An example of meso-scale hydrographic features in the central Baltic Sea and their influence on the distribution and vertical migration of sprat, Sprattus sprattus balticus(Schn.)

A wind-driven meso-scale pattern of temperature, salinity and oxygen was found along a transect in the northern Bornholm Basin (southern Baltic Sea). Strong winds caused currents along this transect, which shifted cold intermediate water (minimum: 3.6 °C) towards the south. The transect was surveyed with a towed CTD-system and hydroacoustics in parallel to investigate the distribution of sprat, Sprattus sprattus balticus (Schn.) in relation to the observed meso-scale pattern. In those parts of the transect where the cold intermediate water was observed, sprat were restricted to water layers below the halocline. In other parts of the transect, sprat moved into higher water layers and occupied a wider depth range. The important factor was temperature, which set an upper limit to the vertical sprat distribution. The development of hydrography, as measured in the field, was evaluated with a hydrodynamic model.