The interesting explanation for why it's the least saline sea in the world, is that there is a narrow outlet channel via Kattegat, and the volume of water from

The interesting explanation for why it's the least saline sea in the world, is that there is a narrow outlet channel via Kattegat, and the volume of water from rivers with outlets in the Baltic far exceeds the amount of water that evaporates, so there is a constant outflow of water through Kattegat. So there's not much salty water backdraft. We have perch and ordinary grass growing in the Baltic - weird!