My hometown had a small river that was muddy as can be. It wasn't known for its "diversity" - legend had it that there were some catfish in there, but usually all you'd ever catch were carp, suckers, crawdads and bluegills.
It seems odd that something that could live in that mud could be sensitive, but apparently that is true of the bluegill. In fact, nowadays they're being used to detect toxins in drinking water.
At least this time it's deliberate. Anybody who set foot near Lake Michigan in Chicago in the late 60's can tell you that the fish were telling us something.