In the 50s and 60s, days of vast rafts of detergent floating down the Thames, the fishing was one-a-chuck from Richmond upstream. Twenty years the Fraser River in Canada was claimed by a Vancouver newspaper to be the dirtiest river in the Northern hemisphere. It had and has salmon runs and sturgeon populations which could make your eyes pop. The Ebro in Spain was where all untreated sewerage went till about 10 years ago and the fishing was the best in Europe.
Clean rivers do not mean rivers full of fish and dirty rivers do not mean rivers with few fish.