They are not from the Baltic. It's the North Sea. More precisely Kattegat, Skagerak and along the western shore.
Morever, the top layer of the Kattegatt has a much lower salinity compared to the ocean due to significant outflow of warm brackish water from the Baltic, carrying all the pollutants to the North Sea through the Skagerrak.
To be fair, the situation has actually been improving since the late 90s with tighter regulation on effluents, however I'd rather wait another 10 year before this patch of sea clears up itself.