Although not yet well understood scientifically, integrated and diversified crop-fish-livestock production systems evolved on empirical knowledge, have a history of over two millennia in parts of South and Southeast Asia. With limited farm area and poor resources available to the small farmers of these regions, such integrated systems, based on generations of experience, provide a balanced diet and often a small marketable surplus.
Shallow water in the rice-field has a lower oxygen content and higher turbidity/temperature than the water in ponds. The fishes which commonly occur in this ecosystem are mud-dwellers which can survive and thrive well in shallow muddy water. Many of them, e.g., Trichogaster, Anabas, Channa, Clarias, Heteropneustes, possess accessory respiratory organs to breathe oxygen from the air. Some of them breed in the rice-field, while others enter the field through tidal action, monsoon, flood or via irrigation canals. Shrimps and fish become trapped in the field, and in this way rice-field fisheries have evolved. In many places, these remain by and large capture-oriented. However, in many parts of Asia, true cultivation of fish is encouraged in rice-fields.
The physico-chemical conditions of soil and water in the rice-field affect the growth of rice more or less in the same way as that of phytoplankton - the first chain of the aquatic food cycle. However, there is a sharp difference between the mechanism of the two types of primary production, i.e., nutrient uptake in the rice is mainly through the root system, whereas in the phytoplankton it is through the whole surface and thus is much faster. Similarly, in submerged or floating weeds, the nutrient uptake is normally faster since the nutrient uptake occurs directly from the water. Thus, both these groups are direct competitors with rice for nutrients, and also to some extent indirect competitors for oxygen in the night. Further, while phytoplankton gives rise to zooplankton, zootecton, insects and molluscs, the submerged and floating weeds which harbour these organisms also adversely affect rice in various ways. In order to control excessive growth of undesirable groups of plants or animals, suitable species of fish are an essential component of the rice-field ecosystem. While controlling pests, the fish also fertilize the field through their faecal matter. Further, mud-dwelling fishes in search of food turn over the soil, making available more nutrients and oxygen to the roots of rice, thus acting like a biological plough.
In order to facilitate fish culture in rice-fields, the farmers make water retention structures, which also ensure storage and conservation of water, favouring rice growth. These may be circular moat-like structures, trenches, ponds or ditches, depending on the configuration/topography of the land.
The wet land of rice-fields is congenial to fish both as a spawning ground as well as a pasture. Many species which breed in rice-fields have adhesive eggs which are laid on green plants. This facilitates oxygenation of eggs by the green plants in the day time; oxygenation is also enhanced both during day and night by rippling of the water. Shallow-water and nest spawners also have favourable breeding conditions in rice-fields.
Rice-fields act as pastures for those fish which come naturally through flood, tide or irrigation water. However, modifications in the system of rice-field fisheries have been made; farmers have started introducing fry/fingerlings of easily-bred fishes for on-growing as long as water is retained in the field.
Man's use of rice-fields for fish production can now be of three types:
harvesting the wild (naturally occurring) fish crop;
harvesting the fish crop after a certain interval of time provided for growth, such as trapping of shrimp and fish gaining entry into the field through communicating water courses; and
for raising fry of desirable fish species to fingerling size, or fingerlings to marketable size, according to the availability of water in the field.
Thus, fish culture in rice-fields is undertaken as:
a second crop after the single annual crop of rice;
an intermediate crop between the rice harvest and the next plantation; and
a concurrent crop with the growing rice.
Rice-cum-brackishwater fisheries
Traditional culture of brackishwater shrimps and fish in rice-fields is quite prevalent in West Bengal and Kerala in India. In West Bengal, water level in the irrigation canal near the rice-fields is maintained about 30 cm below the level of the fields until the outbreak of the southwest monsoon (June/July), when the fields are manured and rice seedlings planted. In August, the water level in the canal rises because of the accumulation of rain water, and consequently its salinity decreases. The dikes surrounding rice-fields are then cut in places to facilitate entry of wild fry into the fields, where they are reared during the rice cultivation period. They are cropped before the harvesting of rice. The catch mainly consists of Mugil parsia, M. tade, Rhinomugil corsula, Lates calcarifer, Macrobrachium rosenbergii, M. rude, Metapenaeus monoceros, M. brevicornis and Penaeus semisulcatus. The yield varies from 100 to 300 kg/ha/year. In the Malabar area of Kerala, cultivation of rice is restricted to a single crop from July to September, when the surrounding brackishwaters are low in salinity. In October, after the harvest of the rice and during high tides, millions of fry enter with the tidal water into the fields, where they are trapped and reared until December. The catch consists approximately of 80% of Penaeus indicus, P. semisulcatus, Metapenaeus monoceros, M. dobsoni, Macrobrachium rude, and Palaemon styliferus; the rest of the crop comprises mullets, pearl spot and chromides.
In the lowland brackishwater area of the Solo River in Indonesia, rice-cum-milkfish ponds are constructed by digging circular moats inside the rice-fields. These are irrigated by a combination of rain and water from the irrigation canals.
Rice-cum-common carp culture
Common carp cultivation in rice-fields is highly developed in Japan, Indonesia and China. In Indonesia, the field is prepared for the rice crop in July by cleaning the field, repairing the dikes and tilling. After this, 7-day-old carp fry are stocked during flooding of the field at a stocking rate of about 30 000 fry/ha; the fry are harvested in late July after 20 days. In August, the second batch of fry is stocked after 3 days of rice transplantation. The fingerlings are harvested in September after 37 days. Again, after the first weeding of the rice in September and 3 days of fertilization, about 6 000 fingerlings/ha are stocked; these are harvested after about 60 days in October. The field is then drained, weeded, fertilized, and again flooded and stocked with advanced fingerlings (smallest fish from the third harvest) at 1 000– 2 000/ha. These are harvested after 3–4 weeks in November as a fourth crop of fish, when the field is drained and rice harvested. The field is then left fallow for one month.
In China, common carp and golden carp were originally used in the rice-fields, but more recently silver carp (Hypophthalmichthys molitrix), bighead (Aristichthys nobilis), snakehead (Channa sp.), tilapia, etc., have been cultivated. The deep-water method of culture is undertaken in the northern provinces of Szechwan, Kweichow, Kwangsi and Hupeh; rice-fields may have as much as 0.5 to 0.8 m water wherein the growth of the fish is about 250 to 500 g within 3 months; the yield varies from 35 to 75 kg/100 m2. The shallow-water method is used in the southern provinces, where the depth of water in the fields may be 6 to 8 cm only. In order to allow the fish adequate space to swim and hide, several ditches are dug in a criss-cross pattern, each about 20 cm wide and deep with 0.6 to 1 m deep pockets. Normally, fingerlings of carps and tilapias are stocked into the fields about a week after transplantation; they are reared there until after the harvest. The rate of stocking varies from 3 000–9 000 fingerlings/ha depending on the fertility of the soils. At certain places, fishes are fed intensively. In Kwangtung province, 2.7 ha of rice-fields have produced about 4 000 kg/ha and 5 230 kg/ha of rice from the first and second crops respectively, together with 937 kg/ha of fish.
Cultivation of other fishes in rice-fields
Puntius. In Indonesia, raising of 10 day-old fry of Puntius gonionotus is undertaken when the field has been ploughed and harrowed once. Stocking density varies from 40 000 to 80 000/ha and fish are grown for about 1 to 1.5 months, with the provision of feed varying from occasionally to twice a day. Fry are also cultivated after the rice plantation. The rate of stocking then varies from 120 000 to 150 000/ha, and the rearing period is about 4 weeks until the first weeding. Large fry are obtained when raised for 3 weeks between the first and second weeding at a stocking density of 10 000–15 000 fish of 3–5 cm length per hectare. For cultivation of big fish, the large fry (3 to 5 cm) are stocked at the rate of 2 000–3 500/ha. These fishes stay in the moat during the second weeding, and are reared for 80 days until rice flowering. Mixed stocking of common carp and Puntius is also undertaken 5 days after planting. The stocking rate is about 1 000 fry/ha of each species. The culture period lasts for about 60 days.
Osteocheilus. Production of fry of Osteocheilus hasselti in rice-fields is undertaken in Indonesia after the rice has been harvested; the average depth of water in the field is about 30 cm. The stocking rate is about 150 000/ha of 7 day-old fry; the period of rearing is 30 days; during this period 6–8 t/ha of green manure or 3–4 t/ha of farm manure is applied to the field. However, when they are to be reared together with rice, fry are stocked 1 month after rice planting. The stocking rate is then about 25 000/ha of 3–5 cm fry; the period of rearing is 9–11 months, after which the field needs to be tilled in preparation for the next planting period.
Tilapia. Culture of T. mossambica is done in Indonesia between two crops of rice or along with rice. The stocking rate is about 2 000/ha of 4–7 cm fry; fish are harvested periodically after 35 days. The next crop of Tilapia is raised at a stocking density of about 2 000/ha of 8–12 cm fish, grown for 20 days. Mixed stocking of common carp and Tilapia is sometimes done between two crops of rice or along with rice; stocking rates for Tilapia are about 1 200/ha of 8–12 cm size and for carp 450/ha of the same size. Manuring is done with nightsoil, farmyard manure and horse dung.
Trichogaster. In Thailand, Trichogaster pectoralis is mainly cultured in disused rice-fields, modified to form 5 ha ponds by constructing a peripheral ditch and an enlarged dike to maintain water depths greater than are necessary for rice production. The peripheral ditch, 3 m wide and 80 cm deep, is filled with water, and adult Trichogaster of 100 g selected from the previous harvest are stocked at the rate of one pair of spawners for each 16 m2 of water surface. The pond is then flooded to a depth of 50 cm over the central platform area. Subsequent spawning results in a fry density of approximately 185/m2. Emergent vegetation in the central part of the field is mowed and applied as green manure. Water is changed, but the fish are not fed. Harvesting is done after 6–7 months of growth when the fish reach about 100 g; fish production is about 0.7 t/ha.
Helostoma. Rice-fields are used for spawning of Helostoma temmincki in between two crops of rice in Indonesia. Normally, about 100 kg of green manure and about half this amount of farmyard-manure is used per hectare. Thirty pairs of Helostoma, each weighing about 200 g, are kept in about 10 ha of rice-field. In about 40–50 days, approximately 100 000 fry of 1–2 cm are obtained. When the Helostoma fry are 20 days old, 2–3 cm goramy fry are stocked and fed with fine bran.
Osphronemus goramy. The fry of goramy are produced in Indonesia in rice-fields which retain 40–50 cm of water. In a field of about 10 ha, 2 000 2–3 cm fry are reared, fed with pupae of white and red ants; fry of 5–8 cm are produced in about 2 months.
The manure value of pond mud is usually considerable. A large quantity of nutrient remains locked in the sediment and not utilized in the rapid nutrient cycling of the water column in fish ponds. Thus, pond mud may be scraped/removed and can be profitably utilized for horticultural or other agricultural crops. Removing the soft sediment by some mechanical/manual/biological means also improves the productivity of the pond. Further, irrigating terrestrial crops with pond water is much better than using running water, since impounded water is more fertile and adds nutrients for the growth of plants.
An example of the pond dike system of integrated farming is to be found in the Pearl River Delta in China. The nucleus of the system is the pond; the excavated soil provides material for dike preparation and renovation. Before being filled with water the pond is prepared for fish culture with lime and tea-seed cake. Under natural conditions, the pond bottom is enriched by silt and organic matter through dike erosion and the organic load of the water. This process is interrupted 2–3 times a year when the pond mud is used to fertilize crops on the dike and to build up the top of the dike. Pond mud is also used to make mud beds for mushroom cultivation on the floor of the silkworm sheds in winter, when silkworms connot be raised. Mulberry and sugar-cane, planted in alternate years, are the main crops. Mulberry cuttings planted on the dikes are fertilized with pond mud and irrigated with nutrient-rich pond water. Mulberry provides leaves for silkworms and bark to make paper; pruned branches are used as support for vegetables and also as fuel. The silkworms reared in sheds provide cocoons for yarn production; the cocoon waste and dead larvae provide feed for the fish and enrich the fish pond. The silkworm excrement is also used in the ponds as fish feed. As indicated above, mushrooms are cultivated on mud beds on the floor of the silkworm rearing sheds during the off-season for silkworm production. After raising mushrooms the nutrient rich mud bed is used to fertilize those sections of the dike used for vegetables, fruit trees and grasses. Small groves of bamboo are also a fundamental part of the system. Sugar-cane, some of which is either annually or biennially rotated with mulberry, is another essential sub-system providing young leaves to feed fish and pigs, old leaves for shading crops and for roofing thatch, and roots for fuel. Besides sugar for human consumption, refinery wastes are used for animal and fish feed. Pigsties constructed on the dikes provide faeces and urine which fertilize the pond.
The rearing of ducks near or in the pond utilizes the pond surface. Thus on the one hand fish-cum-duck culture increases production of animal protein from the same unit area, and on the other it offers a profitable solution to the problem of disposal of duck droppings. However, ducks do prey upon fry and small fingerlings, and therefore only large fingerlings should be stocked in this culture system.
Under sub-tropical and tropical conditions, the effect of duck droppings is far better than in the temperate region. The average annual production of fish in a fish-cum-duck farm in the Taiwan Province of China is about 3 500 kg/ha. Management techniques involved include polyculture, repeated stocking and repeated selective harvesting of fish, and high density stocking of ducks at 500–1 500/ha. In such ponds the duck droppings produce abundant fish food organisms, and are also utilized directly by fishes as feed. The left-over duck-feed washed into the pond is eaten by fishes, and thus high fish production is obtained. In Nepal, a 1 ha fish pond can produce about 1 t of fish and 1 t of ducks in a year. Fish-cum-duck farming in India has been tried at Krishnagar with successful results. Normally, under this system no supplementary feed is supplied to the fish, nor are fertilizers applied to the ponds. Droppings from 200 to 300 ducks are adequate for fertilization of 1 ha water with 2–3 m depth, when composite fish culture is applied. The management techniques of this system of culture are similar to those of general carp farming, except that the person responsible should have basic knowledge of duck husbandry. Fish production of over 4 t/ha/year has been obtained with fish-cum-duck culture at Krishnagar, India. While China, Hong Kong, Viet Nam and Indonesia commonly raise ducks with fish, other countries in the region have also successfully introduced this system of integrated farming. The high productivity of Laguna de Bay in the Philippines is partly attributable to the duck population around it.
Raising poultry and fattening pigs in combination with fish farming is becoming popular in many countries. Fish culture combined with pig raising is a traditional system is China. In Taiwan, tilapia culture in combination with pigs or ducks has been popular since 1972. Because of low prices for rice, and high labour costs, many farmers found rice farming unprofitable. They therefore converted about 5 000 ha of their rice fields into fish ponds, and built pigsties or duck houses beside the ponds. It is estimated that the excreta from 50–70 pigs, with or without fermentation, can supply adequate fertilizer and feed for fish in a 1 ha pond.
It has been found in China that 50 kg of pig manure produces 37.5 kg of green fodder, which when fed to herbivorous fish yields 2.185 kg of fish. However, when 50 kg of pig manure is directly applied to a fish pond, it yields only 1 kg of filter-feeding fishes. Therefore, pig manure is first utilized for production of English rye grass, which is subsequently used to feed the herbivorous fish.
