While fish communities have been studied in Czech reservoirs for decades, monitoring has increased since implementation of the EU Water Framework Directive (2000/60/EC). Despite this, many smaller drinking water reservoirs have yet to be surveyed. Between May and July 2016, we undertook a pilot‑study examining the fish communities of five Moravian reservoirs (Ludkovice, Bojkovice, Hubenov, Nová Říše and Landštejn). Fish were sampled using boat electrofishing along the littoral zone and Nordic gill nets in the pelagic zone. Both methods were evaluated separately and combined as CPUE and relative abundance and biomass ( %). Nineteen fish species and one hybrid were recorded, with roach Rutilus rutilus dominating overall by abundance, but carp Cyprinus carpio, pike Esox lucius and wels Silurus glanis dominating by biomass in electrofishing samples. Predatory species represented up to 60 % of biomass in the combined sample, with an F/C index ranging from 0.5–3, implying a high abundance of predators. Electrofishing tended to underestimate the abundance of bleak and ruffe, while gill nets underestimated the biomass of predatory species. In some reservoirs, the abundance of pike and wels was high (though it is difficult to say if these fish were stocked or the result of natural reproduction). In others (e.g. Landštejn), zander Sander lucioperca were not registered by either sampling method, despite intensive stocking. Overall, gill netting alone appears inadequate for gaining a true picture of the fish community in reservoirs and we suggest a combination of electrofishing and gill netting in future.

Ergasilosis is a common parasitic disease of fish caused by species of the family Ergasilidae. During the fish growing season, parasitic abundance (including developmental stages) changes in relation to zooplankton development. In this study, we evaluated the seasonal dynamics of ergasilosis in relation to zooplankton development at two reservoirs (Hubenov, Koryčany) in the Morava River Basin (Czech Republic). Samples of fish and zooplankton were obtained at monthly intervals between April and October 2014. In total, 189 fish of 11 species were caught using electrofishing and seine nets. Overall, epidemiological characteristics were higher in the Hubenov reservoir, which also hosted higher numbers of pelagic Copepods. Hubenov also supported a higher number of predatory fish species, which presumably helped to reduce the number of zooplanktonophagic cyprinids. Our results suggest that each reservoir represents a unique ecosystem with its own pattern of ergasilosis seasonal development. Differences between reservoirs are closely related to zooplankton development, which is in turn affected by the fish stock in each reservoir. As the nauplius and copepod stages of arthropods form part of the zooplankton assemblage, they will be found at highest numbers in reservoirs where predatory fish suppress zooplanktonophagic fish species. We suggest to analyse zooplankton with the determination of the genus Ergasilus. Parasite, ergasilosis, food web, biomanipulation, Hubenov, KoryčanyRepresentatives of the subclass Copepoda play an essential role as part of the food chain in aquatic ecosystems. These parasitic organisms form an integral part of the zooplankton assemblage during their free-living stage and represent a much sought-after food item of zooplanktonophagic fish (Dussart and Defaye 2001). Over the year, both quantitative and qualitative changes occur in zooplankton composition, with seasonal dynamics mainly affected by changes in abiotic and biotic factors, reservoir trophic status and fish predatory pressure (Wolfinbarger 1999;Pichlová and Brandl 2003;Sommer et al. 2012;Dvurechenskaya and Yermolaeva 2014). If predatory fish suppress zooplanktonophagic fish sufficiently, for example, the quantity of zooplankton increases, positively affecting the quality of raw drinking water (Tatrai et al. 2005;Bernes et al. 2015;Jurajda et al. 2016). On the other hand, such a reduction in cyprinids may increase the occurrence of parasites whose development is bound to that of zooplankton as the freeliving nauplius and copepod stages of parasites such as Ergasilus sieboldi Nordmann 1832, for example, form part of the zooplankton assemblage. In the life cycle of E. sieboldi are six free-swimming nauplii stages and five copepodite stages, from which free-swimming adult males and females develop. The development of the parasite usually lasts from 10 to 70 days depending on water temperature. Adult females then attach to the gills of many fish species, feeding on the gill epithelium and blood, eventually impairing the