Estimation of size‐specific mortality rates in zooplankton populations by periodic sampling1

Many populations of small, mobile organisms cannot be analyzed with standard demographic techniques. A method is introduced for estimating patterns of size‐specific mortality for such species from periodic samples. The technique does not require that individuals be marked or recaptured and may be extended to age and other quantitative characters so long as the class distribution of the population and the rate of flux of individuals between classes can be accurately determined. The most serious difficulty in applying the technique seems to be the ability to sample adequately populations that are patchy in space. However, even if the problem of patchiness cannot be eliminated, so long as the size‐frequency distribution can be accurately described, the technique generates the correct pattern of size‐specific mortality and will provide minimum estimates of mortality for the different classes. Preliminary results presented for four species of planktonic cladocerans suggest that more widespread application of the technique may allow an empirical test of the assumptions on which zooplankton community theory is based.