POPULATION DYNAMICS OF THE FRESHWATER SNAIL BIOMPHALARIA PFEIFFERI IN EASTERN ZAIRE

The population dynamics of the freshwater snail Biomphalariapfeifferi, an intermediate host of the parasitic worm Schistosomamansoni, was studied in the Virunga stream in Eastern Zaïre. Methods are developed to estimate age-specific survivorshipfrom regular quadrat-sampling data without assuming a constantmortality rate, and to approximate age-specific fecundity whenthe fecundity function is known in the laboratory. The population dynamics in the field was found to be very differentfrom that in the laboratory: it is basically discontinuous,with one main generation per year; a massive mortality occursafter hatching; fecundity is reduced considerably; and thereare marked seasonal variations in both survivorship and fecundity.Time-dependent demographic parameters were estimated by monthinstead of by cohort. They show that the environmental conditionsare favourable to an increase of the population only duringa short period of the year. The intrinsic rate of natural increase,r, was found to be well correlated with the changes in egg density. The environmental factors determining the population dynamicsin the Virunga are discussed; current speed seems critical. (Received 4 February 1987;