As seawater is denser, the water in the Baltic is stratified, with seawater at the bottom and freshwater at the top. Limited mixing occurs because of the lack of tides and storms, with the result that the fish fauna at the surface is freshwater in composition while that lower down is more marine.
Due to seawater's higher density, the Baltic Sea is stratified, with saltwater at the bottom and freshwater at the top.  Limited mixing occurs due to a lack of tides and storms, resulting in surface fish being freshwater while those deeper down are more marine.