Fishery is one of the main environmental factors affecting fish population. In this paper we study the dynamics of population’s dimensional distribution under the influence of environmental variations with and without a fishery impact. The characteristics of population under consideration correspond to pelagic or semi-pelagic population, such as the populations of pollack (Theragra chalcogramma) inhabiting the northern seas of Pacific Ocean. To study the effect of fishery upon a population the problem of harvesting optimization with the amount of profit for a given time period as a criterion function is solved. The impact of population replenishment on the population and catch dynamics was studied and the smoothing influence of dimensional structure on the changes of environment and fishery was revealed. The seasonal changes of environment only increase the population stability.

In spring and early summer 2005, a new Mediterranean diatom species, Chaetoceros trondsenii, was noted in phytoplankton of the northeastern Black Sea. The parameters of the created phytoplankton species dynamics model were determined in experiments with nitrogen and phosphorus supply. Computational experiments with the model gave the following results: for Chaetoceros trondsenii establishment in an ecosystem, an elevated stability of the water column (water rate exchange in the upper mixed layer should be lower than 0.1 day⁻¹), a relatively high initial number of this species, and low values of nitrogen and phosphorus concentration at the boundary of the seasonal thermocline are necessary. The hypothesis of the introduction of the species into the Black Sea by the raised intrusion of Mediterranean water in winter is more preferable than the hypothesis of releases of the species with ballast water.