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The depth distributions of predation and resources are explored as two major reasons for vertical migrations in zooplankton. Migratory behavior is expected when two conditions are fulfilled. First, the risk of mortality due to predation is significantly higher in the upper than in the
lower strata, and remaining in hypolimnion is advantageous. Second, growth and reproduction are both higher in migratory animals than in nonmigratory ones that remain in hypolimnion day and night. The first condition is fulfilled when predation is exerted by a visually-oriented planktivore
that cannot affect its prey below a threshold light illumination. The second is fulfilled when the concentration of edible food is much higher in the epilimnion, and the energy investment for migration is lower than the energy gain resulting from nocturnal feeding in the epilimnion. Nonmigratory
behavior is expected when one of the two conditions is not fulfilled, i.e., visual predation is unimportant, or edible food concentration in the hypolimnion is not significantly lower than in the epilimnion. The animals are predicted to remain in the epilimnion in the first case, and in the
hypolimnion in the second case. These predictions are supported by two sets of data. The first set describes migratory and nonmigratory populations of the copepod Cyclops abyssorum in lakes with and without planktivorous fish, respectively, and with similar depth distributions of food.
The second set describes migratory and nonmigratory populations of Daphnia hyalina in lakes with different depth distributions of edible food but similar numbers of visually-oriented planktivorous fish.
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