SHORE AVOIDANCE IN ZOOPLANKTON - A PREDATOR-INDUCED BEHAVIOR OR PREDATOR-INDUCED MORTALITY

The distribution ot two Daphnia species dominant in near-surface waters of a eutrophic lake, Lake Ros, showed offshore-inshore gradients in density with a dramatic decrease towards nearshore waters, suggesting a strong 'shore-avoidance' behavior. Much smaller body sizes and much smaller clutches in the nearshore fractions of the two populations, however, showed that the lower population density of each species in the nearshore areas was rather a direct effect of high mortality selectively generated in more conspicuous larger individuals and in individuals with greater clutches that are more vulnerable to predation by littoral fish, mostly abundant perch juveniles. Hydroacoustic and trawling data revealed that young perch move offshore in the evening to feed on pelagic zooplankton, but their feeding cannot be as efficient later in the dark when they eventually reach the most distant Daphnia patch in the center of the lake. High predation risk in the littoral neighborhood may be considered an ultimate factor behind the evolution of shore avoidance behavior in zooplankton.