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In the rice–fish ecosystem materials move in a benign cycle and the energy flows in the direction favourable to both rice and fish. The ricefields nourish the fish, and the fish nourish the rice. Like other theories, the theory of rice–fish mutualism has only been adopted slowly. The meaning of the word mutualism has, in recently years, been extended from its original meaning in classical ecology. It has now taken on the meaning of functional mutualism in addition to the original organization sense. A mutual relationship is one in which two different species live together and promote and accelerate their growth.

Although rice and fish are mutually beneficial, they are not totally dependent on each other. Their coming together is based on scientific principles and the anticipation of greater economic returns. As the system is further developed, and rice–fish culture is widely recognized as the best way to increase yields, their mutualism will become more of a necessity.

Chinese ecologist Ma Shijun said it is necessary to simulate mutualism of different species of plants and animals according to one's needs. The theory of rice–fish mutualism was founded on both conceptional and practical principles.

Rice–Fish Ecosystem

Ecology, in a direct sense, is a branch of science that studies habitat. It is, indeed, very closely related with the development of the national economy. In nature, animals, plants, and microorganisms come together to form a unified entity, or ecological system. The close relationship among animals, plants, and microorganisms and between these organisms and the environment, is made possible by the flow of energy and the circulation of material.

Ecological systems are both large and small. The biosphere is a large system; a ricefield or pond is a small ecosystem. In addition to these natural ecological systems, there are other ecological systems, such as the rice–fish system. All agricultural systems are, in fact, anthropogenic.

The nonbiological factors in the rice–fish ecosystem include light, water, water temperature, pH, carbon dioxide, oxygen, and some inorganic matter. The biological factors in the ecosystem include producers, consumers, and decomposers. The main producers are plants with roots and large and small phytoplankton. In another words, there are three categories of producers in ricefields: rice plants, weeds, and algae. They are all involved in the circulation of carbon through photosynthesis and respiration, and they provide organic matter to consumers and decomposers.

There are also many consumers. They include zooplankton (protozoa, rotifers, and crustaceans); benthos (nematodes, molluscs, annelids, and water insects); fish reared in ricefields (common carp, crucian carp, bighead carp, nile tilapia, and grass carp); mosquito larvae, insects, and worms harmful to rice; natural enemies of harmful insects and worms (spiders and parasitic wasps); and the natural enemies of fry (chilopods, scorpions, dragonflies, frogs, otters, water rats, eels, loach, water snakes, sandpipers, ducks, kingfishers, gulls, and egrets). Many animals are both primary consumers and secondary or tertiary consumers. For example, water snakes feeds on frog, frogs feed on fry, and fish feed on plankton. Many animals are harmful to rice but useful to fish, and vice versa. For example, although frogs feed on fry, they also feed on many of other insect and worms that are harmful to rice. The composition of the producers, consumers, and decomposers in the rice–fish ecosystem is complicated and merits further investigation.

Cycling of Material and Energy Exchange

To create a rice–fish ecosystem in a ricefield, it is necessary to pay attention to the appropriate time and size of the system to ensure that the rice and fish are truly mutually beneficial. Material must be made to circulate in a benign cycle and the energy flow must be in a direction favourable to both rice and fish (Figure 1). The rice–fish ecosystem is created by adding fish fry or fingerlings to the ricefield. In this system, the cycling of matter and the movement and storage of energy becomes more rational. The difference, compared with a natural ecosystem, is that the rice–fish system is controlled and adjusted by the farmer. Of course, to perfect such an ecological system, many improvements are needed.

In the rice–fish ecosystem, rice is the dominant biological community. It absorbs large quantities of light, carbon dioxide, water, and inorganic elements and manufactures organic matter by photosynthesis. The large quantities of weeds, plankton, and photosynthetic bacteria in the ricefield undertake the same processes. However, they do not provide useful products. On the contrary, they compete for fertilizer, space, area, and sunshine with the rice and in some cases are the intermediate hosts of rice pests. Of course, weeds and plankton are all primary producers that help fix and store energy. The primary consumers are mainly zooplankton, herbivorous animals, and plant pests. The secondary consumers are mainly carnivorous animals.

Fish in the rice–fish system can be primary consumers, secondary consumers, or tertiary consumers. This creates the problem of which fish to raise to make the system more efficient. It is also the leading factor that affects the density of other biological species and communities. Repeated experiments and comparisons have demonstrated that grass carp are the best fish to use for rice–fish culture.

In ricefields, grass carp eat a large quantity of weeds. There are more than 30 kinds of common weeds in ricefields. Eleocharis yokoscensis, Hydrilla sp., Potamogeton crispus, Vallisneria spiralis, Najas marina, Potamogeton distinctus, and Lemna minor are eaten by grass carp. In general, weeds can reduce rice yields by 10–30%. Therefore, if weeds could be totally eliminated, rice yields should increase by over 10%. Our experiments have indicated that early ricefield without fish have 13–15 times more weeds than fields with fish. When the fish are harvested there are about 33–435 kg/ha of weeds, but in ricefields without fish there are 450–6520 kg/ha of weeds when the rice is harvested, even when weeding is done three times. The weeds eliminated by grass carp (coefficient of feed 1:80) provide about 5 kg of fish output. Furthermore, fewer weeds are available to compete for fertilizer. This stimulates increased rice output, purifies the water, and improves the environment.

By eating the plankton, weeds, and benthos in the ricefield, the grass carp grow quickly. The more they eat, the more excrement they discharge. A grass carp (6.5-13 cm) is estimated to eat 52% of its own weight and to excrete 72% of the amount of grass it eats. If 400 grass carp are reared for 110 days, fish excrement amounts to about 26475 kg/ha. This excrement is rich in nitrogen and sulphide and therefore increases the fertility of the field.

In most cultivation systems, most of the weeds in the ricefield are pulled out and discarded. This causes a large loss of soil fertility and wastes the solar energy captured by the weeds. In addition, much of the bacteria, plankton, zooplankton, and part of the benthos, are usually discharged with the water. This accounts, either directly or indirectly, for loss of soil fertility and solar energy. From the point of view of circulation of matter and energy, this is a natural phenomenon that is unavoidable. However, from the point of view of maximizing bioproductivity, it is obviously a waste of matter and energy. The raising of fish in ricefields captures part of the matter and energy that would otherwise be wasted and transforms them into fish products. At the same time, the fish stimulate rice output. This is a very economical practice. It is desirable to continue to seek ways to improve the system and to strive for the highest possible yield using the least amount of energy and matter to produce the maximum economic returns.

The introduction of grass carp into the ricefield changes the composition of the biological species and communities and their mutual relationships. Grass carp and rice become codominant factors in the system.

In the rice–fish ecosystem, nonbiological and biological factors are important. Growth and development of rice requires light, heat, carbon dioxide, water, and nutrients. Of these factors, air, water, and nutrients undergo the most dynamic changes and exert an extremely large influence on the growth of the rice plants. For example, carbon dioxide is an indispensable raw material for photosynthesis. During the day, the amount of carbon dioxide in a ricefield with fish is higher than in a ricefield without fish. The fish respire carbon dioxide and feed on plankton that compete with the rice for nutrients. Generally, there is an increase of 1.5–8.2 mg/L (average 5.1 mg/L) in dissolved oxygen in ricefields with fish. The minimum level of dissolved oxygen at night is also tolerable for grass carp. Furthermore the fish tend to raise the dissolved oxygen content level because they stir up the water and increase the contact between water and air. The activities of the fish can also make the distribution of oxygen more uniform. Because they move the soil, the fish also improve the oxygen supply to the soil, which favours the breakdown of organic matter and reduced material in the soil. This is why many rice–fish fields that are not exposed to the sun and are not weeded still yield 10% more than fields in which fish are not reared.

Economic Returns

The first and foremost objective of raising fish in ricefields is to increase rice output while reducing the labour required for weeding. Rice yields are increased (by about 10%) in rice–fish fields. In addition, grass carp make full use of the water and feed provided by the ricefield, harmful insects and other rice pests are reduced, and the system retains and creates more fertilizer.

This new model of a rice–fish ecosystem is becoming increasingly popular. From 1980 to 1983, Hubei and Hunan Provinces devoted about 33 300 ha of ricefields to this new model. If the area devoted to raising fingerlings is assumed to be 2000 ha, the area for growing food fish 2670 ha, the average output of fingerlings 4875/ha, and the output of food fish 525 kg/ha, the two provinces produced 9.75 million fingerlings and 1.4 million kg of food fish with an output value of CNY 3.2 million. If the increase in rice output is assumed to be 10%, the added output would be worth CNY 1 million. The total value of the rice and fish would be CNY 4.2 million.

In 1984, China had nearly 0.7 million ha of ricefields devoted to rice–fish farming. This was an increase of more than 80% from 1983. The increase in rice output was estimated at 285 million kg and the output of fish at 47 000 tonnes. Sichuan Province, which leads the country, devoted 0.3 million ha to rice–fish farming, and Chongqing City alone reserved 77 330 ha for rice–fish farming. Hunan Province had 0.2 million ha for rice–fish in 1984, 33.7% more than in 1983. In addition, many households have reported collecting 7500 kg of rice and 750 kg of fish from 1 ha ricefield.

If these improved methods of raising fish in ricefields could be applied to 6.7 million ha over the next 3–5 years, the rice output could be increased by 2 billion kg and the catch of fingerlings would amount to 20–50 billion, an abundant source of supply to raise adult fish. This would help China reach the goal of producing 4–5 million tonnes of fresh fish each year.

Document(s) 38 of 41