Summary Natural diets of tench (Tinca tinca L.), carp (Cyprinus carpio L.) and bigmouth buffalo (Ictiobus cyprinellus Val.) were studied to determine food competition among them in polyculture stocks of two different densities. Tench diet consisted mainly of zooplankton (43.8%) and bottom sediments (21.2%). In contrast, carp diet consisted mainly of bottom items including plant debris and detritus (68.8%), which dominated over zooplankton (19.1%). In bigmouth buffalo, most food bulk comprised zooplankton (80.7%). Thus, the resulting food competition (i.e. similarity) was most pronounced between tench and carp (60.8%) and between tench and bigmouth buffalo (47.4%). When comparing growth performance of the three cultured species, the ratios between stocking and harvesting size were: in normal stocking density (392 kg ha−1) 1.72, 3.67 and 2.13, and in doubled stocking density (777 kg ha−1) 1.07, 2.33 and 2.16 in tench, carp and bigmouth buffalo, respectively.

In November 2001, October and November 2002, the concentrations of Hg, Cd, Pb, Cr, Cu, Zn and Ni were determined in four localities of the lower course of the Jihlava River (Czech Republic). The river water, sediments of the riverbed, zoobenthos and tissues of the fish (gill, gonad, skin, dorsal muscle) were analysed. The chub (Leuciscus cephalus) was used as the ichthyo-indicator. At the same time, the stability of the fish community was evaluated using electrofishing and applying basic ichthyological methods. The work links up with previous investigation of the upper course of the Jihlava River (three localities) carried out by the authors in the same way in 1999. The aim of present study was the completion of previous investigation in the whole longitudinal river profile by monitoring selected heavy metals. The highest Cr (14.290 -77.070 mg/kg), Zn (13.600 -92.930 mg/kg) and Ni concentrations (12.290 -36.680 mg/kg) were found in sediments and their highest loading by all of the monitored metals (without Ni) was detected in the last downstream locality (Pohořelice). Zoobenthos was most contaminated by Zn and Cu (7.480 -62.690 and 8.050 -21.810 mg/kg). In the body tissues of the chubs, the highest concentrations of Cd, Hg, Cu, Zn and Ni were determined in the gills, while Pb was also high in gills but also in gonads, the skin and in the muscle tissue. Concentrations of the analysed metals in the chub muscle were (in mg/kg): Hg 0.040 ± 0.014 -0.133 ± 0.063, Cd 0.003 ± 0.002 -0.010 ± 0.004, Pb 0.045 ± 0.038 -0.768 ± 0.038, Cr 0.046 ± 0.023 -0.106 ± 0.092, Cu 0.203 ± 0.123 -0.634 ± 0.194, Zn 4.25 ± 0.84 -6.69 ± 2.95 and Ni 0.062 ± 0.018 -0.103 ± 0.030. Significant differences in the chub muscle were found concerning Hg, Pb, Cu (P < 0.01) and Cd (P < 0.05). Taking into consideration the actually valid FAO/WHO limits for human consumption of chub muscle from the investigated river section, only Hg (PTWI 2.3 kg in locality 7) and Pb (PTWI 2.0 kg in locality 4) could constitute some risk for human health. The index of diversity of the fish community was 0.825 -2.110, equitability index 0.380 -0.793, abundance 312.6 -2,106.5 fish/ha and biomass 46.7 -451.5 kg/ha. The water quality was characterized as betamesosaprobic (saprobity index 1.77 -2.18). The results of the study reveal detailed ecological data concerning heavy metals contamination of the whole Jihlava River ecosystem (water, sediment, zoobenthos, fish). The outcome of this study extends our knowledge about metals accumulation in different fish tissues (gill, gonad, skin and muscle).

Key-words:Gobiidae, food, lowland reservoir, rip-rap bank, the Dyje RiverThe tubenose goby (Proterorhinus semilunaris) is a gobiid species currently extending its area of distribution in Central Europe. The objective of the study was to evaluate the annual pattern of its feeding habits in the newly colonised habitats of the Mušov reservoir on the Dyje River (the Danube basin, Czech Republic) with respect to natural food resources. In the reservoir, tubenose goby has established a numerous population, densely colonising stony rip-rap banks. Its diet was exclusively of animal origin with significant dominance of and preference for two food items -chironomid (Chironomidae) larvae and waterlouse (Asellus aquaticus), which contributed 40.2 and 27.6%, respectively, to the total food bulk ingested. The index of preponderance for the two items was also very high, amounting to 73.8 and 26.5, respectively. In the annual pattern, a remarkable preference for chironomid larvae was recorded in the summer period whilst waterlouse were consumed predominantly in winter months. The proportion of other food items was rather marginal -only corixids, copepods, ceratopogonids and cladocerans were of certain minor importance with proportions of 5.4, 4.3, 4.1 and 3.9%, respectively. Certain signs of cannibalism were also recorded, with 0.9 and 0.2% of the diet consisting of their own progeny and eggs, respectively. RÉSUMÉ Schéma saisonnier de l'alimentation du gobie demi-lune (Proterorhinus semilunaris) non-natif dans le réservoir de plaine Mušov (République tchèque) Mots-clés :Gobiidae, alimentation, réservoir de plaine, berges enrochées, Rivière Dyje Le gobie demi-lune (Proterorhinus semilunaris) est une espèce de gobie augmentant actuellement son aire de distribution à l'Europe Centrale. L'objectif de cette étude était d'évaluer le schéma annuel de l'alimentation dans des habitats nouvellement colonisés du réservoir Mušov sur la rivière Dyje (Bassin du Danube, République tchèque) par rapport aux ressources nutritives naturelles. Dans le réservoir, le gobie demi-lune a développé une importante population colonisant les berges enrochées. Son régime alimentaire est exclusivement d'origine animale avec une dominance significative et une préférence pour deux sources

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The natural hydrobiological conditions of southern Moravian floodplains have been drastically altered by regulation of the lower Dyje river and its tributaries. Natural floodplains were characterized by temporary pools and swamps, each inhabited by a typical community of aquatic invertebrates, and permanent water bodies (rivers and abandoned channels). Some aquatic taxa migrated onto 'terrestrial' localities during flood periods. Following regulation, floods were eliminated and most of the wet floodplain habitats disappeared, together with their typical species. The results of faunistic studies carried out in different permanent and temporary water bodies of the floodplains are reviewed. Altogether 188 taxa of planktonic and 206 taxa of benthic and semiplanktonic animals have been recorded. Of the planktonic community, rotifers (96 taxa) and copepods (41 taxa) have very favourable conditions for their development, especially in the spring. The main season for cladocera (53 taxa) development (summer) is less favourable due to the short duration of flooding (in flooded meadows) or poor oxygen conditions (in temporary pools with a high content of decaying organic matter). Among the zoobenthos, mainly facultative aquatic macroinvertebrates (larvae of insects) appear in these localities.