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Zooplankton perform diel vertical migration (DVM) to avoid predators at the upper water layer, but often stay in the upper water layer throughout the day seeking food in spite of the presence of predators. This difference in migrating behavior has been explained by differences in environmental conditions or genetic differences. We examined theoretically how nutritious conditions of zooplankton individuals relate to determining different migrating behavior. A simple optimization model, maximizing the population growth rate, demonstrates that zooplankton individuals change their migrating behavior depending on the amount of accumulated energy. Such energy accumulation and its investment in reproduction are repeated every reproductive cycle. Therefore, unless the reproductive cycle is synchronized among individuals, different migrating behaviors will be observed within a population even if no genetic differences exist. Our model demonstrates that such coexistence of the two migrating behaviors is possible in natural Daphnia populations, and suggests that internal conditions of zooplankton individuals may be important as a factor for determining migrating behavior of zooplankton.