Vertical migration of zooplankton: a game between predator and prey.

Many zooplankters in lakes and oceans assemble in the upper waters at night and sink to the lower layers in the day. Planktivores follow zooplankters. This diel migration is studied by analyzing a habitat selection game between predators and prey, based on the predation hypothesis: in the daytime zooplankton avoid predators that hunt by sight at the cost of reduced grazing on phytoplankton. When the efficiency of predation by sight is low, both predator and prey concentrate in the upper layer (night phase) , and, when it is high, most zooplankton stay in the lower layer and the fish population is distributed between both layers (day phase) . The equilibrium distribution of zooplankton discontinuously changes with the predation efficiency at a threshold value. Since the predation efficiency varies with light intensity, diel migration pattern is expected, and the following results are shown. 1) The change in zooplankton distribution at dawn and dusk is more sharp than that of fish distribution. 2) The partition ratio of zooplankton between the 2 layers in the day is determined only by the ratio of predation efficiency of fish in both layers, and is independent of the total amount of zooplankton or fish. 3) The threshold light intensity at which zooplankton distribution changes discontinuously decreases with the feeding intensity of fish and increases with the scarcity of food of zooplankton. 4) The total predation rate of fish attains its maximum shortly after sunset or before sunrise. The total grazing rate by zooplankton on phytoplankton is higher at night than during the day.-Author