In a well-prepared pond, natural production will provide a significant part of the postlarval shrimp’s nutritional needs and improve initial growth rates. This contribution of natural pond biota decreases over the course of the production cycle, with increasing shrimp size and biomass (for review, see Hunter, Pruder and Wyban, 1987; Allan and Maguire 1992; Palacios et al., 1994; Hunter 1996; Lawrence and Lee 1997; Moriarty 1997; Focken et al., 1998; Villamar 1999; Nunes and Parsons 2000; Gautier et al., 2001). Meiofauna are more abundant in a pond prepared with a good fertilization programme (Allan, Moriarty and Maguire, 1995).

Pond preparation between crops is an important part of the shrimp production cycle. The basic objectives of pond preparation are to (Suresh, Aceituno and Oliva, 2006):

Organic matter accumulation in shrimp ponds results in the deterioration of the pond ecosystem (Avnimelech and Ritvo, 2003). After harvest, the ponds are allowed to dry out and pond sediments are then removed (FAO, 2007b) (Figure 31).

Ponds should be dried for at least 7 (CIBA, 2003) to 14 d (Boyd, 1990, 1992). Diab and Shilo (1986) found that the microbial nitrification rates, which are low when the soil is flooded, increase in the drained soil. Air exposure oxidizes the black sludge within a few hours. This can be followed in the field through the colour change of the sediments from black to brown, with deeper sediments taking longer to oxidize. Where the pond cannot be dried, chemicals such as chlorine compounds and burnt or hydrated lime can be used on the wet areas. Wet areas can be treated with burned or hydrated lime at 0.25 kg/m2 or saturated chlorine solution at 1 litre/m2 to effect disinfection.

Treatment of wet patches with sodium, potassium, or calcium nitrate at 0.1 to 0.2 g/m2 also can enhance oxidation. Sludge can also be flushed from the ponds, but must then be directed to a settling pond for treatment and not released to the environment (Avnimelech and Ritvo, 2003; Suresh, Aceituno and Oliva, 2006).

Tilling and liming are often done at the same time. The purpose of tilling is to enhance the oxidation of organic matter by pulverizing the soil to increase the exposure of wastes to sunlight and air. Tilling the upper 5–10 cm of soil is most important, for waste from the previous crop accumulates in this layer (Suresh, Aceituno and Oliva, 2006). Liming should be performed when the pond bottom is still wet, because some moisture is required for microbial activity (Boyd, 1995). Lime is applied to regulate soil pH at 7.5–8.5 (Table 10). Liming can decrease phosphorus availability in ponds through the precipitation of low-solubility calcium and magnesium phosphate minerals. It is therefore important to leave sufficient time between lime application and pond fertilization. Shrimp farmers use lime during pond preparation to prevent disease (FAO, 2007b).

Pond filling and fertilization are usually done together. 200 µm mesh filters are used to remove pathogens and their vectors. A series of progressively finer filters installed between the intake point, reservoir and ponds is employed at many farms. In modern farms, sediment ponds provide initial water treatment, maintain water levels and provide a reservoir of water. Chlorinate with 25–30 mg of calcium hypochlorite per litre before use. Chlorine is usually applied at concentrations ranging from 3 to 30 mg/litre to eliminate Vibrio spp., Monodon baculovirus(MBV) and viral vectors, particularly wild crustaceans (Chanratchakool, 1995, Hedge, Anthony and Rao, 1996, Boyd and Massaut 1999). During sterilization, the pond is oxygenated using a paddle wheel for 2–3 h and then allowed to settle for 4–5 d. Chlorination is expensive, will also eliminate beneficial organisms (0.25 mg/litre adversely af­fects phytoplankton) and is not fully effective in killing all organisms (Husnah and Lin, 2002).

Tea seed cake and pesticides (e.g. rotenone at 20–25 mg/litre) are commonly used to eliminate fish in the ponds (Figure 32). Application rates for tea seed cake ranged from 150 (Minsalan and Chiu, 1986) to 300 kg/ha (Suresh, Aceituno and Oliva, 2006). The cakes are soaked in water overnight and the resulting liquid is applied uniformly over pond surfaces one week before stocking so there are no residual effects on the postlarvae.

Either organic or inorganic fertilizers are used (Table 11b). The aim of fertilization is to achieve a healthy bloom of zoo– and phytoplankton in ponds before stocking so that natural food organisms are available to the newly stocked postlarvae (Suresh, Aceituno and Oliva, 2006). Target nitrogen levels through fertilization range between 0.5 and 2 ppm and for phosphorus, between 0.1 and 0.2 mg/litre. The fertilizers available will differ regionally (see FAO, 2007b). As phytoplankton is 50 percent C, 9 percent N and 1 percent P, the ideal fertilizer ratio is a C:N:P of 50:10:1 (Fox, 2008b).

Various organic materials can be used as fertilizer (Table 11b). Research has found minimal risk from bacterial contamination (Salmonella and other human pathogenic bacteria) when using organic fertilizers (Dalsgaard and Olsen, 1995; Dalsgaard, 1998).

Organic fertilizers are widely used in shrimp pond preparation, with applications up to 1 000 kg/ha initially, followed by applications of 200–300 kg/ha at one to two week intervals (Boyd, 1989) or maintained thereafter with inorganic fertilization. Organic materials provide nutrients other than nitrogen and phosphorus, as well as substrate for the growth of microbial food organisms suited to the shrimp’s natural behaviour. Zooplankton feed directly on particles of manure, enabling a rapid increase in their abundance (Boyd, 1989). With their low inherent concentration of nutrients, organic materials are best applied together with inorganic fertilizers. 2.67 kg urea contains as much nitrogen as 100 kg of poultry manure.

Molasses is also used (50–100 kg/ha) as an organic fertilizer, as it provides a readily available source of carbon and stimulates the growth of beneficial bacteria. Molasses can be used as a tool to prevent an increase in ammonium and nitrite (Samocha et al., 2007). Erler et al. (2005) demonstrated that the addition of carbon, in the form of molasses resulted in greater growth of P. monodon and improved FCR. Molasses promotes heterotrophic bacterial growth that assimilates ammonia–nitrogen directly into cellular protein (Ebeling, Timmons and Bisogni, 2006).

Organic fertilizers also include feedstuffs and feeds: soybean meal, chicken feed, palm by-products (Gautier, 2002), cottonseed meal, rice bran, alfalfa meal and other processed grains or hays (Wurts, 2004).

Inorganic nitrogen and phosphorous fertilizers can be applied to shrimp ponds to stimulate algae growth to increase zooplankton production (Table 11a). 1 kg of urea and 1 kg of triple super phosphate (TSP) contain nitrogen and phosphorus equivalent to about 100 kg of chicken manure (FAO, 2007b). A wide N:P ratio is thought to encourage blooms of diatoms (Fox, 2008b). A typical ratio for brackishwater ponds is around 12–20:1, N:P. To calculate the amount of urea and TSP required to achieve 20 kg N/ha and 1 kg P/ha, urea is 45 percent N and TSP is 46 percent P2O5 (20.1 percent P). For a 10 ha pond, you need 444 kg urea and 108 kg TSP. Various fertilization applications are used, depending on available fertilizers and management preferences. In general, there is no need to fertilize after the biomass exceeds 300 kg/ha or feeding exceeds 30 kg/ha/d (Fox, 2008b) or the sechi disk reading is below 30 cm.