Relationship between seasonal dynamics in zooplankton density and Ergasilus infection in two reservoirs

Abstract

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