Application of an electrified benthic frame trawl for sampling fish in a very large European river (the Danube River) – Is offshore monitoring necessary?

The organization of fish assemblages in offshore, deep channel habitats is poorly known in very largerivers compared with shoreline, littoral areas. We report on the parameters and testing of an electrifiedbenthic frame trawl (EBFT), developed for monitoring the distribution and abundance of benthic fishesin the Danube River, Hungary. We also compare the results of the benthic main channel survey witha shoreline electrofishing (SE) data set. Altogether 33 species were collected offshore during the 175trawling paths (500 m long each). Both sample based and individual based rarefaction showed that nighttime SE detected significantly more species with increasing sampling effort than day time trawling ofoffshore areas. However, offshore surveys detected sterlet Acipenser ruthenus, which could not be detectedby SE, even using extreme high sampling effort. Offshore trawling also proved the common occurrenceand high abundance of the strictly protected endemic Danube streber Zingel streber in the river, whichproved to be extremely rare in SE catches. The EBFT caught larger/older individuals of many species thanSE, and indicated diverse size/age structure for many species offshore. Our survey revealed that offshoreareas are intensively used by a variety of species, which occur relatively even, but with variable abundancein the Danube River. We suggest that even a relatively small (i.e. 2 m wide 1 m high) EBFT can be a veryuseful device for monitoring offshore fish assemblages in very large rivers and provide important datafor bioassessment and conservation purposes

Effects of a Nuclear Power Plant Warmwater Outflow on Environmental Conditions and Fish Assemblages in a Very Large River (the Danube, Hungary)

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