It wouldn't necessarily kill the fish right off. Fish lived in the Hudson River in NYC all through the worst years of pollution. But you wouldn't want to drink it.
I would guess this is either pollution or dumping.
Thanks for the comments.
Good point. In northwestern IN. there is a lovely little river called the Little Calumet. From its outlet to Lake Michigan all the way up past Hammond IN it is considered unsafe to touch let alone drink from (heavy metals, clorinated hydrocarbons, petroleum products, storm drain/sewage mix-you name it, chances are its there), and yet there are still fish in it! You can see them swim to the surface from time to time.
The Army Corp of Engineers has debated for over the last 20 years on how in the world they are going to dredge the toxic witch`s brew from the bottom and not contaminate the lower half of Lake Michigan, not to mention how or where to store the millions of tons of poisoned muck. So you should not be suprised at what fish could live in.