There are physical and chemical differences among the lakes and those lakes are HUGE. Not every fish is capable of spreading to other lakes. Unfortunately, the ones we don't want seem to have no trouble.

It's true that water does drain from one lake to the other, but that does not mean that the fish will do the same thing. The conditions were satisfactory so that they didn't need to migrate elsewhere to survive.

I was reading in a book when it is said, I believe, that the blue walleye was native to Lake Erie prior to its extinction. My thought was if the water in the Great Lakes drains into each other, wouldn't that cause to migrate to other waters. I can see an exception with Lake Ontario because it is closest to the Atlantic Ocean, plus Niagra Falls; but the others, no. What would cause a fish to be endemic to one Great Lakes and not the other?

Thanks