A simple, but natural, marine pelagic predator-prey system affording an ideal model for experimental investigation of the effect of fish predation on zooplankton behavior exists in Jackles Lagoon in the state of Washington. This model predator- prey system will be used to test, by direct experimentation, the hypothesis that diel vertical migration by zooplankton is a tactic for evading predators. This will be done by manipulation of populations of a planktonic copepod, Acartia hudsonica, and its natural predator, the planktivorous fish Gasterosteus aculeatus, in enclosures. Experiments and supplemental field observations will establish the conditions under which migration behavior is induced in the copepod population by fish predation, and to identify the specific mechanisms of induction. This is the first experimental test of the predation hypothesis and the results will have wide implications, not only for understanding diel vertical migration behavior of pelagic oarganisms in lakes and seas, but also for addressing more general aspects of predator-prey interactions, such as refuge use by prey, in aquatic pelagic ecology. %%% Predator-prey interactions involving vertebrates in pelagic systems are poorly understood for many reasons including the difficulty in obtaining replicate samples, differential movements of predators and prey in sheared circulation, and mismatches in the "living volume" occupied (scale mismatch). If the predators are confined to a volume that can be studied from the point of view of prey, the volume is generally so small that behavior of the predator is substantially altered. A volume large enough to be realistic for the predator cannot be sampled adequately for prey. In Jackles Lagoon, however, the planktonic community is low in diversity, and alteration of behavior on the part of prey has been noted in response to increasing and decreasing concentrations of predators. For many years it has been suggested that diel vertical migration by crustacean prey in pelagic marine systems is a response to visually searching predators. This study will quantitatively test that supposition.
