Multiple defence strategies of <Emphasis Type="Italic">Daphnia</Emphasis><Emphasis Type="Italic">galeata</Emphasis> against predation in a weakly stratified reservoir

In the presence of size-selective fish daphnids were shown to exhibit two alternative inducible defence strategies: They may either escape predation by active migration or adopt a life history strategy, e.g., reproduce earlier and at a smaller size. Depending on the type of habitat, migration may either be vertically (in deep stratified lakes) or horizontally (in shallow lakes with macrophytes) oriented. Concerning behavioural defence strategies, daphnids living in medium-deep, weakly stratified water bodies with a poorly developed littoral face a dilemma, since the littoral provides no shelter and the availability of a deep-water refuge is unpredictable. We studied the population dynamics, life history changes (size at maturity) and daytime vertical distribution of Daphnia galeata in a weakly stratified reservoir in relation to predation by juvenile fish during 6 years. While temperature gradients were usually small, oxygen concentrations suggest that a low-oxygen refuge for daphnids was available in every year to some extent. Our results indicate that, depending on predation intensity and stratification patterns, daphnids exhibit both, behavioural and life history defences. In years with a high biomass of young-of-the-year (YOY) perch Daphnia abundance declined rapidly at the end of the clear water stage while at the same time the vertical distribution at daytime shifted to deep strata providing a low-oxygen refuge and the size at maturity decreased. However, while the life history response in some years lasted throughout most of the summer period, a shift in daytime vertical distribution was exhibited for much shorter periods. Both traits were much less expressed in years with low YOY fish densities and no negative correlation between them could be verified. We suggest that under high predation pressure in this relatively shallow reservoir no strictly alternative (either behavioural or life history) strategies exist, but that daphnids make use of the full range of possible anti-predator defences available, at least during short periods when predation is most intense. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera