MODELING PRODUCTION CAPACITY OF AQUATIC CULTURE SYSTEMS UNDER FRESH-WATER CONDITIONS

In the design of flow-through systems, it is typically assumed that oxygen is the most limiting parameter. This is correct for single-pass systems without aeration, except for water with alkalinities less than 10 mg/liter (as CaCO3) where metabolic carbon dioxide canreduce the pH below acceptable limits. As the rearing intensity is increased by the use of pure oxygen or aeration, the build-up of carbon dioxide becomes more important. The build-up of carbon dioxide has three primary effects: (1) increase in dissolved carbon dioxide concentrations, (2) decrease in pH, and (3) reduction in the mole fraction of unionized ammonia due to the decrease in pH. In a closed system with no carbon dioxide removal, the maximum carrying capacity is controlled by pH at low pHs, by carbon dioxide at intermediate pHs, and by unionized ammonia at high pH. Under pHs typical of salmonid systems, it is impossible to exceed an unionized ammonia criterion. In an open system with 100% carbon dioxide removal, the maximum carrying capacity is limited by unionized ammonia computed at the influent pH. Published information on unionized ammonia is seriously biased by inappropriate analytical procedures used in the measurement of pH and possible loss of carbon dioxide from the sample.