CBS Chicago reports that anglers found the dead fish at an old dock, a popular local fishing spot. Photos of the scene were then passed on to the Illinois Department of Natural Resources for investigation. Meanwhile, gulls and fish-eating ducks quickly devoured the perch, cleaning up the mess. But though the fish may be gone, the mystery of their deaths remains.

According to WGN, authorities believe a phenomenon known as upwelling, or turnover, may be to blame for the die-off. Upwelling occurs when deep, cold water rises to the surface, displacing the warmer, nutrient-depleted water above. As the cold water ascends, fish are often drawn upward as well, which may explain why the perch surfaced. READ MORE