Habitat Use of Juvenile Fish in the Lower Danube and the Danube Delta: Implications for Ecotone Connectivity

Abstract

In the lower Danube and the Danube Delta the juvenile fish assemblage has been surveyed in August 2001. In the main channel of the Danube downstream of the town Galati (Romania) and in the Sfintu Gheorghe branch in the Danube Delta altogether 22 sites were sampled by beach seining and electrofishing respectively, covering nine mesohabitats. Altogether 109 beach seine samples and 318 electrofishing points were taken during daylight, to analyze the distribution and abundance of fish species and ecological guilds compared to regulated rivers, to derive conservation targets for the fish community of Central European rivers. A total of 22 412 fish was caught representing 42 species, five of them non-native. Bleak and monkey goby were most abundant and frequent, contributing together 61% to the total catch. The highest fish densities were observed in connected side waters, but substantial amounts of phytophils in the main channel and regular distributions of rheophils and lithophils between main channel and side waters indicated a high ecotone connectivity. Large woody debris was identified as most important habitat structure for riverine cyprinids, whilst its replacement by riprap favors gobiids. Accordingly, habitat alterations and embankments are prerequisites for gobiid fish to invade Central European rivers. River rehabilitation and mitigation measures should provide large woody debris as most important natural in-channel structure and improve the lateral connectivity of main channels to ecotone complexity beneficial for fish.