The composition and development of the parasite community of overwintering young-of-the-year (YOY) roach (Rutilus rutilus) was studied in outdoor artificial ponds. Cumulative mortality of the roach from November until May was about 20%. The prevalences of the epizoic ciliates Trichodina sp. and Apiosoma sp., and of the monogeneans Dactylogyrus suecicus, Dactylogyrus nanus, and Gyrodactylus sp. in general tended to peak in the beginning of April at a water temperature of about 8 C. However, mean parasite intensity remained constant over this interval. Prevalence of metacercariae of the digeneans Diplostomum cf. spathaceum and Tylodelphys clavata remained constant with values around 60% and 10%, respectively. Larvae of the nematode Philometra obturans and the cestode Ligula intestinalis and females of the copepod Neoergasilus japonicus were found sporadically. A concept of 2 thresholds, a critical condition factor (attained by a temperature-induced energy deficiency), and a condition-dependent critical infection intensity, which determine winter mortality of YOY, is suggested.
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