Optimal population ecological adaptation model is used to demonstrate the vertical migration of Calanus sinicus and Paracalanus parvus population of Bohai Sea ecosystem. Life cycle, generation time, fecundity, and egg production quantity are the main factors to determine these two species' vertical migration. For a Calanus sinicus type life history, fitness is maximized by diurnal vertical migrations. When advection risk in surface waters is low and by spending less time in the upper waters as ...