When I started working for Yorkshire Water the river water quality they were intaking at Elvington Water Treatment works was monitored by pumping the water through small tanks with trout swimming in them.  There were electrodes on the side of the tank to detect if the trout were making unusual swimming movements indicating pollutants in the water.  They were phasing in a UVICON? ultraviolet pollution monitor in parallel with the fish monitor.  The fish monitors are now not used.  I think it's just UV monitoring along with clarity and conductivity.