An invasive fish species.
Asian Carp, an invasive fish species were brought from China, to keep ponds clean and control parasite hosting snails. Eventually these Asian Carp escaped due to flooding and entered the Mississippi river system destroying ecosystem after ecosystem. If they get to the Great Lakes where they are headed they will cause damage to fisheries and sport businesses in the Great Lakes.
There are many solutions below will be the two biggest solutions and I will explain the other smaller one. For example one idea to make a permanent barrier in the Chicago Canal was suggested but then rejected because the damage of no trade would outweigh the damage done by the carp.