Effects of feed addition on water quality
Most of the published information on feed-based static water aquaculture concerns the American channel catfish industry. I will therefore use the development and intensification of that industry as a general example. When prepared pelleted diets are added to fish ponds the result is eutrophication. Because the amount of added feed is unlimited, water quality degradation is much more important in fed than fertilized ponds. The degree of eutrophication caused by artificial feeding is proportional to the amount of feed provided. The proliferation of plankton resulting from the addition of fish feed causes increasingly wide fluctuations in DO due to plankton photosynthesis and respiration. In these cases oxygen depletion can become so severe that it may result in fish mortality.
A further consequence of wildly fluctuating DO levels is daily elevations of pH, which results in short term exposure of fish to potentially lethal levels of NH3. The chemistry and toxicity of ammonia are described in Chapter 9.
Furthermore, as culture intensity increases, nitrite (NO2-) accumulates in pond waters. Nitrite is the intermediate product of nitrification and can accumulate to toxic levels in aquaculture systems. The mode of toxicity of NO2- is conversion of hemoglobin to methemoglobin, which cannot carry oxygen. The resulting disease is called methemoglobinemia. The blood of affected fish is brown in color. Losordo (1997) recommended that NO2- levels be kept below 5 mg/L, but channel catfish exposed to this level for 24 hours showed a conversion of 77% of their hemoglobin to methemoglobin (Tomasso et al. 1979). It is impossible to assign a maximum tolerable concentration for NO2- to fish because its toxicity is highly variable depending upon pH, chloride, and calcium levels. Nitrite toxicity in ponds can be controlled by chloride addition (Perrone and Meade 1977; Tomasso et al. 1979) and fish in brackish or seawater are resistant to methemoglobinemia (Almendras 1987). Recommendations on effective NO2-:Cl- ratios range from 3:1 (Schwedler et al. 1985) to 20:1 (Losordo 1997). Because Ca2+ also protects fish from NO2- toxicity (Wedemeyer and Yasutake 1978), CaCl2 is more effective than NaCl for this purpose.
Generally, feeding rates of up to 30 kg/Ha per day can be maintained without danger of fish mortality caused by nighttime oxygen depletion. This level of feeding results in a maximum fish crop of around 2000-3000 kg/Ha, depending upon the trophic status of the aquaculture species, which is generally considered to be unacceptably low for economical production. Aeration is the most common and most effective method of preventing fish kills caused by excessive plankton respiration resulting from feeding rates in excess of 30 kg/Ha.