Recirculating systems are potentially appropriate when water quality and quantity are insufficient, for example if water must be cooled or warmed, or if discharge standards are strict . While the culture of fish in recirculating systems is biologically possible, the economics must be very carefully studied. These systems require high value species and experienced operators for commercial success. They may be too deep, too large, or have irregular bottoms or obstructions. Cages are relatively small, rigid structures usually between one and approximately 20 cubic yards.
Net pens are usually large mesh enclosures up to several hundred cubic yards.
Both cages and net pens should be anchored securely in an area where water flow and depth are sufficient to provide aeration and waste removal. Many successful aquaculture businesses use cage or net pen culture. One or two cages placed into an existing pond can provide an excellent and relatively inexpensive introduction to aquaculture as well as a source of high quality fish for the owner. Annual yields can exceed 400 pounds per cubic yard of cage or net pen, depending upon the skill of the fish farmer and quality of water. Use of public waters for cage or net pen culture requires careful investigation of social, regulatory, and technical issues.