Studies on the Structure of Benthic Fish Assemblages With an Electrified Benthic Trawl in the River Danube Between Calarasi and Braila, Romania

 Zoltán Szalóky1, Ágnes Irma György1, József Szekeres1, István Falka2, Béla Csányi1



1 VITUKI – Hydrobiological Laboratory, Budapest, June 2012

2 National Institute for Research and Development in Enviromental Protection, Bucharest, Romania




The aim of this work is to collect baseline deepwater fish data of the three river stretches within the Lower Danube, between Calarasi and Braila (375 - 175 rkm), with the objective to assess the possible influence of planned navigation improvement works on the environment. We applied new methodology of fish sampling within the deeper part of the river. Thus, this work could contribute to the development of more effective large river sampling procedures. The assessment of the sites is based on the analysis of 29 fish species and 1498 individuals. There were no significant differences between the fish assemblages of the sampled sites. The most abundant and most frequent species were found to be white-finned gudgeon (R. albipinnatus) and streber (Z. streber). According to our results, increasing water depth and distance from shoreline had a negative impact on the occurrence of the majority of fish species and on the diversity indices, while a diversely structured riverbed had a positive effect on species richness and abundance. Fish size distribution and thus age groups were more diverse in habitats near the shorelines. The conservation value of the investigated area is high, due to the high frequency of protected (CD, local protection and IUCN 2011) species. The protection and maintenance of shoreline habitats is a high priority task designed to ensure the survival of Danubian fish species. Nevertheless, more detailed assessments (including littoral night electrofishing and seasonal trawling) are crucial in obtaining an accurate representation of fish assemblage relative to shoreline diversity, eroded, undercut river banks and underwater root structures.

Keywords: large rivers, Lower Danube, fish sampling methodology, fish assessment, river works, electrofishing, electric benthic fish trawl





The project "Improvement of navigation conditions on the Danube between Calarasi and Braila" aims to ensure a minimum water depth of 2.5 m of the fairway recommended by the Danube Commission during the entire year, including the dry season, within the 375 - 175 rkm section of the River Danube. During the first phase of the project works will only be executed at three critical points at Bala area and Caragheorghe sand, Epurasu Island Area and Caleia Branch (Ostrovul Lupu). The interventions include removing the so-called bottlenecks, obstacles to navigation during low water level by deepening and widening the fairway with the help of dredging and dyking. To explore the impact of the project on biotic and abiotic factors, a monitoring program, „Monitoring of Environmental Impact of the Works for Improvement of the Navigation Conditions on the Danube between Calarasi and Braila, Km 375 – km 175" was assembled. The first objective of the program was to collect baseline data on biotical and non-biotical factors at the critical points.

In regard to the fish fauna, these interventions may primarily affect the rheophilic benthic fish species in the river. However, most of the faunistic surveys of fish conducted in the member states of the European Union are confined to the shallow, littoral areas of non-wadable rivers. The European Standard (CEN 14011) specifies that surveys shall be conducted only during daylight hours, for fish assemblages in the littoral zone. The standard hand-held and boom electrofishing method is suitable only for sampling shallow shoreline areas of a river and is not effective in deeper areas, therefore, this method is not appropriate for representative sampling of benthic fish assemblages.

Until now, assessment of the Danube fish fauna, with special attention to species listed in the Council Directive 92/43/EEC (CD species) and which are protected and rare, has been done mainly by sampling only the shorelines of the river according to the above-mentioned standard method. Since the occurrence of benthic fish species, such as the CD species Zingel streber, Z. zingel and Barbus barbus, is lower during daytime at littoral areas, thus their frequency is often underestimated. To explore the actual distribution and spread of protected and CD species in deeper habitats, a novel method was developed in the Hungarian section of the Danube, which is suitable for sampling the deep areas of such large rivers.

Our aim was to collect baseline deepwater fish data along the representative river stretches at the three critical sections, and to characterize and determine the real structure and the possible differences in fish assemblages between the examined sites. At the same time, this study may contribute to the design of more effective combinations of sampling techniques for large lowland rivers. Investigation of fish fauna and community analyses are a particularly challenging task in large rivers (Nelva et al., 1979, Persat and Copp 1990, Cowx et al., 2001). Although fish sampling and data interpretation have been discussed for a long time (e.g. see Tomlinson, 1971, Lagler 1971, Holčik 1972, Ricker 1975, Hartley 1975, Hamley 1975, Weiss 1976, Persat and Cop 1990), which resulted in the development of standardized procedure (CEN Standard 2003), there is still no widely accepted combination of sampling procedures for large rivers. There is no single method to sample and study shoreline habitats and mid-channel deep sections with similar efficiency, which is of particular importance in the case of large rivers.



Materials and Methods

The fish sampling was conducted by means of a self-developed, electrified frame trawl. The method's main concept is to collect the fish stunned by the electric field near the bottom by the trawl. The usage of the electrified trawl is presented in Figure 1.

We examined three sites where constructions are planned, one near Călăraşi another near Gropeni in the River Danube and the third in the Bala-sidearm. The trawling was carried out in the deep waters (>2 m), at 29 sections during June and July 2011 (Table 1.). An 100 x 200 cm inox frame trawl was drawn by a motor boat while an electric field was generated regularly at the mouth of the net. The anode was the frame itself while the cathode was fixed to the pulling rope in front of the frame. The surveys were conducted by a Hans Grassl 65II GI type electrofisher with direct current. The abiotic attributes of fish habitats (water depth, water temperature, weather characteristics, riverbed material) and the operating parameters of the device were recorded. The fished sampling units were 500 m long; each was delineated using a GPS unit and presented on maps (Figure 2-4.). Captured fish were identified, counted by species and measured to the nearest millimeter standard lengths (L).

We applied discriminant and variance analysis to find the differences in the compositions of fish assemblages of each studied section. The analysis was done by Statistica 10. The level of sampling effort was tested by a rarefaction curve, where the cumulative number of species was plotted against the cumulative number of sampling units. Relative abundance and frequency of occurrence data were log10(x) transformed prior analysis. Plotting was done by Grapher 8. Correlation between some environmental variables and species habitat use, as well as, indices of species diversity (Shannon, Simpson, Evenness, Margalef, Equitability) was examined by Spearman Rank correlation analysis. Environmental variables were average water depth (DW), standard deviation of water depth (SDDw) and the distance from shoreline (DS). The relationship between the size distribution of frequent species and DS was plotted on Box Plot charts. For the details of described statistical procedures see McCune and Grace (2002), Karadžić et al. (1999), Lepš and Šmilauer (2003) and Karadžić and Marinković (2009).



Figure 1: Electrified frame trawl in the Danube