DACTYLOGYRIASIS IN HYPOPHTHALMICHTHYS MOLITRIX:SEASONAL DYNAMICS AND FREQUENCY DISTRIBUTION OF DACTYLOGYRUS VAGINULATUS ZHANG ET NTU,1966(MONOGENEA)

The seasonal population dynamics of Dactylogyrus vaginulatus Zhang et Niu, 1966, a monogenetic parasite on gills of silver carp (Hypophthalmichthyw molitrix)were studied during the period from 1992 to 1994. After appeared in winter, the population of this parasite increased quickly. In Wuhan, the peak of the prevalence reached 100% and abundance was observed above 370 parasites per fish respectively in April of the following spring. The population then decreased sharply while the water temperature rose up. The prevalence fell to zero in early June,and a single worm of the monogenean could not be found on fish from June to November. The distribution of D. vaginulatus in the host population was aggregated. Heavy infected fish may harbor parasites up to 1728. The intensity was independent of the size of fish (P0.05). Since D. vaginulatus had few effects on the plumpness of the whole population of silver carp (r=-0.204, n=30, P0.05), the major loss caused by the worm appeared due to the death of heavily infected fish.