Four brook-dwelling populations of Ancylus fluviatilis were studied. Data on life-cycle, specimen number, biomass and egg capsule biomass are reported. By means of experimental data on ingestion, assimilation, growth and egg-production, energy balances are calculated for whole populations.Mean annual biomass turned out to be 26.58-64.88 mg C/m2. Mean annual biomass turn-over ratio (production/biomass) was 2.64-5.95. Cohort biomass turnover ratio was 4.11-6.09 for the whole population. Young specimens show considerably higher production efficiencies than adult ones. Production by the smallest size classes is a very considerable part of whole population production. Net production efficiency of the whole population was 11.2-11.8%, gross production efficiency 5.6-6.5%. An important factor for variation between different populations is temperature, but other factors, such as food supply, may be important, too. Experimental production estimates were compaerd with the Allen curve method and the instantaneous growth method.Differences due to different methods of production measurement are discussed. The range of values for different efficiencies lies within the range reported for other freshwater snails. Interpopulation variation in turnover ratios nearly equals variation reported for different species of univoltine snails.

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