A lake in Northern Wisconsin that I fish (and shall be left nameless because the WI DNR says that there are no SMB in there) had Rusty Crayfish introduced to it somehow. They pretty much completely denuded the lake - very few submerged aquatic weeds in there now. At night if you take a flashlight and shine it on the bottom of the lake there is a crayfish walking around for every square foot of pond bottom, maybe more.

Smallmouth look like pellet hogs. Last one I kept from there was 19" long, 19" around and weighed 6#.