The Baltic sea has a low salt content. Together with the Black Sea these are the largest brackish (medium salt-content) reservoirs in the world. The fish live under what is called physiological stress. The mussels on the West coast of Sweden are three-four inches in diameter, whereas in the Baltic they reach barely an inch in size. This is because of the low salt content.
Back to the subject: any herring caught North of a line drawn from Klaipeda in Lithuania and Kalmar in Sweden is called 'Baltic Herring' or 'strömming' in Swedish. Any herring caught west or south of that line should be called just herring or 'sill' in Swedish.
There is a difference! The Baltic herring is meagrer and smaller, and in some recipes from the Sweish Smorgasbord, it can not be substituted with fattier herring.