POPULATION-DYNAMICS AND PRODUCTION OF DREISSENA-POLYMORPHA (PALLAS) IN RIVER SEERHEIN, THE OUTLET OF LAKE CONSTANCE (OBERSEE)

Population dynamics and production of Dreissena polymorpha (Pallas) we re studied in River Seerhein, the outlet of the upper basin (Obersee) of Lake Constance. Mean abundance was 60,000 animals . m-2 and mean bi omass was 9.9 kg fresh weight . m-2 in October 1988. More than 90 % of the standing crop were harvested by wintering waterfowl, mainly Tufte d Ducks (Aythya fuligula) and Pochards (Aythya ferina). Biomass had de creased to 0.35 kg fresh weight . m-2, and abundance amounted to 5,000 m-2 in spring 1989. Thereafter the population increased by immigratio n of one-year old individuals (1+) and by settlement of larvae (0+) to 60,000 individuals . m-2 in October 1989, corresponding to a biomass of 6.7 kg freshweight - m-2. Both standing crop and production were do minated by the 1 + cohort. Calculated annual production was 138 g C(or g) . m-2 in 1989.