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Changes in infection and distribution of parasites in fish of different length (age) were analyzed. It was found that most parasites colonized fish during the first weeks of their sojourn in the ponds, but only some of these species remained in fish in the second and third years of their life. Protozoans appeared to be mainly associated with fry; the occurrence of some dactylogyrids decreased with increasing fish length, others showed the opposite tendency; ectoparasitic crustaceans and leeches showed preference to bigger fish. The probable reasons for these phenomena are discussed.
The occurrence and reproductive activity of 10 monogeneans, parasitizing four herbivorous fish (common, grass, silver and big head carp) in pond culture were analyzed in relation to three seasons of the year (spring, summer, autumn). The population dynamics of autochthonous species showed the features characteristic for other European monogeneans: a high level of frequency and reproductive ability in the spring, probably connected with a distinct rise of ambient temperature in this season. Among acclimatized species, Dactylogyrus lamellatus and D. nobilis seem to be well adapted to the temperate climatic conditions, manifesting a high level of occurrence and reproductive activity at the beginning of spring. The occurrence of D. aristichthys, D. hypophthalmichthys and D. suchengtaii seem to retain the behavioural features connected with the climatic conditions of the region of their provenance. They probably do not reproduce in colder seasons, and develop better in the higher temperatures of summer.
Seasonal changes in prevalence and mean intensity of several parasite species representing four systematic classes, and occurring in four herbivorous fish (common, grass, silver and big head carp) in pond culture were analysed. Frequency of metacercariae of Diplostomum sp. appeared not to be dependent on the season of the year. The seasonal occurrence patterns of two cestode species, Khawia sinensis and Bothrio-cephalus acheilognathi, are described with regard to the process of recruitment of larvae by fish, their maturation and removal of old specimens from fish. Two species of Crustacea (Ergasilus sieboldi and Argulus foliaceus) and two species of Hirudinea (Piscicola geometra and P. pojmanskae) appeared to be most frequent on fish in autumn.
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