2 points
Also if I am not mistaken - aren’t there ships that go both on the Great Lakes and the open ocean? That could be a possible criterion. Most lakes don’t have ocean-going ships.
1 point
That was how I was assuming the rationale went. You do see ocean going vessels on the Great Lakes.
https://en.wikipedia.org/wiki/St._Lawrence_Seaway
And I think that’s how everyone wound up with the zebra mussels. (Bilge discharge from those ships)
