Acoustically measured diurnal vertical migration of fish and zooplankton in the Baltic Sea - seasonal variations

Acoustic measurements were conducted at different seasons in the last five years at a fixed point of the Baltic Sea at a frequency 30 kHzcon currently with temperature and salinity sampling. Echosounding records were used to determine the seasonal and diurnal migration patterns of scattering layers in accordance with thermohaline conditions. Different seasonal patterns of nocturnal aggregations of organisms dependent on the temperature gradient in the thermocline were found. This paper illustrates the diurnal distributions of sound scattering layers and analyses the way they are linked to the physical structure of the water.