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

Szalóky, Zoltán and György, Ágnes Irma and Tóth, B. and Sevcsik, A. and Specziár, András and Csányi, Béla and Erős, Tibor (2014) Application of an electrified benthic frame trawl for sampling fish in a very large European river (the Danube River) – Is offshore monitoring necessary? FISHERIES RESEARCH, 151. pp. 12-19. ISSN 0165-7836

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Abstract

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.

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