Pond fertilization helps to increase the amount of natural food (phytoplankton and zooplankton) in the pond through the supply of nutrients which are either lacking or are deficient in the pond ecosystem. The feeding strategy for Indian major carps under extensive and semi-intensive pond culture conditions depends largely on the application rate of inorganic and/or organic fertilizers.
The success of a pond fertilization strategy is dependent on drying, tilling and liming of the substratum. After each production cycle the pond is dried, excess mud and silt is removed and the pond bottom is tilled and treated with lime. Fertilizer is applied after partial filling. Liming of the pond is usually done on the basis of soil type and pH and is applied to obtain a water pH of between 7 and 8. A suggested liming rate for carp culture ponds based on soil pH is given in (Table 4).
The optimum rate of fertilization varies from one region to another because of differences in soil chemistry and the concentrations of dissolved nutrients. The application of fertilizers should be repeated on the basis of soil and water chemistry and until a Secchi disc reading of approximately 30 cm is obtained.
Inorganic fertilizers enhance primary production in the pond while organic manures can in addition, directly stimulate higher trophic levels. Organic manures contain practically all necessary nutrients required for biological production, encourage bacterial growth and increase the effectiveness of many inorganic fertilizers by providing the necessary organic base. Organic manures are highly suitable for mrigal culture; besides their value as fertilizers, they represent an immediate source of food for the fish. The most notable disadvantages of organic manures are their low and variable nutrient content, the slow rate of release of nutrients that can only occur after microbial breakdown and the variable C:N ratios, resulting in unpredictable primary and secondary production. Ammonium compounds with a high C:N ratio are sometimes added to organic manures. A variety of agricultural wastes, dung of various farm animals (cow, pig, buffalo, sheep, horse, poultry and duck) and domestic wastes are used as organic fertilizers for carp culture ponds. The nutrient composition of major organic and inorganic fertilizers used in carp culture is summarized in Table 5 and 6, respectively.
Inorganic fertilizers usually contain fixed levels of nutrients. Due to the high solubility of inorganic fertilizers, the nutrients are readily available soon after their application to the pond and are rapidly utilized. The dose of nitrogen (N), phosphorus (P2O5) and potassium (K2O) usually varies from 100 to 200 kg N, 50 to 100 kg P2O5 and 25 to 50 kg K2O/ha/year on the basis of low, medium or highly productive ponds. The efficacy of rock phosphate has been tested in polyculture systems with common carp and Indian major carps and best results were obtained at an application rate of 100 kg/ha twice monthly (Sahu and Jana, 1996). It has also been suggested that the bioturbation activity of bottom feeders such as mrigal and common carp induced a greater release of phosphorus from bottom sediment, which resulted in higher plankton production (Sahu and Jana, 1996).
Fertilizer application rates and regimens depend on several factors, such as the management system (extensive or semi-intensive), stocking density, fish biomass, nature of the soil and type of fertilizers used (organic, inorganic or combination). A number of fertilization schedules (as practiced in different types of ponds) for carp culture are summarized in Table 7. However, the information provided in Table 7 is a very general. A number of experiments have been conducted in Bangladesh and India to determine the efficacy of different types of fertilizers (organic, inorganic or in combination) on production of mrigal (Govind, Rajagopal and Singh, 1978; Manjunatha, 1979; Dhawan and Kaur, 2002; Saha et al., 1978). Best growth of mrigal was obtained with cattle dung treatment (Manjunatha, 1979). The application of pig dung at a rate of 18 tonnes/ha/year has also been found to be effective in significantly increasing growth of mrigal and common carp in polyculture (without supplementary feeding) along with catla and rohu (Dhawan and Kaur, 2002).
Mrigal larvae (after absorption of yolk sac) are stocked at high densities (6.25 million/ha) into specially prepared nursery ponds for a period of two weeks until they reach 25-30 mm in length. Prior to stocking the nursery ponds are prepared by drying, removal of aquatic weeds, eradication of predatory and nuisance fish and are fertilized. An initial application of 10 000 to 15 000 kg/ha of cow dung, approximately 15 days before the release of the post yolk sac larvae, has been found to be the best and cheapest option for carp nursery ponds. Higher doses are less effective in producing zooplankton. Mohua oilcake can also be used as an initial fertilizer but at a lower rate (5 000 kg/ha). The application of 5 000 kg/ha of cow dung 7 days after stocking promotes sustained production of zooplankton. This procedure is adopted only when one crop of fry is to be harvested from the ponds. If two or more crops are to be harvested, nursery ponds may be fertilized with 2 000 kg/ha of cow dung 7 days before each subsequent stocking. Super phosphate mixed with cattle dung can also be used for fertilizing nursery ponds. The initial dose of fertilizer (kg/ha) applied on the day preceding stocking consists of 150 kg of super phosphate plus 50 kg of triple super phosphate plus 700 kg cow dung plus 700 kg of oil cake. Alternatively a 3:1 mixture of cow dung and super phosphate can be used at a rate of 555 kg/ha/year (Bardach, Ryther and McLarney, 1972). Higher inputs of inorganic fertilizers for advanced nursing of carp are economically attractive.
After 15 days, the fry are thinned out and transferred to fingerling rearing ponds which are also prepared more or less along the same lines as the nursery ponds. Fingerling rearing ponds are generally fertilized with cow dung at a rate of 11 230 kg/ha, 10 days before stocking. After 2 months the ponds are fertilized with ammonium sulphate plus single super phosphate plus calcium ammonium nitrate at a ratio of 11:5:1 at 690 kg/ha. After 3 months the fish have normally reached an adequate size for stocking into production into ponds.
Fingerlings are transferred to production ponds where they are stocked at various densities and ratios depending on the fertility of the pond and management techniques. Old ponds with excessive bottom deposits are de-silted and limed at a rate dependent on soil pH (Table 4). Fertilization practices vary greatly in production ponds. One of the many recommended fertilization schedules consists of a dose of 1 000 kg or more of cattle dung, 560 kg to 1 200 kg of poultry manure and 5 000 kg of green compost per hectare. However, the fertilization schedule has to be modified depending on soil characteristics. Cow dung ranging from 20 to 30 tonnes/ha/year may be required if organic carbon levels in the soil are low. The fertilizer is normally applied in installments. The first installment should be around 17 percent of the total quantity and is applied a fortnight before stocking, and the rest in 10 equal monthly installments. The use of inorganic fertilizers varies according to the available P and N in the soil. The standard combination of N:P:K (18:8:4) is generally recommended but this has to be modified according to the availability of N and P. After determining the total quantity of inorganic fertilizer required, it should be applied in 10 equal monthly installments alternating with organic manure. The use of fertilizer is stopped if algal blooms develop.
Apart from the above conventional fertilization schedules, a number of other fertilization and natural food production methods have been attempted. For example, the placing of bamboo poles into the ponds to increase periphyton production (Azim et al., 2002; Gangadhara et al., 2004). Integrated poultry, pigs or cattle cum fish farming systems are dependent on feed waste and animal excreta. The fish feed on spilled animal feed and also directly on fresh animal excreta. It has been estimated that in fish-cum-dairy farming three-four cows/buffaloes can provide sufficient manure and feed for a one hectare pond (one cow/buffalo excretes approximately 12 000 kg of dung and 8 000 litres of urine per year). Similarly, 35-40 pigs or 500–600 chickens or ducks are adequate to provide the necessary nutrients and feed per hectare. Under these conditions no other feed or fertilizer is applied to the pond.