Frontiers in Marine Science (Nov 2015)
The electrified benthic frame trawl (EBFT): A new approach for sampling benthic fish species in a large river – The experiences of the Joint Danube Survey 3
Abstract
In large rivers, such as the Danube, standardized and representative sampling of fish is still problematic. Until now, night time shoreline electrofishing proved to be the most comprehensive fish assessment method, however, it is not suitable for detecting sterlet (Acipenser ruthenus), the only native sturgeon species still present in the upper and middle Danube. Besides, other rather rare native bottom dwelling fish species (e.g. Zingel streber) are also usually underrepresented in shoreline catches. Hence, we have developed in 2010 the electrified benthic frame trawl (EBFT) which, due to its robust frame, can overcome bottom structures and can give important results on offshore benthic fish species (i.e. presence-absence, distribution, spawning habitats, etc.). In 2013, this method was adopted by the fish Core Team of the Joint Danube Survey 3, providing an opportunity to test the equipment in diverse conditions along the whole river for the first time. Data were collected parallel with standard electrofishing at 22 sites from river km 2214 (Austria) to river km 15 in the Sulina arm (Romania) sampling 500 m long stretches parallel to the riverbank. We found that offshore areas were intensively used by a variety of species. Most abundant and most frequently occurring were the highly invasive round goby (Neogobius melanostomus, 9 ind/500 m) and the whitefin gudgeon (Romanogobio vladykovi, 4 ind/500 m). The number of Z. strebers caught (138 individuals from 16 sites) was much higher in the EBFT samples than in the shoreline catches (34 individuals from only 6 sites). The EBFT was also successful in catching sterlets but only 9 individuals were caught and only below the inflow of Tisa River. The survey proved the relevance of using the EBFT during large river fish assessments, and it also revealed the alarmingly low abundance of sturgeons. Based on the results further development (e.g. lifting device) and optimization possibilities of the EBFT may be also considered.
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