The CA-model was modified in order to take into account the birthrates seasonal dependency and the influence of water streams on movements of organisms. The model was verified within the production-to-biomass and the relative average quantity criteria. A difference in the verification results and the assessments of physical values is about 20 %. The simulation of a possible pollution influence was carried out. The assessment of the critical pollution that leads to the total death of individuals and acceptable pollution with no observable influence are presented.