Putting walleye into Rock lake sure ruined the jumbo perch fishing so I agree with you. First the jumbo (&all) perch went downhill, then the pike went down for a little while too; probably due to the fact their main food source they were used to werent there anymore. It ruined Rock as far as I'm concerned.
I don't get why they do it when a lake already has a healthy population of other species that fishermen love to target.