In the Netherlands they used trouts to monitor water quality. They let trout swim in a aquarium where they let in river water destined for drinkwater. The trout swims against the stream and when it noticed a polution it flees. The inlet is closed and then a sample is taken which is tested. They did it in 1984, but I don’t know if they still do it. Because I think you can do it now with an automated system which includes a gaschromatograph.