Food and Agriculture Organization of the United Nationsfor a world without hunger
Fisheries and Aquaculture Department
Grass carp culture began in the areas along the Yangtze and Pearl Rivers in the southern part of China. Compared to common carp, the culture of grass carp started much later. According to historical records, the culture of grass carp was closely related to the will of the current governor. In the Tang Dynasty (618-904 A.D.), the family name of the emperor happened to be pronounced the same in Chinese as common carp, the only fish cultured then. The royal family prohibited common carp to be sold and killed by the people. Therefore, grass carp was chosen by the farmers as a substitute for aquaculture together with silver carp, bighead and black carp; this was because the seed of these fish were easily available in the areas along the Yangtze River and the Pearl River. The culture of grass carp remained relatively small in scale due to the dependence on the natural supply of seed. Success in induced breeding technology significantly promoted its culture. The fish has been introduced to more than 40 other countries; sometimes it is referred to as the white amur. About 10 000 tonnes/yr in 1950, the global production of farmed grass carp had reached over 100 000 tonnes/yr by 1972, exceeded 1 million tonnes/yr by 1990, and has been above 3 million tonnes/yr since 1999. China is by far the major producer (3 419 593 tonnes in 2002, 95.7 percent of the global total).
Grass carp is a native Chinese freshwater fish with a broad distribution from the catchment area of the Pearl River in southern China to that of the Heilongjiang River in northern China. It has been introduced to about 40 other countries and there have been limited reports about the natural populations occurring in those areas; for instance, a natural population exists in the Red River in Vietnam. It inhabits lakes, rivers and reservoirs. It is a basically herbivorous fish that naturally feeds on certain aquatic weeds. However, the fry/larvae feed on zooplankton. Under culture conditions, grass carp can well accept artificial feed such as the by-products from grain processing, vegetable oil extraction meals, and pelleted feeds, in addition to aquatic weeds and terrestrial grasses. Grass carp normally dwell in mid-lower layer of the water column. Comparatively, it prefers clear water and can move swiftly. It is a semi-migratory fish; the mature broodstock migrate to the upper reaches of major rivers to propagate. Flowing water and changes in water level are essential environmental stimuli for natural spawning. The fish can reach sexual maturity under culture conditions, but cannot spawn naturally. Hormone injection and environmental stimuli, such as flowing water are necessary for induced spawning in tanks. Gras carp grow rapidly and reach a maximum weight of 35 kg in the wild.
Various production systems are currently used for the culture of grass carp the major ones include semi-intensive and intensive culture ponds, and pens and cages in open waters.
At present artificial propagation is the major supply of seed for the culture of grass carp, although natural seeds are still available in some rivers of China. Seed collected from the wild is mainly used for maintaining the genetic quality of the broodstock. Broodstocks used for artificial propagation are usually raised in captivity from seeds from the wild or from breeding stations where good natural stocks are maintained.
Well-matured breeders are released into the spawning tank (round cement tank with diameter of 6-10 m and water depth of around 2 m) after being injected with inducing hormone (usually LRH-A). Water circulation is maintained throughout the spawning period.
Eggs are transferred to hatching raceways or jars, either manually or by gravity. Hatching raceways (which are round or ellipse-shaped structures) are commonly used for large-scale production. The width of the raceways is normally 0.8 m and the depth is 0.8-1.0 m. The inlets are mounted on the bottom of the raceways with openings in the same direction and at an angle of around 15° to the bottom, to promote water circulation. Screens are mounted on the inner wall for discharging water during the operation. Water can be totally drained out through the outlet on the bottom. Current flow is maintained during the hatching period to keep the eggs and larvae suspended in the water column.
In India, dry or wet stripping methods are used for the seed production of grass carp. Pituitary extract or synthetic agents such as ovaprim are used for induction (John Stephen Kumar, pers. comm. 2004).
Earthen ponds (usually 0.1-0.2 ha and 1.5-2.0 m deep) are used for the nursing of grass carp. Ponds are chemically cleared, normally with quicklime, to eliminate all harmful organisms after totally drying. The usual dose is 900-1 125 kg/ha.
Organic fertilizer, animal manure and/or plant wastes (‘green manure‘) is commonly applied to increase the natural biomass of algae and zooplankton 5-10 days before the stocking, according to the water temperature. The quantity of organic fertilizer used is usually 3 000 kg/ha for animal manure or 4500 kg/ha for green manure. Green and animal manures can be used simultaneously but the quantity of each should be reduced accordingly.
Monoculture is practiced in the nursery stage, with a stocking density normally ranging between 1.2-1.5 million/ha, depending on the length of rearing and targeted size. The nursery operation usually takes 2-3 weeks in China. Organic fertilization is carried out at frequencies and rates sufficient to maintain high pond fertility and therefore a good supply of natural food organisms (especially zooplankton) for the fish. The quantity ranges from 1 500-3 000 kg/ha once every 4-5 days for animal manure or green manure, depending on existing water fertility. Soybean milk can also be used as both direct feed and fertilizer to replace organic fertilizer in the nursery stage. The normal quantity is 3-5 kg (dry soybean)/100 000 fish daily. This usually means production costs are high. A paste-form of soybean cake or other by-products from grain processing is applied from the 5th day after stocking, usually at a rate of 1.5-2.5 kg/100 000 fish daily. A paste of water peanut, water lettuce and water hyacinth can also be used to replace the above-mentioned feed and fertilizers at the rate of 25-40 kg/100 000 fish daily. 0.5 percent of table salt needs to be added to the paste of water peanut to remove its saponin toxicity. Normal survival rates in nursery ponds are 70-80 percent, although it may reach over 90 percent under good management.
The fish usually reach the size of about 30 mm in length after 2-3 weeks of rearing. These are called summer-fingerlings in China and are ready for the fingerling rearing stage. Conditioning, through careful netting and holding the fish at high density for a while (several hours) is required before the transfer of summer-fingerlings to the fingerling pond. This practice is designed to fish tolerance to stress before they are transported.
Summer-fingerlings are not suitable for direct stocking in grow-out ponds; they need to be reared to the fingerling stage (13-15 cm in length or larger) first. The technique for fingerling rearing is rather different to the nursery operation, especially when grass carp are stocked as the major species. The major differences include the following:
In Vietnam, the rearing of grass carp before the grow-out stage is divided into two periods. Fry are first raised to 4-5 cm, with a stocking density in the earthen nursery pond of 200-250 fry/m². The rearing period is normally 1.5-2 months. Then the fish are further raised for about 2 months to a size of 12-15 cm at a much lower density. The fish is mainly fed with soybean powder, rice bran, maize powder and aquatic plants (Azolla sp.) after reaching 3 cm in body length.
The nursery rearing of grass carp in India is carried out in intensively fertilized ponds, adequately enriched with zooplankton and unicellular algae. Generally the survival of fry is about 70-80 percent in well-managed nursery ponds. In addition to the natural feeds developed, supplementary feeding with powdered groundnut oilcake and rice polishings or bran is also practiced (John Stephen Kumar, pers. comm. 2004).
The most commonly adopted ongrowing techniques for grass carp include polyculture in ponds and pen and cage culture in lakes and reservoirs.
Semi-intensive to intensive polyculture in ponds in China
For polyculture in ponds or pens, grass carp can be stocked either as the major species or a secondary species together with other carp species. The total stocking density is 750-3 000 fish/ha with a stocking size of 125-250 g. Aquatic weeds and terrestrial grasses form the major feed for grass carp in grow-out culture. Feeding commercial feeds such as pellets and by-products from vegetable oil extraction and grain processing are becoming more popular as a means of replacing aquatic weeds and grasses to save labour costs in pond culture. The yield of grass carp is usually 1 000-3 000 kg/ha, which accounts for 15-40 percent of the total production.
Intensive culture in cages in China
In intensive culture systems in cages, grass carp are usually stocked as major species. Cages are usually about 60 m², with a depth of 2-2.5 m. 250-500 g fish are stocked at 10-20/m³, depending on the targeted production. In addition, 30-50/m³ Wuchang fish (bluntnose black bream, Megalobrama amblycephala), are also stocked at a size of 80-125 g. Silver and bighead carp are also stocked at 1 percent of the total, as 'cage cleaners'. The fish are fed with aquatic weeds/terrestrial grasses and pelleted or other commercial feeds. The culture period is usually 8-10 months and the yield is normally 30-50 kg/m³. Grass carp usually account for 60-70 percent of the total production. Cage culture of grass carp through the use of commercial feeds involves relatively high production costs. Feeding efficiency is not always as high in cage culture as in pond culture so, where terrestrial grass and aquatic weeds are locally abundant, collecting them and applying them in cage culture usually requires less labour input as the transportation is limited.
Grow-out systems in other countries
The grow-out of grass carp is mainly conducted in earthen ponds and cages in Vietnam. Polyculture with other species (e.g. silver carp, common carp, rohu and mrigal etc.) is common. Grass carp may be stocked as either major or secondary species. Grass carp usually account for 60 percent of the total stocking density of 1.5-3 fish/m² (dependent on the level of intensity) in ponds and the fingerling size is 5-6 cm (mountainous areas) and 12-15 cm (lowlands). The stocking rate in cage culture is 20-30 fish/m³ but much larger fingerlings are used (normally 50-100 g). Grass carp are usually fed with terrestrial grasses, cassava leaves, banana stems and maize leaves in grow-out culture. Grass carp production usually accounts for 60 percent of total production (7-10 tonnes/ha) in ponds. The marketing size for grass carp is 1-1.5 kg and 1.5-2.5 kg in ponds and cages respectively.
In India, grass carp are cultured as an important species in pond-based composite systems consisting mainly of Indian major carps and Chinese carps. The grass carp stocking density depends mainly on the availability of aquatic weeds and terrestrial grasses but is usually 5-20 percent of the total. Aquatic weeds (Hydrilla, Vallisneria, Wolffia) and terrestrial grasses such as Napier grass and other hybrid grasses are the major feeds in grass carp farming. Normally, grass carp reach 0.5-1.5 kg in 8-10 months (John Stephen Kumar, pers. comm. 2004). The total production from such systems can reach 8-10 tonnes/ha/yr.
Grass carp can be reared with commercial feeds or natural food, such as aquatic weeds and grasses. They prefer relatively low fertility. Production is mainly limited by water quality. The commercial feeds used for grass carp are relatively low in protein (28-30 percent) and their raw materials include soybean cake/dregs, rapeseed cake and wheat bran etc. Aquatic weeds can be collected from natural water bodies. Terrestrial grasses can be grown on the pond dyke with organic manure.
Both selective and total harvesting are practiced for grass carp. Selective harvesting is usually conducted in the early morning (because temperatures are relatively low and for morning sales) during late summer and autumn. Individuals of marketable size are selected after netting (a single netting for each harvest). Total harvesting is carried out at the end of the culture period. Several nettings are usually carried out before total drain-down of the pond. All the fish are harvested at the end of the year, either for marketing or for restocking (individuals below marketable size) for the next production cycle.
Grass carp is normally sold live or fresh. A small quantity of the production is processed by ready-to-eat food stores; in this case the most commonly used processing method is deep frying.
The production cost of grass carp vary according to the culture practice used but are normally about USD 0.50/kg of fish produced. Feed costs comprise the largest portion of production costs.
Farmed grass carp are rather susceptible to various diseases. Major diseases and methods of control are listed in the table below.
In some cases antibiotics and other pharmaceuticals have been used in treatment but their inclusion in this table does not imply an FAO recommendation.
Suppliers of Pathology Expertise
Assistance can be provided from the following sources:
The global value of global grass carp aquaculture production was USD 2.92 billion in 2002, an annual expansion rate between 1993 and 2002 of 7.5 percent/yr. The slower growth rate in terms of value, as compared to volume, was mainly due to changes in the valuation of the Chinese RMB yuan against the US dollar.
The major producer of this species is China where, traditionally, grass carp are consumed fresh. Most of the production is marketed fresh, either as whole fish or as pieces. Very little production is processed. At the present time, grass carp is mainly a locally consumed product but some of those produced in Guangdong province (southern China) are marketed in Hong Kong. There is no specific data on the quantity of exported grass carp in Chinese statistical information. However, 41 798 tonnes and 4932 tonnes of live fish (species not specified) were exported to Hong Kong and Macao from the mainland of China in 2002, according to the national statistic yearbook of imports and exports of aquatic products. Grass carp must have comprised large proportion of this total.
Grass carp is a low price commodity that is affordable to middle and low income classes in China and other countries. There has been a slight decline in the price of grass carp in the past few years in China. Currently, retail prices are usually USD 0.7-1.0/kg. There are no specific regulations relating to the marketing of the grass carp because the fish is basically for local consumption.
Grass carp has a long history in aquaculture and is one of the most important species cultured in inland water bodies in China. There have been great efforts devoted to research on this species; the most important achievement has been success in the development of induced breeding technology. This ensures a constant supply of seed for large-scale farming. Another important aspect of research was the study of nutritional requirements and the development of cheap pelleted feed. As this species is easily susceptible to disease, there have also been a lot of studies on disease control under culture conditions. The best-studied disease of grass carp is Haemorrhagic Disease, which has a viral agent. Effective preventive measures, especially a vaccine have been successfully developed and applied. Culture techniques and models for pond, cage and pen culture have also been well developed.
After silver carp, grass carp currently has the largest production in freshwater aquaculture globally. However, the rate of expansion in China (by far the major producer) has been declining in the last several years. Due to the introduction of new species and changes in people's preferences, grass carp is getting less popular now. Chinese people still prefer to eat whole fish, but whole grass carp are a little too large for the small Chinese families (3 persons mostly) to consume in one meal. It seems that grass carp culture has more potential for development in other countries, especially developing countries. Its fast growth rate, large size, lack of fine inter-muscular bones and, most importantly, feeding habits make the fish an ideal species for culture in these areas. Rapid expansion of its culture outside China may imply that this great potential is being realised. However, appropriate processing technology is required for the fish to enter international markets.
Grass carp not only grow quickly but have a low requirement for dietary protein. They can be produced at low cost by feeding them with aquatic weeds, terrestrial grasses and by-products from grain processing and vegetable oil extraction. Seed can be produced through induced breeding at a large scale and very low cost. The culture of grass carp can be well integrated into crop farming and animal husbandry, to maximize the utilization of natural resources. On the other hand, it is a large fish without fine inter-muscular bones. It is acceptable to consumers in many countries and it very likely has good potential for development. The market for grass carp is close to saturation in the eastern part of China, where aquaculture is well developed now. However, there is still a considerable potential market in central and western China and many other developing countries.
Pond based polyculture of grass carp does not have much negative impact on environment. The integration of grass carp - grass cultivation - pig rearing is an ecologically sound production model. However, large-scale intensive culture of grass carp with commercial feeds in cage/pen in shallow open-water may pollute the environment by discharging various wastes, which might accelerate the process of eutrophication. Besides, grass carp is more easily susceptible to some diseases. Poor management in fish health might results in extensive use of different chemicals and drugs, which may affect the quality of the fish and pollute the water at the same time. For the convenience and reducing labour input, farmers are using more and more pellet feed in grass carp culture in pond and cage/pen in open water. Wasted feed and discharge of nutrients may cause adverse impact on the environment.
Several issues need to be addressed in considering responsible aquaculture practices for grass carp culture:
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