Why would a species of fish increase from an increase in sewage waste?

This is a phenomenon that is seen with certain industrial effluents. When the level of waste is such that it is not toxic, you also have to look at the food value. Many industrial effluents can actually act as food for the small critters and plants, thus making it more attractive for fish. The other factor is warmth. Effluent is usually warm and fish prefer warm. What is not known is this . Do the fish increase in population total or do the migrate to the warm nice water.from other areas.