No abstract

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

-Performance of the prescribed gear of the European Standard (i.e. 1.5 m deep Nordic gillnet) was compared with gillnets deep enough to cover the whole water column (WWCG) for characterizing fish assemblages in a large and shallow lake (Lake Balaton, Hungary). Differences in species number, relative abundance and biomass, and length distribution of fish were examined in both inshore and offshore habitats of the lake. Nordic gillnet samples provided comparable information to that of WWCG for both species richness and abundance and biomass of benthic species. However, substantial differences were found between the two net types in the representation of pelagic fish species (i.e. bleak, razor fish and asp). Overall, the WWCG provided different information with more precise (less variable) data on fish assemblage structure than the Nordic gillnet. Results suggest that identification of neither horizontal nor vertical gradients in within lake fish assemblage structure can be guaranteed with the Nordic gillnet alone even in such shallow lakes, like Lake Balaton. It was concluded that benthic Nordic gillnet samples should be complemented with information on fish occurring in the upper water layers (i.e. pelagic species) even in very shallow lakes either by using pelagic or floating gillnets or by whole water column gillnetting.