The Baltic Sea is a semi-enclosed, relatively shallow shelf sea with some deeper basins of more than 200 m depth. There is a gradient in salinity with declining salinity towards east and north. Winter sea-ice normally covers the northern and eastern parts of the Baltic Sea.
In the Baltic Sea and vicinity, herring, sprat, and cod are the main prey items, and many prey species are benthic or demersal (Read 1999, Boerjesson et al. 2003).