A SEMICLOSED RECIRCULATING-WATER SYSTEM FOR HIGH-DENSITY CULTURE OF RAINBOW-TROUT

Water recirculating systems for fish culture are potentially desirable for conserving water and reducing heating requirements, maximizing pr oduction of fish under water and space limitations, minimizing effluen t problems, and maintaining better control over environmental factors. A semiclosed recirculating-water system for intensive culture of rain bow trout Oncorhynchus mykiss is described. The system used self-clean ing, rectangular, cross-flow rearing tanks (water volume, 9 m(3) each) , multistage oxygenators, microscreen filters, and a sidestreamed, flu idized-bed biological filter. Rainbow trout were reared under continuo us culture conditions, with periodic stocking and periodic selective h arvesting. Makeup water entered at 47.3 L/min, producing a new-water t urnover time of 9.2 h. Steady-state and maximum fish biomass densities and loading rates were estimated to be 66.0 and 74.6 kg/m(3) and 2.50 and 2.83 kg . L(-1). min(-1), respectively. Steady state gross produc tivity was estimated to be 6,257 kg/year (120 kg/week). Overall food c onversion (feed fed/fish weight gained) was 1.33. The system performed satisfactorily and provided data for refining future designs. Subsequ ent modifications of equipment and operating procedures may have made the system economically viable under some pricing scenarios.