WWW.WAS.ORG • WORLD AQUACULTURE • JUNE 2014 9 World aquaculture is dominated by the use of simple earthen ponds. Unlike other common aquaculture systems, ponds provide many of the resources needed to grow aquatic animals in one self-contained unit. The main resource is phytoplankton that use solar energy to drive photosynthesis. Phytoplankton produce new organic matter, generate oxygen as a byproduct of photosynthesis, and assimilate carbon dioxide, ammonia, and other mineral nutrients from the water. Algal photosynthesis provides three essential resources for aquaculture production: 1) potential food for cultured animals, 2) oxygen to support life, and 3) treatment of wastes so that they do not accumulate to toxic levels. Relying upon sunlight to drive photosynthesis to maintain water quality represents the lowest cost and most sustainable approach to fish or shrimp production, which explains the popularity of ponds as aquatic animal production systems. However, utilization of solar energy comes at a cost and with certain limitations. The capacity of ponds to treat wastes— and, therefore, the upper limit to aquaculture production—is ultimately limited by the finite energy available from sunlight and the relatively low photosynthetic efficiency of algae (only 1 to 2 percent of incident solar energy is converted to chemical Partitioned Pond Aquaculture Systems Craig S. Tucker, David E. Brune and Eugene L. Torrans energy stored in algal biomass). Aquaculture production per unit area in algal ponds is therefore significantly lower than that achievable in systems using energy subsidies (in the form of feed) from externally supplied fossil fuels. Another consequence of low photosynthetic efficiency is that relatively large areas are needed for waste treatment relative to the water area (or volume) needed simply to confine the cultured animals (Sidebar). Aquatic animals moving freely within traditional aquaculture ponds are therefore distributed at much lower densities than in more intensively managed culture systems, leading to a number of management inefficiencies. Aquaculturists have proposed or developed alternative outdoor culture systems that attempt to address the limitations and inefficiencies of traditional aquaculture ponds. In this article, we describe our personal involvement with the development of some of those alternatives. These systems have one design feature in common: the water body is physically divided into areas that allow better control of certain processes, such as confining fish, producing oxygen, treating wastes, or culturing secondary species. Because various ecosystem functions are physically separated, we call these systems “partitioned ponds.” Aquaculturists have proposed or developed alternative outdoor culture systems that attempt to address the limitations and inefficiencies of traditional aquaculture ponds. Partitioned ponds are physically divided into areas that allow better control over confining fish, producing oxygen, treating wastes or culturing secondary species. SPATIAL REQUIREMENT FOR FISH CONFINEMENT AND WASTE TREATMENT Channel catfish can be grown in raceways at biomass densities exceeding 135 kg/m3, assuming that water flow is sufficient to provide dissolved oxygen and remove wastes. In a 1-m-deep pond, 135 kg/m3 is equivalent to holding 1,350 kg of catfish in a 10-m2 area. Consider this as an estimate of the ‘living space’ needed by pond-grown catfish. The amount of ammonia produced by fish can be estimated from feed consumption, ammonia production rate, and fish biomass. If 1,350 kg of fish are fed 2 percent of body weight per day and ammonia excretion is 35 g N/kg of feed consumed, Ammonia production = (35 g N/kg feed)(27 kg feed/day) = 945 g N/day. Most ammonia produced by fish is initially assimilated by phytoplankton as a nitrogen source for growth. The nitrogen assimilation rate by phytoplankton can be estimated from the rate of carbon fixation in photosynthesis and the average ratio of carbon to nitrogen in algal tissue (6C:1N by mass). Phytoplankton photosynthetic rates in warm, unmixed, nutrient-rich waters range from less than 1 to more than 6 g C/ m2 per days. Using 3 g C/m2 per day as an optimistic carbon fixation rate provides an estimated nitrogen assimilation rate of (3 g C/m2 per day)(1 g N/6 g C) = 0.5 g N/m2 per day. Therefore, the pond area required to remove ammonia excreted by 1,350 kg of channel catfish is (945 g N/day) ÷ (0.5 g N/m2 per day) = 1,890 m2. The pond area needed as living space for 1,350 kg of channel catfish (10 m2) is about 200 times smaller than the pond area needed to remove ammonia produced the fish (1,890 m2). (CONTINUED ON PAGE 10)
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