Occurrence of Phyllodistomum umblae (Fabricius, 1780) in the ureters of coregonids of Lake Yli-Kitka in northeastern Finland

380 whitefish (Coregonus sp.) and 260 vendace (Coregonus albula L.) were studied from Lake Yli-Kitka in northeastern Finland during 1980-1982. 33% of the whitefish and 38.5% of the vendace were infected with Phyllodistomum umblae (Fabricius, 1780), the average intensity of infection being 4.2 and 2.3 per infected fish, respectively. No clear seasonality was observed in either the prevalence and intensity of infection or in the level of maturation and the length of P. umblae in both fish species, which points to a continuous invasion and maturation of these trematodes. The prevalence and intensity of P. umblae infection was highest in the middle-sized whitefish. Similar results were obtained when the mean number of worms per fish and the level of overdispersion (S2/x) are plotted against the length of the fish. With regard to vendace the prevalence and intensity of infection per infected fish and per fish studied increased as the fish length increased: only the variance-to-mean ratio remained constantly low. Overdispersion indices (S2/x) revealed that in the whitefish P. umblae is more overdispersed than in vendace when studied using data from seasonal periods with size-groups as homogenously infected as possible. The negative binomial distribution gave a good fit in the case of vendace and with one exception also for whitefish, and the values of parameter k are well within the limits most often found in parasitological studies.