How did eurasian watermilfoil (an exotic species) get in to the Great Lakes?

Question

How did eurasian watermilfoil (an exotic species) get in to the Great Lakes?

Answer 1

My initial understanding was that it was discharged into the Great Lakes in the ballast water of a Russian ship.  Though some researchers are now also point the finger at the aquarium trade and some migratory bird populations.

I grew up on the shores of Lake Michigan and it's heartbreaking to see some of the more severely damaged habitats resulting from this weeds arrival.  As if the zebra mussels weren't bad enough!

The third link below is for a map from the USGS -- it shows just how far ranging this invader has spread throughout North America.