WWW.WAS.ORG • WORLD AQUACULTURE • DECEMBER 2012 65 1998). Waters of coastal shrimp ponds have high concentrations of minerals from natural sources. Zeolite exchanges sodium for other cations, but ammonium is a minute fraction of the cations in seawater and brackishwater. Bacterial species in amendments are common ones already present in ponds; to increase their growth one only needs to add substrate or have sufficient oxygen. If bacteria are not functioning in the presence of adequate substrate, there is a problem with the pond environment. Nevertheless, shrimp farmers refuse to believe this and efforts to reduce use of these products have been for naught. Aeration is widely used in shrimp aquaculture and, in contrast to channel catfish farmers who tend to use too little aeration, shrimp farmers often use more aeration then necessary. Most shrimp farmers in Asia do not bother to measure DO concentration in ponds; they use so much aeration that DO depletion is seldom an issue. The aerators used in Asia are much less efficient than those used in channel catfish farming in the United States but farmers are unwilling to invest in higher-priced aeration units, just as channel catfish farmers balk at investing in additional aeration capacity. I believe that a major problem in aquaculture is the reluctance of producers to invest in technology shown to be effective through sound research. They are, however, easily coerced by vendors, consultants, or neighbors to spend large amounts on almost any product that is alleged to increase production. University professors are accused frequently of being impractical but aquaculture producers suffer the same affliction. As a group, they seem unable to understand that the maximum is seldom if ever the most profitable level of production and they are more likely to believe a good salesman than a reputable scientist. In the late 1990s, it was discovered that marine shrimp aquaculture also can be conducted in low-salinity, inland ponds (Roy et al. 2010). There are extensive areas in many countries with surface waters, ground waters, or both of suitable salinity for shrimp culture. However, these waters often must be supplemented with potassium (Boyd et al. 2002, Boyd and Thunjai 2003) and possibly with magnesium (Pine and Boyd 2010, Wudtisin and Boyd 2011) to avoid shrimp stress and mortality attributable to imbalance of major cations. Intensification of Culture Systems Pond aquaculture is becoming more intensive through exportation of waste loads from ponds through water exchange and removal of settled solids, application of greater aeration rates to oxidize waste loads within ponds, and use of biofloc systems in which wastes are recycled as food for culture animals. High rates of water exchange are no longer considered advisable in shrimp culture because of the increased risk of disease. Environmental regulations also may restrict use of water exchange. The aeration rate in earthen ponds is limited by erosion of embankments by aerator-generated water currents. Lining insides of embankments or entire pond bottoms with plastic sheets allows higher aeration rates and facilitates removal of sludge with suction devices. Of course, sludge removal is not environmentally friendly because wastes from ponds are exported to natural waters. The biofloc method for culturing shrimp (McIntosh 1999, Avnimelech 2012) usually is conducted in completely lined ponds with intensive aeration. A portion of the wastes in biofloc systems is converted into bacteria biomass and fed upon by shrimp. Biofloc systems are becoming rather common in several countries and species other than shrimp can be cultured in them. In catfish farming in the United States, efforts are being made to develop ways of confining fish during culture and use the traditional pond as a water treatment area. In the late 1980s, at least one farmer in Arkansas developed a system in which catfish were produced in raceways (external to ponds), and raceway discharge was recirculated through the former production ponds for purification. The partitioned aquaculture system (PAS) (Brune et al. 2004), the split-pond system (Tucker and Kingsbury 2010), and the in-pond raceway system (Brown et al. 2011) are possible alternatives to traditional pond culture of catfish and other species. It is not clear whether or not these systems will allow greater production than traditional ponds when the total water surface area is considered, but management can be greatly improved by confining fish. Traditional ponds have been and still are the major production system for freshwater fish and marine shrimp. An estimated 8,700,000 ha of freshwater ponds and 2,330,000 ha of brackishwater ponds presently are used for aquaculture (Verdegem and Bosma 2009). It is doubtful that the emerging systems mentioned above will come to represent a large percentage of total production in the near future, but they probably will become much more important. More intensive production — whether in (CONTINUED ON PAGE 64) TOP TO BOTTOM: Emergency aerator powered by the PTO unit of a tractor. Small Taiwanese-type of electric paddlewheel aerator. Typical electric paddlewheel aerator used in commercial channel catfish ponds.
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