That was terrible journalism
Local CBS in Chicago found a picture on a forum or on facebook, and ran with it. Then Field and Stream picked it up and just rehashed (not bothering to even interview anybody) and then Fox picked up Field and Stream's article and rehashed it, again not bothering to interview anybody.
No dead fish were ever found by 4 separate people that went looking, including Dale Bowman of the Sun Times, and Illinois DNR. There were numerous "perch boils" or "perch upwellings" of LIVE perch this winter and early spring in the slips. Huge aggregations of LIVE perch would boil up to the surface. If you took a picture of it, they look dead. But the videos that were taken clearly show they are alive.