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[Lernaea cyprinacea (Copepoda, Lernaeidae) from crucian carp Carassius carassius from the Dlinnoje Lake in the middle stream of the Vychegda River].

Till 1996 the Vychegda River's flood lakes at the territory of the biological station of the Syktyvkar University were overflowed every spring by water so that fishing took place from time to time. In 1996 a low flood level and intensive fishing resulted in the change of the infection rate of crucians by Lernaea, as well as in the change of the size and sex ratios of the crucian population in the Dlinnoje Lake. Thus, the size pattern of the crucian population became simpler, and it led to the changes in parasite-host relation. As a result, the intensity of the invasion of crucians by Lernaea cyprinacea have increased and the distribution of the parasite in the host population does not fit by the negative-binomial distribution. Crucian females and males play an equal role in the maintenance of the parasite population. In 1984 crucian females of medium and large body length played the main role in that process. Functions of other host groups (fishes of small body length, males) were different. Up to 2001 a significance of "k"-exponent of the negative-binomial distribution, describing the number of copepods in the mature part of crucian population, changed with a small range that suggests a stability of parasite-host relationship in the investigated crucian groups. We suppose that after a period of an unstable state in 1997-2000 the population of the parasite reached a new stable state.