SRAC Publication No. 454 November 1992 Southern Regional Aquaculture Center Recirculating Aquaculture Tank Production Systems Integrating Fish and Plant Culture James E. Recirculating aquaculture systems are designed to raise large quanti- ties of fish in relatively small vol- umes of water by treating the water to remove toxic waste prod- ucts and then reusing it. In the process of reusing the water many times, non-toxic nutrients and or- ganic matter accumulate. These metabolic by- products need not be wasted if they are channeled into secondary crops that have economic value or in some way benefit the primary production system. Systems that grow addi- tional crops by utilizing by-prod- ucts from the production of the primary species are referred to as integrated systems. Plants are an ideal secondary crop in integrated systems because they grow rapidly in response to the high levels of dissolved nutrients that are generated from the micro- bial breakdown of fish wastes. In closed recirculating systems, which employ very little daily water exchange (1 to 5 percent), dissolved nutrients accumulate and approach the concentrations that are found in hydroponic nutri- ent solutions. Nitrogen, in particu- l University of The Virgin Islands; 2 North Carolina State University 3 Alabama Cooperative Extension Service Rakocy 1 , Thomas M. Losordo 2 and Michael P. Masser 3 lar, occurs at very high levels in recirculating systems. Fish excrete waste nitrogen directly into the water through their gills in the form of ammonia. Biofilter bacte- ria convert ammonia to nitrite and then to nitrate (see SRAC Publica- tion No. 451 on critical considera- tions). Ammonia and nitrite are toxic to fish, but nitrate is rela- tively harmless and is the pre- ferred form of nitrogen used by higher plants, such as vegetables. Integrated systems can be used for the hydroponic culture of high value cash crops such as tomatoes, lettuce and sweet basil. Recirculat- ing systems may also be used for the culture of aquatic plants. Aquatic plants Aquatic plants grow rapidly in recirculating systems that are lo- cated outdoors, in greenhouses or in buildings with adequate artifi- cial light. Plants are typically grown in shallow tanks that are separate from the fish rearing tank. Three types of aquatic plants can be cultured: floating, emer- gent and submerged. Floating plants, which are naturally buoyant, grow with their roots suspended in the water and their leaves in the air. They repro- 1 duce vegetatively (without seeds) by dividing in half and grow so rapidly that they can completely cover the surface of the tank within a short time. When this oc- curs, about one third of the plants Leaf lettuce grown in deep flowing chan- nels is an ideal crop for integrated recir- culating systems.