SRAC Publication No. 452 Southern Regional Aquaculture Center March 1992 Recirculating Aquaculture Tank Production Systems Management of Recirculating Systems Michael Recirculating systems for holding and growing fish have been used by fisheries researchers for nearly three decades. Attempts to ad- vance these systems to commercial scale food fish production have in- creased dramatically in the last few years. The upsurge of interest in recirculating systems is due to their perceived advantages of: greatly reduced land and water re- quirements; a high degree of envi- ronmental control allowing year-round growth at optimum rates; and, the feasibility of locat- ing in close proximity to prime markets. Unfortunately, most commercial systems, to date, have failed be- cause of poor design, inferior man- agement, or flawed economics. This publication will address prob- lems confronted when managing a recirculating aquaculture system so those contemplating investment can make informed decisions. For information on theory and design of recirculating systems refer to SRAC Publication No. 451- Recir- P. Masser 1 , James Rakocy 2 and Thomas M. Losordo 3 culating Aquaculture Tank Produc- tion Systems: An Overview of Critical Considerations and SRAC Publica- tion No. 453- Recirculating Aquacul- ture Tank Production Systems: Component Options. Recirculating systems are mechani- cally sophisticated and biologi- cally complex. Component failures, poor water quality, stress, diseases and off-flavor are com- mon problems in poorly managed recirculating systems. Manage- ment of these systems takes exper- tise and dedication. Recirculating systems are biologi- cally intense. Fish must be reared intensively (0.5 lb/gal or greater) for recirculating systems to be cost effective. As an analogy, a 20-gal- lon home aquarium, which is a miniature recirculating system, would have to produce at least 10 pounds of fish, a sobering thought when one contemplates the man- agement needs of recirculating sys- tems. System operation To provide a suitable environment for intensive fish production, recir- culating systems must maintain uniform flow rates (water and 1 air/oxygen), fixed water levels, and uninterrupted operation. The main cause of flow variation is the constriction of pipes and air diffusers from biological growth (fungi, bacteria and algae), which proliferates in response to high lev- els of nutrients and organic mat- ter. Constrictions reduce flow rates, This can cause increases or decreases in tank water levels; re- duce aeration efficiency; and re- duce biofilter efficiency. Flow rate variation can be avoided or re- duced by oversizing pipe diame- ters and configuring system components to shorten piping dis- tances. If flow rates gradually de- cline, then pipes must be cleaned. A sponge, cleaning pad or brush attached to a plumber’s snake works well for scouring pipes. Air diffusers should be periodically cleaned by soaking them in muri- atic acid. Flow blockage and water level fluctuations can also result from the clogging of screens used to re- tain fish in the rearing tanks. Screen mesh should be the largest size that will still retain the fish (usually 3/4 to 1 inch). The screened area around pipes should