Schroeder, G.L., Fish and Aquaculture Research Station, Dor, Hof Hacarmel, Israel

Oxidation ponds or sewage lagoons often present characteristics of natural water bodies in an extreme state of ecological imbalance. Excess nutrients of the decaying waste give rise to plankton blooms which subsequently die. In such conditions dissolved oxygen concentrations (DO) in the water are often low and the pH depressed. The addition of fish to ponds which twice a week received, per ha, wastes with a 5 day BOD of up to 800 kg (20°C) and 5600 kg solids, reduced plankton and benthic populations, increased the average DO, and raised the pH. All of these changes improve the effectiveness of a waste treatment pond for reducing BOD and removing nutrients from the water. Bacteria concentrations were as much as 15 times lower in treatment ponds stocked with fish as compared with unstocked ponds. Lower bacteria concentrations however reduced the rate of loss of COD in the solids of the waste. © 1975.