This is the result of the significant difference in egg density (high egg density in
the western population compared to lower egg density in the eastern
population) restricting dispersal into the deep, central Arkona Sea
(SD 24) and thus further east into the Basin of Bornholm (SD 25) or
Gdansk Deep (SD 26) (Petereit et al., 2014). 
The shallow connection
between basins causes egg mortality during drift by preventing dense
or “salty” enough hydrographic conditions for positive buoyancy
(Petereit et al., 2014).
 The salinity gradient is probably one of the most
important factors maintaining population structure of the Baltic cod.
 
A second factor could be due to homing behaviour described for cod
from the Danish Straits and western part of the Baltic Sea (Nielsen
et al., 2005; Svedäng et al., 2007) which limits the choice of spawning
areas.