Water recirculating systems for fish culture are potentially desirable for conserving water and reducing heating requirements, maximizing production of fish under water and space limitations, minimizing effluent problems, and maintaining better control over environmental factors. A semiclosed recirculating‐water system for intensive culture of rainbow trout Oncorhynchus mykiss is described. The system used self‐cleaning, rectangular, cross‐flow rearing tanks (water volume, 9 m3 each), multistage oxygenators, microscreen filters, and a sidestreamed, fluidized‐bed biological filter. Rainbow trout were reared under continuous culture conditions, with periodic stocking and periodic selective harvesting. Makeup water entered at 47.3 L/min, producing a newwater turnover 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/m3 and 2.50 and 2.83 kg·L–1·min–1, respectively. Steady‐state gross productivity was estimated to be 6,257 kg/year (120 kg/week). Overall food conversion (feed fed/fish weight gained) was 1.33. The system performed satisfactorily and provided data for refining future designs. Subsequent modifications of equipment and operating procedures may have made the system economically viable under some pricing scenarios.
