THE EVOLUTION OF INTEGRATED AQUACULTURE SYSTEMS 1

Oxidation ponds have been used for many years for treatment of domestic wastewater. Nutrients and carbon dioxide produced by the decomposing organic wastes enhance the growth of unicellular algae which, in turn, provide oxygen for the bacterial decomposition. When such ponds have an effluent, however, the algae themselves represent a loading of organic matter that fails to meet discharge standards, so the algae must be removed from the water. Conventional harvest techniques are uneconomical, particularly in view of the low value of the product. An alternative is to allow the plants to be consumed by filter‐feeding or grazing fish or invertebrates that harvest the plants at no cost and, in the process, produce an animal crop that has potential commercial value. Polyculture systems, involving freshwater fishes low on the food chain grown in ponds enriched with a variety of organic wastes, have been used for centuries in China. The practice has spread to many other countries and the concept has begun to be used in estuarine mariculture. Usually the ponds are stagnant, with no effluent, and the objective is fish production, not waste treatment. Recently, similar waste recycling‐polyculture systems have been developed experimentally on a flow‐through basis with the dual objectives of both fish (and/or invertebrate) production and advanced wastewater treatment (specifically nutrient removal). Several recent examples of quite different freshwater systems and a fewer number of marine systems are briefly described. Problems and constraints of such practices are also discussed.