Diverse responses of planktonic crustaceans to fish predation by shifts in depth selection and size at maturity

We investigated the behaviour and life histories of large zooplankton in the Esch-sur-Sûre reservoir (Luxembourg). We found that the decrease in size at maturity, as well as diurnal refuge in deep waters, were the adaptive responses (concurrently or alternatively) adopted by large zooplankters to cope with the increasing predation risk throughout the summer. Daphnia galeata initiated a more severe trade-off to cope with high summer predation relative to other cladoceran species. Diaphanosoma brachyurum and Daphnia cucullata seem less susceptible to fish predation as indicated by the low alteration of their size structure and their vertical distribution. The copepod Eudiaptomus gracilis took refuge in deep waters and achieved important modifications of its mature size.[PUBLICATION ABSTRACT]