In the Baltic Sea, two cod stocks evolve independently. The young fish of two economically important flatfish species, flounder and plaice, live within limited space, possibly due to the different salinity within this inland sea. Scientists explain how these hydrographic conditions affect the distribution of fish eggs and growth of economically important fish stocks. The findings are relevant for fisheries management since they show that the individual stocks of a species do not necessarily benefit from each other, and -- for example -- that one stock cannot recover from overfishing with the help of the other. Regulations must therefore take into account local specifics, and the monitoring of fish stocks in their various stages of life....