Abstract: Marine animals, fish and zooplankton, migrate vertically in a diel cycle in search of food and to avoid predators. Their crusades are determined mainly by the light amount, but also seasonally variable hydrological conditions have a big influence on the distribution of marine fauna and their aggregative and migratory behaviour. This paper presents the results of numerous series of acoustic measurements (echosounders and ADCPs) conducted in the Baltic Sea in various seasons at different frequencies (30-300 kHz) during the last 17 years. Long-term variability of echo signal parameters on the background of other environmental factors reveals some key features of marine fauna behaviour, among them a double-cycle of diel migration, a loop shape and hysteresis of the dependence between mean gravity centre depth of the signal envelope and backscattering strength, a diel variability of the distribution of SV-values, and seriously limited vertical migration in windy time. The vertical migration velocity is determined by three methods and the results are compared.