Estimating the size at maturity in field populations of Daphnia (Cladocera)

1. Two methods for estimating the average body size at the onset of reproduction were applied to populations of Daphnia hyalina. The first,'average maturation size', assesses maturity by the relative length of abdominal processes of Daphnia; the second,'size at first reproduction' (SFR), calculates the size class in which 50% of the maximum proportion of egg-bearing females is reached. 2. The influence of environmental factors on the expression of the difference in length between the first and the second abdominal process was tasted in the laboratory. Neither temperature nor chemical cues from predators affected the difference in length, but the processes were indistinguishable at very low food concentrations. 3. During one season, D. hyalina exhibited pronounced changes in fecundity, proportion of egg-bearing adults and SFR. The pattern obtained by the two methods was similar in many cases, although maturation size estimated by the first method was slightly smaller than SFR obtained by the second method. Neither method gave reliable results during times of starvation (clear-water phase)