3.1 Freshwater Production Systems
3.2 Mariculture Production Systems
3.3 Production Facilities
3.4 Species Cultured
3.5 Extent of Rural Aquaculture
3.6 Social and Economic Aspects
3.7 Environmental Issues
3.1.1 Pond fish culture
Rural freshwater fish culture in China mainly refers to pond fish culture. In 1997, pond fish culture area reached 2 million ha (not including paddy field culture area), with an output of 8.9 million mt, accounting for 72.1% of the total freshwater aquaculture production. Technological system for pond fish culture is basically the ancestors' traditional Chinese fish farming technology refined and improved through knowledge and experience gained from research and development efforts of last five decades. The system has the following outstanding features:
i) Rearing short food-chain fish: Fish reared in China is largely herbivorous or omnivorous. Their food chain is very short. Fertilizer, grass, wastes from farm products processing industry can be used as fish feed. So their feed sources are abundant and wide, and culture cost is low and resulting in good economic benefits.3.1.2 Culture-based fisheriesii) Self-sufficiency in seed production: Now over 20 species are being artificially bred. Hatchery produced seed of all the major culturable species are available in almost all rural areas where aquaculture is an important activity. Culture activities therefore can be done in a planned way and in accordance with demands.
iii) Mixed and polyculture with high density: Several species are cultured in the same pond. This experience was explored and accumulated by the Chinese fishfarmers in a long period of practical production activities. Different species of fishes are cultured in the same pond according to their biological characteristics, food and feeding habits, and the water column inhabited by different species, so as to fully utilize the water space and the available natural feed to maximize production per unit area.
iv) Integrated culture: Pond fish culture is carried out by integrating it with other farming activities such as livestock rearing (chicken/duck/pig/cattle etc.) and/or crop cultivation/horticulture (vegetables, mulberry, fruits etc.). It is a comprehensive and integrated method of production with fish culture as the main activity and growing different kinds of crops, cash crops, grass as feed, and raising livestock and poultry on and around the pond banks. Fermented waste of farmed animals can be used as fertilizer or as fish feeds, the sludge from the pond bottom can be used as quality fertilizer for crops on land, and crops and grass can be used as feed for farmed animals and fish. Such integration allows best uses of all available resources and results in higher household income for the farmer.
v) Water quality management: For healthy growth of fish, the pond water quality is maintained at the optimum level by balancing the pond ecosystem. This is done through carefully managed feeding regime, water inflow and outflow, and aeration.
Fish culture in large water body, such as lake fish culture, began in the Western Han Dynasty in China (206 B.C.-A.D. 24), but the Dongqianhu Reservoir built in 744 is the earliest record of reservoir for fish farming. However, community fish culture in large water bodies began in the 1950s. At the beginning, it adopted stocking and extensive culture method, which resulted in enhancement of natural resources and improved the productivity of water bodies. Fish species and number for stocking are determined according to size and type of water body. For example, the ratio of fingerling stocking in fertile lakes is generally silver carp and big-head carp 80%, grass carp and black carp 5%-10%, common carp, crucian carp and bream 10%-15%, etc. At the same time, 0.5 kg crab seed and 0.5 kg eel fry can be stocked per 20 ha water area. Fish screen facilities and daily management must be stressed in such water area. Owing to the fact that large-sized fingerling is required for stocking in large water body, this gives an impetus to rearing of fingerlings in surrounding areas and has encouraged the development of a complete and separate production system. For the purpose of tapping the potentials of lake and reservoir fish culture, net enclosure and net cage cultures are gradually developing in large water bodies depending on their suitability.
3.1.3 Cage culture
The Chinese modern net cage culture started in the early days of 1970s and major efforts were devoted to developing and extending the method in 1980s. Net cage culture is now being used in lakes, reservoirs, rivers, ditches and shallow sea water and can be largely divided into 4 types: (1) to use natural feed (planktons) to culture large-sized silver carp and big-head carp fingerlings for stocking large water bodies; (2) to use natural feed (planktons) to culture table fish; (3) to use artificial feed to culture table fish; and (4) to culture high value species such as eel, mandarin fish, sea bream, etc. through high density intensive feeding system. Net cage culture in developed countries is usually an intensive culture system, but the culture system in China is either traditional or semi-intensive.
3.1.4 Paddy-fish culture
Paddy-field fish culture in China has a history for more than 2 000 years. In the early days of 1970s, China carried out extensive research on ecology and biology of culturing fish in rice-fields. This led to the development of various methods of rice-fish culture, based on a symbiotic relationship between fish and rice cultivation, leading to increased economic benefits to the farmer. The major types of rice-fish culture are as follows:
(1) Raise fingerlings in paddy fields in the plains: The fry are directly reared in early rice fields and after they have grown to 4-5 cm they can be transferred to the semi-late rice fields to grow them to market size by the time the rice is harvested. This is the easiest and most effective way which brings the maximum benefits to fishermen.The above-mentioned culture methods are extensive, though in some cases supplemented feed are given to the fish.(2) Planting rice on a ridge and rearing fish in a ditch: The best specifications are a ridge - containing two rows of rice seedlings and a ditch of one meter in width and one meter in depth. Some farmers even plant melons and soy-beans on banks and rear duckweeds in water. And thus a kind of multi-layer planting and culture mode is established.
(3) Rearing fish in a wide ditch: Wider ditches of varying sizes are prepared in the rice-field and are connected with the water inlet for fish culture. The total area for fish culture may vary from 5-10% of the rice-field.
(4) Culturing fish in an adjacent pond which is connected with the rice-field. The fish pond serves as a water reservoir for the rice-field.
(5) Another way of rice-fish culture is to cultivate rice and fish in rotation in the same field.
Before 1950s all mariculture systems in the country were extensive type. Baby fish and shrimps used to be trapped during high tides in ponds with sluice gates built on the mudflats, and various types of substrates were used to help settle spat.
Successful research and developmental activities of last four decades resulted in the improvements and modernization of culture systems. Most oyster, mussel, shrimp and seaweed culture are of semi-intensive type, nowadays. This has been made possible by the success in artificial breeding of many marine species for rearing seed for stocking, in the development of formulated feed, in the health management of cultured organisms, etc.
Mariculture in China is generally divided into mudflat culture and shallow sea water culture. Mudflat culture refers to the culture which makes use of intertidal area. It has had a long history and places stress on culture of oyster, constricted tagelus and bloody clam. Mudflat culture methods are as follows: (1) planting seeds of shellfish in mudflat directly, such as constricted tagelus etc; (2) using stone, bamboo pole etc, as an attachment to collect seeds and spats such as oyster, gracilaria, and cultured in mudflat; (3) pond building in mudflat for prawn and fish culture. In 1997, the mudflat culture output made up 50% of the total mariculture output.
Shallow sea culture is mainly employed in the places near gulfs and islands, where water flows smoothly; water quality is good and there are shelters against stormy waves. By using racks, floating rafts, floating ropes etc., kelp, laver, gracilaria, undaria, oyster and mussel are cultured. Also, fish culture is carried out in net cages.
Rural aquaculture facilities in China are very simple in those districts where farmers give priority to agriculture and aquaculture is only their sideline production. For example, in paddy field fish culture the most basic facilities are paddy fields and their connected ditches for water to flow in and out, sluice gates; and in mariculture are earth dykes and bamboo wooden sticks, etc. But the professional fishfarmers, who culture fish in ponds, use machineries for digging ponds and clearing away sludge, equipment for increasing oxygen supply to the pond water, as well as machinery for preparing feed and for harvesting.
For water supply and draining of ponds, different kinds of general-purpose water pumps are mainly used. In some small districts where pond culture is not very well developed, water supply and drainage are carried out by small pumps for individual pond. However, if pond culture is well developed with clusters of ponds, a pump station is used to pump and convey water through distribution pipes to the surrounding ponds. Heavy machineries such as bulldozers and dredgers are used for excavation/desilting works.
Also used are locally made aerators of various kinds such as impeller, paddle wheel, sprinkling and jet flow type are used to supply oxygen to the pond waters.
The fishfarmers also use locally made simple machines for crushing snails and clams, greenfeed cutting machine used for cutting aquatic plants into small pieces or as a palp for feeding fish or for using as a pond water fertilizer. Machines for making pellet feed in the small fishfarms are also used. For harvesting, winch and rope winding pulley are usually used for drawing the harvesting net from one end to the other end of the pond.
The old structures of production such as production team, production brigade and people’s commune disappeared and therefore large production units no longer exist. With the promotion of production-related contract responsibility system, the rural aquaculture is basically run by households. In most cases, each household contracts a pond covering about one ha. Small-scale lakes and reservoirs are mainly contracted by several households.
Freshwater fish culture mainly refers to 8 species: common carp, silver carp, big-head carp, black carp, grass carp, crucian carp, freshwater bream and mud carp. Owing to the improvement of culture technologies, and through introduction and domestication, controlled breeding, and hybridization, fish species of economic value that can be cultured now have reached nearly 50 species, of which over 20 species are cultured in ponds. In addition, crab, shrimp, soft-shelled turtle, pearl shell are also important varieties in rural freshwater aquaculture. The major species of freshwater aquaculture are listed in Table 4.
Before 1950s, rural mariculture was mainly limited to a few species of shell fish cultured in mudflat, such as oyster, bloody clam and constricted tagelus. After 1950s, with the advancement of science and technology, cultured shell fish species list now includes mussel and scallop. Also, marine seaweeds, prawn, crab and fishes are widely cultured (see Table 5).
In the past 20 years, aquaculture yield has been increasing year after year, from 1 221 792 mt in 1978 to 20 276 988 mt in 1997, a rise of 15.7 times. It is the period in which aquaculture developed at the fastest speed. Freshwater aquaculture yield increased from 762 254 mt to 12 366 559 mt, a growth of 15.2 time and that of mariculture increased from 449 538 mt to 7 910 429 mt, an increase of 16.6 times. Aquaculture yields over the years are shown in Table 1.
The outputs in different provinces are shown in Fig. 2. In 1997, in freshwater aquaculture output, Guangdong, Hubei, Jiangsu and Anhui each exceeded one million tons, so did Shandong, Fujian, Guangdong and Liuoning in mariculture output.
The steady expansion of culture areas was the main reason for steady increase in aquaculture yield. In 1997, aquaculture area of the country reached 5 892 770 ha, of which mariculture area was 937 933 ha, and freshwater aquacultural area was 4 954 837 ha, a growth of nearly twice as against the total cultured area in 1978. Mariculture area increased by 8.3 times and freshwater culture area increased by 82%. In freshwater aquaculture, pond culture area was 1 994 208 ha; lake, 879 661 ha; reservoir, 1 567 973 ha; river, 370 977 ha; and others 142 018 ha. Compared with 1978, pond increased by 76%; lake by 65.6%; reservoir by 29.7%; and river by 41.8%. The expansion of aquaculture area in the recent twenty years is shown in Table 6. Marine aquaculture area is shown in Table 8. It can be seen that in the recent fifteen years from 1983 to 1997, the areas for shallow sea water culture, bay and gulf culture, and mudflat culture were expanded by ten times, seven times and over three times respectively. Because aquaculture can reap profits more quickly, a large number of farmers engaged themselves in aquaculture. Up to 1997, full-time labourers engaged in aquaculture reached more than 3 million. In addition, several hundred thousands of rear-service labourers and several millions of part-time labourers constituted a huge contingent of producers. On average, there was nearly one labourer working on each ha culture area. Per unit output of different kinds of water areas varied greatly. In 1997, per unit output of freshwater aquaculture is listed in order of output: pond, 4 474 kg/ha; river, 1 623 kg/ha; lake, 921 kg/ha and reservoir, 743 kg/ha. Per unit output of marine aquaculture is listed in order of output: shallow sea, 22 155 kg/ha; mudflat, 6 288 kg/ha and bay and gulf, 2 683 kg/ha. This reflects that the per-unit output of rural aquaculture was still not very high. However, a steady growth in production has been maintained by bringing in more and more ha under various culture systems.
Fish is in the dominant position in freshwater aquaculture. Silver cap and big-head carp outputs rank first, making up 37.2% of the total output of freshwater aquaculture, followed by grass carp 21.3%, common carp 14.2%, crucian carp 6.9%, tilapia 3.9%, etc. In marine aquaculture output, shellfish forms an extremely large proportion, amounting to 82.3%, followed by algae 12.2%, fish 3.2%, and prawn and crab 2.1%. To be more exact, oyster output before 1996 was calculated in terms of fresh meat only. However, since 1996, it has been calculated in live weight, so the output figure has increased greatly. In 1997, oyster occupied first place with an output of 2.32 million mt, accounting for 29.4% of the total marine aquaculture; clam was 15.9%; scallop, 12.8%; mussel, 5.1%; and prawn, only 1.33%. The yields of marine and freshwater aquaculture in terms of species are shown in Table 7. And aquaculture output by environment, species and per hectare yield are shown in Table 8a-8e.
In China, the production relations in rural aquaculture are similar to those in agriculture. During the early years of New China, the government led fishfarmers to the socialist road of collectivization step by step, to develop collective ownership economy. By 1957, fishery cooperative economy system of single public ownership of means of production was established in the country. In the second half of 1958, people's communes were set up. Generally, there was one commune in a township. Several advanced fishery producers' cooperatives were merged into one fishery commune, under which there were production brigades and production teams. Because people's communes were too large in scale, it was divorced from the productive and management level of that time, and production enthusiasm of the masses was seriously affected. “Large and Public”, the basic characteristics of people's communes, and equalitarianism in distribution gravely destroyed productive forces. During 1959-1961, there was a serious shortage of food; people enclosed and filled up some lakes and pond areas for food crop production in many places. Fishfarmers' rations were reduced, and a serious shortage of fish feed made aquaculture wilt. In 1961, the pond culture area was only 470 266 ha as against 633 600 ha in 1957, a decrease of 25%; the area of lake culture came down from 672 000 ha in 1959 to 391 333 ha, a decrease of 42%. The production of freshwater aquaculture decreased from 596 000 mt in 1959 to 315 000 mt in 1962, a decrease of 47.1%. Marine culture area dropped down too year by year. In 1959 it was 102 200 ha, and in 1962, there was only 49 833 ha, a decrease of 51.2%. Cultural Revolution beginning in 1966 further emphasized “Large and Public” of people's communes, and stressed that large and medium-sized water area must be run by the state or people’s communes; pond culture must be run by production brigades. Fish culture run by production teams were limited and fish culture by households were prohibited. Aquaculture production came to a standstill and 1969 the total production had not reached the level of 1959. In the following ten years, the total production only increased by 73.2%, an average increase of a little more than 50 000 mt/yr.
However, in 1978 with the introduction of new socio-economic reform policies, the “Contract Responsibility System” was introduced, which broke through the single public ownership and formed a new structure, in which state, collective, individual and private economies existed side by side. From then on, the organizational form of people's commune was abandoned and instead various forms of cooperation and operation which broke equalitarianism were adopted. The government encouraged competition among fishfarmers by carrying out different forms of contract systems. Water areas, mudflats owned by the state or collective were allowed to be run by individuals or households by contract, or jointly run. The government encouraged people to contract to develop large areas of coastal and inland unoccupied water areas and mudflats. Capital, labour, goods and materials, and technology could be regarded as share capital and people were permitted to draw dividends and employ helpers. At early stages of the contract system, the contract period was short. Recently, it has been extended to 40 years. All these mobilized the initiative of the masses to develop aquaculture. But contract and production responsibility system did not change the original system of property right. Therefore some time later, share holding cooperative system was promoted on a trial basis in order to resolve contradictions and abuses caused by the separation of ownership of means of production and right of running. This further stimulated the fishfarmers’ enthusiasm.
The state-run aquaculture enterprises still had a certain position in aquaculture production. In 1997, there were 1 010 aquaculture enterprises in the country with a staff of 80 664 and an output of 1 266 071 mt, making up 6.24% of the total aquaculture production of the country. Of them, mariculture enterprises were 169 with a staff of 10 322 and a production of 246 748 mt, accounting for 3.11% of the total; freshwater culture enterprises were 481 with a staff of 70 342 and an output of 1 019 323 mt, accounting for 8.24% of the total. Although the proportion of state aquaculture production was not big enough all along, the state-run enterprises played a very important role in demonstration, and supply of fish seeds. But unfortunately in 1997, over 400 state-run enterprises out of 1 010 suffered great losses in their business. Now, those state-run enterprises are undergoing reform. The majority of the enterprises are located in the countryside or in the outskirts of cities. Only a few enterprises are engaged in so-called industrial production, mainly in propagation of seeds, however there is little difference in the way of fish culture between the majority of state-run enterprises and fishfarmers. So, on the whole, the state-run aquaculture still belongs to rural aquaculture.
From 1959 on, China implemented centralized purchasing and marketing on aquatic products. The aquatic products produced by people's communes and production brigades were all sold to the state-run supply and marketing companies commissioned by the state except a little amount left for themselves. Purchasing and selling prices were fixed by the state. Black market transactions were strictly banned. In doing so, people's communes and brigades had no right to deal with their own products. As a result, they gradually felt no concern about their fruits of labour any more, and for a long time setting prices were lower than production costs. In view of this, fishfarmers could not maintain their level of production. This highly-centralized planned economy did not change until the implementation of the policy of opening to the outside world.
From 1978 on, the state stipulated that the fish farmers must sell the set quantity of their aquatic products to the government only at a fixed price and the rest were permitted to sell at a negotiated price in the free markets. In 1981, 21 species were controlled by the government and then further cut down to 8 species. The proportion of products which must be sold to the government was also reduced gradually except prawn, the most important culture products for export. However, in 1985 the seller quota system was completely abolished and prices of aquatic products were regulated by the market. The government encouraged state-, collective-owned as well as private enterprises and individuals to go in for increased production and free trading activities. Subsequently, fisheries production started to increase with the new free marketing systems and now fish farmers.
Now, supply and marketing formed a simplified commodity circulation system of multiple economic forms, diversified channels and varied management modes. The marketing system works like this: fish producers - wholesale organization in production areas - wholesale companies in marketing areas - retailers - consumers. Each dealer in the above link will have 10-20 percent profit margin. The price paid by consumers maybe double than that of the farm gate. The average retail price of crucian carp, common carp, silver carp and grass carp was 13.4 yuan/kg; 10.9 yuan/kg; 8 yuan/kg and 10.4 yuan/kg respectively in 1997.
In 1997, the amount of trade in aquatic products at markets in cities and counties reached 12.46 million mt, or the value of trade being 138.6 billion yuan, with a rise of 22.8% and 54.5% over last year respectively. Villages shared 50.6% and 45.2% respectively. The species commonly cultured in large water areas, such as silver carp, grass carp and common carp were dealt with chiefly by individual small retailers in periphery areas. In large and medium-sized cities, it was done mainly by state-run enterprises. In recent years, specially wholesale markets have been booming, greatly promoting the prosperity of aquatic products markets. In 1997, the amount of trade in consumer goods wholesale markets was 2.3 million mt, an increase of 25.4% over 1996. The system of auction have become major ways of business at many wholesale markets in some large cities with cold storage. In 1997, there were 963 service enterprises for aquatic products circulation with a staff of 78 435. State-run enterprises have solid strength, dominating in the whole aquatic products market, particularly in wholesale markets. They are the principal channels of aquatic products. Private enterprises are developing fast in aquatic products markets and hold a distinct advantage in retail markets for their low cost and flexibility in business. Recently, many private enterprises are gradually becoming wholesaler, which has made the competition in market place fierce.
According to the survey of some aquaculture units made in 1995, the economic benefits and production expenditures of various water areas and species groups are shown in Table 9a-9b. It can be seen from the table that the average income per labourer engaged in mariculture was more than 6 000 yuan, but a labourer engaged in kelp culture obtained an income as high as 16 189 yuan; followed by oyster, scallop, prawn and mussel in descending order. In freshwater culture, the average income per labourer of pond culture was over 6 000 yuan. Production of lake culture was low, the average income per labourer was less than 4 000 yuan. In paddy field fish culture, it was only 720 yuan, but this was an additional income for farmers, where paddy was the primary product.
3.7.1 Land and water use
China has vast seas and mudflat areas. The continental shelf within isobath 200 m water depth covers about 1.48 million km2. And it has a network of rivers, lakes, ponds and reservoirs with a total inland water area of about 17.47 million ha, almost one fiftieth of the land area. The annual rainfall is about 6 000 billion m3, of which 2 600 billion m3 forms the water resources of the country. The cultivable water areas are: shallow sea 1 622 560 ha, bay and gulf 180 550 ha, mudflat 797 000 ha and freshwater area 6 749 250 ha. These areas provide favourable natural conditions for developing aquaculture. In 1997, marine aquaculture area was 937 933 ha, which was about 36% of the cultivable sea water area; and freshwater aquaculture area was 4 954 837 ha, which was about 73.4% of the cultivable area. There still exist suitable areas for expansion of aquaculture.
Inland aquaculture is more closely linked with agricultural production. The utilization of water area and land is often affected by such factors as State policy, food supply and prices of aquatic products. At the beginning of 1960s, owing to tension in food supply and shortage in feed, the aquaculture areas of the country were reduced from 2.1 million ha in 1960 to 1.58 million ha in 1963, a decrease of 25%, and it was not restored to the level of 1960 until 1965. During Cultural Revolution, because undue emphasis was put on the slogan “To take grain as the key”, people enclosed areas of lakes or ponds for grain production, which resulted in a radical reduction of fish pond and lake areas. Hubei Province known as a land with thousands of lakes had 1 056 lakes of different sizes, with an area of 826 666 ha, and by 1976, there were only 236 666 ha left, a decrease of 70%. Water area of Dongting Lake in Hunan Province reduced from 435 333 ha to 266 666 ha. Fishing and fishculture were stopped in many lakes in Jiangsu Province and Anhui Province. Consequently, fish production decreased. For example, in Huanggang District fish production decreased from 6 500 mt at the beginning of 1960s to 2 350 mt owing to the enclosure of lakes. In the counties of Wuxing, Deqing, Yuhang and Haining in Zhejiang Province, which are the dense pond fish culture areas, about 3 300 ha ponds were filled up for planting crops, resulted in the production decrease by nearly 10 000 mt.
After the new policy of opening and reform was implemented, the aquaculture area has been tremendously expanded; the total culture area of 1997 increased by 100% and more over 1978, of which marine aquaculture area went up from 100 593 ha to 937 933 ha, a rise of 8.32 times. Freshwater aquaculture area grew by 81.9%. The expansion of aquaculture area varies with demand for fish and fish price. When price is high, some farmers gives up crops and vegetables planting, silkwarm breeding and mulberry growing, and turn to fish culture. When fish price comes down and brought less benefits than cash crops, they filled up ponds to do other things. The market-oriented economy is increasingly exerting a direct influence on the utilization of land and water for aquaculture. It is evident that competition among aquaculture, agriculture, water conservancy, land reclamation, hydroelectricity, navigation and tourism in the use of land and water is intensifying day by day.
3.7.2 Environmental impacts
China, a vast country with varied climatic zones, has the potential for all types of aquaculture production systems. The bigger part of the territory being in the north temperate zone, and a small part in the south being in the tropics, temperate species are dominant. The distribution and development of aquaculture are affected by the distribution of rivers, lakes, and precipitation. The Pearl River, and the middle and low reaches of the Yangtze River have a long history of fish farming. The Pearl River delta and the Yangtze River delta which have a network of rivers, lakes and ponds, are the most important regions for fish culture. Aquaculture is heavily affected by typhoons and cold waves. The season of typhoon lasts from July to September. The area from Liaoning in the north and Guangdong and Guangxi in the south can be hit by typhoons. In the areas where typhoon sweep down, the culture of kelp, shellfish, prawn, etc. are ruthlessly destroyed. Cold waves and strong wind in winter flow from the north to south now and then, and the temperature in the affected areas decrease by more than 10°°C within 24 hours. In the north of China and the Yangtze River basin, strong winds often come together with rain, snow and frost. But in the south, it is semi tropical. Temperature therefore determines the culture systems and the species to be cultured. As for example, an important species like tilapia, which can be cultured all year round in the south, cannot be overwintered in the outdoor in the north.
Along with the rapid development of industry and agriculture, people reclaim water areas for industrial use and cultivation. As a result, culture area is reduced. At the same time, sewage drained from factories and cities, and the application of large quantities of pesticide and chemical fertilizer not only deteriorate water quality, but also destroy culture areas. It is reported that in 19 000 km of 7 large river systems, the water in about 5 000 km is below the required water quality standard for fishculture. In some important culture areas, fish die due to pollution. According to incomplete statistics, in 1995, 510 incidents of water pollution affecting fisheries/fishculture occurred throughout the country, with 138 000 ha culture area and 1 160 km river were seriously affected, causing a loss of 560 million yuan. In the five years from 1991 to 1995, there were over 2 700 pollution incidents that had impacts on fishculture in the country, causing 764 000 ha culture area polluted, resulting in a loss of 350 000 mt aquatic products with a direct economic loss of 3.1 billion yuan. In Zhejiang Province, the losses due to pollution greatly exceeded 10 million yuan in 1996 and 1997. In the coastal area of Jiangsu Province, clams were seriously polluted, which led to the prevalence of hepatitis A in Shanghai.
The development of aquaculture has its positive effect on the environment. For example, paddy field fish culture can fully utilize water bodies, resulting in bumper harvests of both rice and fish. But on the other hand, development of aquaculture also has its unfavourable effects. High-density intensive culture of fish and prawn causes eutrophication of water area and produces fish diseases. Recently, along the coast, particularly in river mouth area, the frequency of red tide occurrence is becoming higher and higher, and the affected areas becoming wider and wider. In 1993, prawn disease broke out in coastal areas, causing decrease in prawn production from 206 866 mt in 1992 to 87 756 mt in 1993. Even now, the impact of the disease lingers on. In recent years, cage culture of fish, in some areas, has decreased mainly because of water pollution caused by net cage culture itself. Owing to the degradation of water environment, red tides increasingly occurred, causing high mortality of cultured animals or made them unconsumable. Economic losses every year because of pollution and red tides amounts to several billion yuan.
