Abstract: The paper compares scattering properties of different types of fish and zooplankton inhabiting the Baltic Sea. They are modelled as elongated objects - prolate spheroids or bent cylinders. The knowledge of the target strength of various marine organisms is indispensable for their biomass estimation and for analysing the variability spatial and temporal in the aggregative behaviour. The dependence of the mean backscattering strength and the normalised moment of inertia on the depth of the centre of gravity, recorded in diurnally migrating Baltic biological layers, is analysed for two different seasons, for night and day separately. These relationships reflect the different vertical migration patterns typical of various seasons, connected with variable hydrological conditions.