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⁻¹). 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 (0.21 d⁻¹) only a third those of individuals in the no‐fish treatment (0.57 d⁻¹). 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.

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.