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Schools of fish, aggregations of plankton and concentrations of gas bubbles cause seattering and attermarion of sound waves. In the Baltic Sea, in some seasons and during some hours, at the thermo-and halocline depth there persist aggregations of small marine organisms registered in the range of lower ultrasound frequeneies. Diurnal vertical migrations of biological objeets (fish and plankton) are easily observed by echosounders. The possible reasons for this well known phenomenon arefatcat:nqxywzbb45blfjlqxioa3up2gm