JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology.Abstract. Diel vertical migration (DVM) of zooplankton is a behavioral antipredator defense that is shaped by the trade-off between higher predation risk in surface waters and reduced growth in deeper waters. We conducted two laboratory experiments to quantify the costs connected with DVM. In the first experiment, Daphnia magna were kept individually in thermally stratified flow-through tubes and exposed to seven different concentrations of fish-exuded kairomones. Above a threshold concentration, the strength of migration increased with increases in the concentration of fish exudates. Enhancement of migration resulted in a lower mean ambient temperature experienced by the animals and marked decreases in individual growth and reproduction rates.In order to separate costs due to low hypolimnetic temperatures in a stratified system from costs due to reduced food concentrations in deeper waters, we conducted an experiment with a 2 x 2 factorial design (fish presence vs. absence and high vs. low food conditions). Differences in mean ambient temperature between Daphnia that performed DVM and nonmigrating animals were found to have a much stronger impact on life history parameters than food effects. A reevaluation of field data on DVM in Daphnia further supports the view that vertical temperature gradients are more important than food gradients.

Life-history parameters were determined for a cohort of 60 clonal Daphnia magna grown individually in thermally stratified, 1 -m-long flow-through tubes under a constant high-food regime (2.0 mg C liter-'). The individuals could migrate freely in the tubes and their depth was noted at regular intervals. Half of the tubes received water from a reservoir containing a fish. The animals in these tubes stayed deep during the day and higher at night, thus showing a "normal" diel vertical migration behavior. The other half of the tubes was supplied with the same water, but free of fish. The individuals in this set exhibited no diel migration, but stayed in the warm upper strata day and night. The migrating animals in the fish-treated set grew at rates (0.2 1 d-l) only a third those of individuals in the no-fish treatment (0.57 d-l). Most of the difference between the treatments can be attributed to the lower temperatures experienced by the migrating individuals. However, an experiment with no thermal stratification showed that the chemical presence of fish in the water can significantly retard Daphnia growth.The hypothesis that diel vertical migration (DVM) behavior of zooplankton is a predator avoidance strategy (Kozhov 1963;Zaret and Suffern 1976; Stich and Lampert 198 1;Gliwicz 1986) has received strong support from laboratory and field studies showing that DVM can be chemically induced by the presence of predators. This induced DVM has been reported for various planktonic prey species responding to different predator taxa in both freshwater and marine environments

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Diel changes in the depth distribution of Chaoborus flavicans larvae in thermally stratified aquaria with a distinct light gradient revealed that the larvae responded behaviorally to the presence of fish. Both the midday and midnight mean depths of the population were greater in the presence of fish. Most fish‐treated larvae found a daily refuge in bottom sediments. It appears that the stimulus for vertical migration is chemical, not visual or mechanical. The fish effect persisted for more than 15 d but was reversible. Those individuals that previously had been exposed to fish factor were considerably more sensitive to light than untreated larvae and showed a panic response when suddenly illuminated.