so do they randomly reproduce or do they need rocky bottoms or running water?
According to Travis and other sources, they do prefer rocky bottoms and running water. However, I think what cannot be stressed enough regarding this species, is it's a big risk and sometimes you just don't know. The lake Bullhead mentioned, Branched Oak, that has a white perch problem is 1800 acres. The other one I mentioned, Pawnee, is less than half that at 740 acres. Perch overpopulated and stunted in Branched Oak, but haven't reproduced and created a problem in Pawnee. I am unfamiliar with the fish habitat in these lakes, maybe Bullhead or Bruce could weigh in on the differences, but what it tells me is you just don't know. There was some extensive research by the state regarding why one became a problem and the other one didn't. I can't seem to find the results of this study at the moment, but will continue to look.
