Abstract: The Baltic fauna is rather meagre, what is connected with their miserable habitat. The area of our investigation is a specific basin because of its land-locked location, brackish water and small depth. It is characterised by enormous diversity of time-variable hydrological conditions. All species of fauna are subjected to the continuous stress of having to adapt to rapid changes in the temperature and salinity of the surrounding water and are exposed to highly variable environment during their every-day vertical migration between the food-rich sea surface and safer deep. The purpose of this paper is to present the results of acoustic investigations of the live interior of the Baltic Sea by means of various acoustic instruments (echosounders and acoustic Doppler current meters). The interpretation of our acoustic images helps to understand the nature and behaviour of the sound scatterers.