In India, three different types of ponds are used for carp culture, viz. nursery ponds (0.02–0.05 ha, water depth 1.0–1.5 m), fingerling rearing ponds (0.05–0.1 ha, water depth 1.5–2.0 m) and grow-out ponds (0.1–2.0 ha, water depth 2–3 m).
Nursery ponds are used to rear larvae (5.0–6.0 mm) for a period of two weeks at a stocking density of 2.5–3.5 million/ha. At a length of 25–30 mm fry are transferred to the fingerling rearing ponds at a density of 0.2–0.3 million fry/ha, in which they are grown up to 80-100 mm for a period of 2–3 months (Jhingran, 1991), before they are transferred to grow-out ponds. Liming and manuring are two essential steps for pond management. Suggested guideline for liming of pond based on pH and texture of pond soil is given in Table 4. The sequential steps generally adopted for carp culture in India are summarized in the box above.
Phosphorus (P) is the most limiting nutrient for the maintenance of pond productivity in freshwater, while nitrogen (N) is the second primary nutrient element. The nitrogen: phosphorous ratio should be 4:1 in the nursery pond. The application of N:P:K fertilizers (18:8:4) at the rate of 500 kg/ha shows good results in the preparation of nursery ponds and also in short-term rearing ponds with slightly acidic (pH 6.0-6.2) and sandy soil (Jhingran, 1991). A variety of organic and inorganic fertilizers are used for pond manuring in India (see Table 5).
Nursery, fingerling and grow-out ponds have different fertilization regimens (Table 6). In nursery ponds, lime is applied at the rate of 250-300 kg/ha. Organic manure, such as cattle dung, is applied at a rate of 10 000 kg/ha about 15 days prior to the introduction of the larvae (Jhingran, 1991). In the case of perennial ponds, mahua (Bassia latifolia) oilcake is applied at the rate of 2 000 – 2,500 kg/ha to eradicate unwanted fish and other biota, where after cattle-dung is applied at 5 000 kg/ha. Mahua oilcake contains 4–6 percent saponin, which causes haemolysis of red blood cells. During the 1st 14 days it serves as a fish toxicant and then serves as organic manure. Alternatively a combination of mustard oil-cake, cattle-dung and poultry manure (6:3:1) at 11 000 kg/ha is also effective for the production of zooplankton in carp nursery ponds (ICAR, 2006).
Liming and fertilization of fingerling rearing ponds at regular intervals is essential for the sustainable production of fish food organisms. Lime is applied at the rate of 250-300 kg/ha in 3 equal monthly instalments. The first dose is applied one week prior to stocking of fry (Jhingran 1991). Both organic and inorganic fertilizers are applied. Cattle-dung is recommended at a rate of 2 500 kg/ha in 4 equal instalments. The initial dose is applied 14 days prior to stocking of fry. Alternatively, inorganic fertilizers like urea (100 kg/ha) or ammonium sulphate (200 kg/ha) and single super phosphate (100 kg/ha) or triple super phosphate at a rate of 35 kg/ha are applied in 3 equal instalments during the rearing period. The first instalment of inorganic fertilizers is applied on second day after stocking and thereafter at monthly intervals, alternating with organic fertilizers (CIFRI, 1985a). Fertilization should be avoided during algal blooms in the pond.
For faster mineralization of organic matter and as a prophylactic measures, quick lime is applied at the rate of 400 kg/ha in grow-out ponds (Figure 4) (especially for alkaline soil). The first dose, 100 kg/ha is broadcast over the pond surface one week prior to stocking of fingerlings and the rest is applied at equal monthly intervals. A combination of organic manures and inorganic fertilizers is more effective than either of these alone. Fertilization schedules depend largely on the fertility of pond soil (Table 6). Initially, 20 percent of the organic manure is applied 15 days prior to stocking of fish and the remaining 80 percent is divided into 11 instalments and is applied at monthly intervals. Inorganic fertilizers are also applied in 11 equal monthly instalments (CIFRI, 1985c).
Bio-fertilization of freshwater fish ponds may be more economical for small farmers. Azolla, a free foliating water fern fixes atmospheric nitrogen in association with diazotrophic cyanobacterium (Anabaena azollae). The application of Azolla in fish ponds at the rate of 40 tonnes/ha/year fulfils all nutrient requirements for intensive carp culture (Reddy et al., 2005). Bio-processed organic manure or bio-gas slurry can also be applied at the rate of 30-45 tonnes/ha/year (ICAR, 2006). Use of bio-gas slurry has certain advantages such as reducing BOD and effecting a faster rate of nutrient release into the water column.
