperature condition and their possible effects on the structure of Danubian fish assemblages due to the outflow of the Paks NPP's warmwater channel, in Hungary. Seasonal surveys conducted both upstream and downstream from the outfall showed that its hydromorphological effects were generally local and comparable to natural or other anthropogenic hydromorphological changes. The effect of the returned cooling water was more apparent in the seasonally recorded surface water temperatures and depended highly on the spatial positioning of the sampling sites. However, environmental and spatial variables accounted only for a low amount of variance in case of both shoreline and offshore fish assemblage data. Overall, we found that the outflow exerted only a local scale effect on the structure of Danubian fish assemblages. Rather, fish assemblages varied largely both inshore and offshore, which dynamics overruled any effects of the artificially elevated temperature. Our study highlights the importance of the assessment of hydrogeomorphological variability of rivers and their influence on fish assemblage variability when examining spatial effects of thermal pollution.

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