A Lagrangian ensemble model which can describe the population dynamics of Neocalanus cristatus was developed to simulate the Northwestern pacific lower trophic ecosystem. This model includes development stages, vertical migration, and NEMURO for other compartments. Each particle of the Lagrangian model represents copepods of the same cohort age, which means one particle represents a set of copepods of the same birthday. Each particle also has variables that represent age, development-time, growth-rate, structural weight, and food satiation. This model was coupled with a lower trophic level ecosystem model of NEMURO as an interactive copepod component. The model can simulate the annual cycle of Neocalanus cristatus. Most of copepodite stages begin during the spring bloom, followed by a decrease in biomass during summer, then by migration of Neocalanus to deep water where they diapause during autumn and winter, and finally ends with egg production in winter. The model simulation successfully described the annual life cycle of the copepods in the Northwestern subarctic Pacific. By analyzing the simulated results of vertical trajectories of particles, individual growth, and stage development, we can conclude that during the spring bloom they graze large-phytoplankton without starvation and in summer they begin to prey on small-zooplankton (ZooS) to maintain their weight. These results support observations of their food selection corresponding to the seasonal food supply. Moreover we are applying this model to vertical 1-D NEMURO, so that we will be able to represent the vertical variations of seasonal abundance, depth dependent features of stage development, and patterns of reproduction caused by ontogenetic vertical migration.