The use of organic fertilizers or manure in aquaculture is an ancient practice and, despite its drawbacks, continues to be used by aquaculturists as an efficient and economical means of increased production in aquaculture ponds, a concept close to organic farming. In fact, the limited research that has been carried out on the subject has served to highlight its advantages. Some of the relative advantages of organic fertilizers in aquaculture are similar to those demonstrated in terrestrial farming. They improve the soil structure and fertility, particularly in newly constructed ponds deficient in deposits of silt and other organic substances. They promote the growth of zooplanktonic organisms, which form the nutritious and preferred food of many aquaculture species. Manures often facilitate the utilization of chemical fertilizers, when appropriate fertilizer practices are adopted. Proper manuring generally has a longer-lasting effect on pond production. In rural areas of most Third World countries, organic fertilizers are more easily available and relatively less expensive to use. When pond culture is integrated with crop and animal production, as will be described, efficient recycling of farm wastes and overall production economies become possible.
One of the major problems in the use of organic fertilizers is the extreme variability of composition. In the case of animal manures, the quality depends not only on the animal species, but also on the nature of its food, handling, storage, climatic conditions, etc. Another problem is the effect of over-fertilizing with organics. Rapid decomposition of the manure, and the increased bacterial population associated with it, can result in oxygen depletion and fish mortality. Over-manuring is also believed to promote the incidence of diseases.
The most commonly used organic fertilizers are animal manure, green manure, composts and domestic sewage, besides organo-chemical manures. The composition of fresh manure from a number of animal species is quoted by Martyshev (1983) from the Agronomist Hand-book on Manures (in Russian), apparently
based on conditions in the former USSR (Table 7.16). The nutrient contents of different animal manures used in China are shown in Table 7.17 (FAO, 1977). The most common manure in use in China is undoubtedly from pigs and the composition of pig manure is given in Table 7.18 (FAO, 1977). In South Asia and Israel, cow dung is commonly used for pond fertilization. Its composition usually ranges from 78–79 per cent water, 0.5–0.7 per cent N, 0.1–2 per cent P and 0.5 per cent K with an organic matter of 17 per cent and C : N: P ratio of 17 : 1 : 0.2 (see also Table 7.16).
Animal manures applied in a pond in fine particulate or colloidal state stimulate hetero-trophic growth of bacteria by providing the necessary surface area for their attachment and facilitating their mineralization. The mineral fraction is also directly available for photosyn-thesis, but apparently because of limited light penetration in manured ponds, phytoplankton production is reduced. However, the production of zooplanktonic organisms is generally more rapid and they feed on the nannoplank-ton and bacteria produced in the pond. Hetero-trophic production is at a maximum level at the soil/water interface in fish ponds and this is conducive to abundant benthic growth. In brackish-water ponds, growth of benthos is of special significance as many of the species grown in such ponds are primarily benthic feeders.
Various methods of application of animal manures are adopted. The traditional way of scattering or dumping them in heaps in ponds is not very commonly practised now. Manure may be applied on the dried pond bottom, particularly in new ponds that have little siltdeposition. The widely recommended method is frequent (if possible daily) application of manure in an easily dispersible form, such as liquid cowshed manure or powdered poultry waste. In large ponds it will be possible to disperse manure from mesh baskets towed around from a powered boat. It is also possible to mix the manure with water and disperse it from a small work boat. Diluted manure can be applied uniformly on the pond surface with a pump fitted to the boat. Different doses are suggested, but it is difficult to determine the most suitable one because of the many unknown variables, including the precise composition of the manure, the organic content of the pond and the climatic conditions. Since no standardized treatments are available, dosages have to be worked out in each area, based on properly monitored trials. Woynarovich (1975) suggested the application of about 1 ton of organic manure per hectare per year in fresh-water ponds that are deficient in organic matter at the bottom. When distributed at the bottom before inundation, it can be expected to contribute to the development of a rich bottom fauna. In Eastern Europe the best application rate is reported to be 5 tons/ha in stagnant or mildly flowing water. In China, animal manure is often composted with plant materials before application. Where it is used directly, most farmers these days keep it in fermentation tanks for a few days before introduction into fish ponds. The rate of fertilization is about 3.7 tons/ha per year, but in addition they may also apply 60kg crushed plant material and other agricultural or processing wastes (FAO/UNDP, 1979). In southern parts of China, a high rate of 5.6–10 tons/ha per year, in three applications, was reported by Tapiador et al.(1977).
Composts made from cow dung and green plants are generally used in the former USSR for fertilizing nursery ponds. Special water-proof pits are dug, in which layers of green grass are placed, alternated with layers of dung. The compost pile is covered with a layer of unslaked lime (about 70 kg), flooded with liquid dung manure and covered with earth. The usual ratio of green plants, dung and liquid manure is 4 : 2 : 1. For every 100 kg plants, 100 litres of water are added so that the decomposition process is accelerated. Such compost is applied at the rate of 7.5 tons/ha. It is thoroughly sieved in an 8mm mesh sieve to remove all undecom-posed materials and then mixed with water and applied from a boat at the rate of 3000– 3500l/ha on the first day, followed by one-quarter to one-fifth of the quantity on the second day. The remaining liquid is diluted and applied twice a day in the morning and evening.
In China, concrete pits are sometimes used for composting (fig. 7.4). The pits are usually circular, measuring 2.5 m in bottom diameter, 1.5 m deep and 3.0 m in top diameter (Delmendo, 1980). Each pit is filled with a layer of a mixture of river silt and rice straw (7.5 tons and 0.15 tons respectively), pig or cow manure (1 ton) and aquatic plants or green manure crops (0.75 tons) in 15 cm layers. The top is covered with mud and a water column 3–4 cm deep is kept at the hollowed surface to create anaerobic conditions and thus minimize losses. The compost is turned over in six to ten weeks, after which it is ready for use. In the first turning over, 20 kg superphosphate are added and thoroughly mixed with the organic material, adding water to ensure moist conditions. The chemical composition of the compost as a percentage of wet weight is 0.30 N, 0.30 P, 0.25 K and the organic matter 7.8 to 10.3. The carbon/nitrogen ratio is 15–20 : 1 (FAO, 1977). Compost is applied at the rate of 5–10 tons/ha in three applications (the first being the largest) in six to eight months of the fish rearing period.
In certain parts of South Asia, composting is done in a corner of the pond in bamboo enclosures. Plant matter such as leaves, grass cuttings and aquatic vegetation is composted in layers about 30cm high with 7.5 cm layers of manure in between and dusted liberally with super-phosphate and lime (Hora and Pillay, 1962). For a rapid decomposition of vegetable matter, 25 kg nitrate of soda to about 1000 kg compost have to be applied. To maintain the humidity of the compost heap, water is sprinkled on to it. The compost has to be turned at intervals of about five weeks. When composted in the pond corner, farmers often expect the manure to diffuse into the pond, but this is a long process.
It has to be removed from the pit and spread in the pond. Further, it has the disadvantage that anaerobic decomposition may produce methane which may accumulate in the pond. Generally, a rate of 5 tons of compost per ha of pond area is applied in small lots. But, as indicated earlier, the dosage varies very considerably. A very important pre-condition for the effective use of compost, or for that matter any fertilizer, is the absence of a dense growth of macrovegetation. The fertilizer can easily be utilized by such vegetation, stimulating their rapid growth and consequent choking of the pond. As a result the growth of microorganisms will be minimized.
As will be discussed in integrated farming of ducks and fish the droppings of ducks fall directly into the ponds from duck houses built on the ponds, or are washed into the ponds from enclosures on the banks of the ponds. Fresh duck manure contains about 57 per cent water and 26 per cent organic matter. On average, 100 kg contain about 10 kg carbon, 1.4kg P2O5, 1kg N, 0.6kg potash (K2O), 1.8kg calcium and 2.8 kg other materials (Woynarovich,1980). Chicken manure is also an efficient fertilizer and contains a high percentage of organic matter (26 per cent) with a low water content (56 per cent). It is high in nitrogen (1.6 per cent), P2O5 (1.5 per cent), K2O (0.9 per cent) and Ca (2.4%). Fresh as well as dry powdered chicken manure has been used very successfully for live food production in fish ponds.
Human sewage is also used for fertilizing aquaculture ponds in some countries. It has been a traditional practice in China and has been adopted in parts of India, Malaysia, other Asian countries and Europe, experimentally or on a small scale. Even in China and Hong Kong, however, the system of using raw sewage is gradually disappearing. Night soil is now stored in properly designed closed fermentation chambers for a four-week period to destroy pathogenic organisms before application in ponds. Another method is to subject sewage to anaerobic digestion in a biogas plant, to kill parasitic micro-organisms. In some biogas plants a mixture of night soil and animal manure is used together with 10–15 per cent grass and crop residues. The mixture may some-times have 10–30 per cent night soil, but the percentage can also be much less, as low as 10 per cent. The carbon/nitrogen ratio varies between 1 : 15 and 1 : 25 and the ratio of solids to liquid between 1 : 15 and 1 : 20 (FAO, 1978).
In certain parts of East and North India, the use of treated sewage for fertilizing fish farms is expanding, particularly in areas near urban centres where the disposal of sewage is an everincreasing problem. Treatment generally consists of sedimentation, dilution and storage. Sedimentation may be done in two stages: a primary stage to settle most of the heavier solids and a secondary stage to increase mixing and homogenization as well as improve natural purification processes. It has been estimated that about 33 per cent of the BOD can be reduced by the sedimentation process. Before application, the sewage is diluted with water to maintain a proper dissolved oxygen balance. During storage, microbic digestion occurs, reducing pathogenic organisms if present in the sewage.
Green manuring is another method of pond fertilization which is adopted in certain areas, particularly where the pond soils are of poor quality. Leguminous plants which fix atmospheric nitrogen are probably the best green manure. Some farmers grow such plants as an alternative crop on dried pond bottoms. After they are fully grown, the plants are cut and ploughed into the soil before the ponds are filled with water. This improves the fertility of the soil and water very considerably. Green manuring is also sometimes done to increase the nitrogen content of the water, using different types of plant matter.
Although the value of live foods in meeting the nutritional requirements of aquaculture organisms is well recognized, the practical problems of maintaining a steady supply of adequate quantities of the required foods at appropriate times are often too difficult to solve. It is not easy to ensure the quality and quantity of natural food to provide the essential components such as proteins, carbohydrates, lipids and vitamins, in the required proportions. The quantities of food needed to feed the increasing biomass of the cultured organisms vary so much with climatic and hydrobiological conditions that it becomes almost impossible to regulate food production to synchronize with requirements through known fertilization or management practices. Because of this, and aquaculturist has to resort to artificial feeds for intensified production. Depending on the nature of the culture operations and local conditions, he has to select ‘supplementary feeding’ or ‘complete feeding’ with artificial feeds.