The spatial distribution of crustacean zooplankton in lacustrine environments is affected by multiple physicochemical and biological processes, the nature and the relative importance of which vary among ecosystems and at different spatial and temporal scales (Lampert, 1993; Angeli et al., 1995a; Pinel-Alloul, 1995). Fish predation has been cited as one of the most important factors influencing zooplankton community structure (Brooks and Dodson, 1965) and distribution over space and time (Urabe, 1990). To counteract fish predation, zooplankton have developed avoidance strategies, such as diurnal horizontal migration (DHM) in weedy littoral zones (Davies,