Chapter 5
POND CULTURE OF FOOD FISH

Fei Yingwu

The intensive pond culture of food fish is mainly carried out in small, man-made ponds with a water depth of 1–2.5 m. The fish are fed commercial food and stocked at a high density to achieve high, stable production. Chinese aquaculturists have summerized their long experience in intensive fish culture into eight main points: water (deep and flexible), seeds (good stock, healthy) feeds (fine and adequate), polyculture, density (high but renewable), rotation, prevention (preventing diseases and eradicating enemies) and management (elaborate). These eight principles (harvesting and stocking) have effectively accelerated the development of pond fish culture in China. In 1983, the total freshwater fishery production in China was 1,840,700 t; 1,428,100 t (77.6 per cent) of this was from cultivation. The total fish production from ponds in 1983 was 1,030,000 t or 72.1 per cent of the total freshwater fishery production from cultivation. The total area devoted to freshwater cultivation in 1983 was 46,239,700 mu (15 mu = 1 ha); culture ponds accounted for 14, 473,900 mu (31.3 per cent) of this area. Pond fish culture obviously plays an important role in the freshwater fisheries of China.

Rearing period refers to the time required to raise fish from the fingerling stage to a stage when the fish can be harvested. In China, rearing periods are determined by local conditions: climate, culturing methods, and market demand. Silver carp and bighead gain a substantial amount of weight in their 2nd year. By using proper culture techniques, silver carp and bighead fingerlings with a body weight of 50 g can reach 1–1.5 kg by the next Autumn. Black carp and grass carp grow rapidly during their 3rd year. A 2-year-old grass carp fingerling with a body weight of 0.5– 1 kg may reach 3.5–5 kg by the following year. The rearing period in Jiangsu, Zhejiang, Hunan, and Hubei provinces for silver carp, bighead, wuchang fish, tilapia, and crucian carp, is 2 years; for black carp and grass carp, the rearing period is 3 years. Fish-rearing periods are much longer than those in livestock and poultry production; therefore, the economic benefit is lower. To correct this situation, experiments and reforms are now being conducted to shorten the fish-rearing period.

Requirements and Renovations of Ponds

Pond conditions greatly affect fish growth and fish yield. Under favourable conditions, the yield may be 2 or 3 times higher than in ponds with unfavourable conditions.

Requirements of the Grow-Out Pond

Area

An area of 7–10 mu is considered optimal for an intensive culture pond. The fish have sufficient space for swimming and feeding and the water is adequately disturbed by the wind to prevent a shortage of dissolved oxygen and to regulate water temperature. In addition, the decomposition of manure and the propagation of plankton can be promoted.

Water supply and water quality

Fresh water should be added to the pond at regular intervals to adjust water depth, control water quality, and replenish the dissolved oxygen supply. The water source should be a river, lake, reservoir, or other large body of water because the dissolved oxygen content (DOC), pH, and temperature are likely to be stable and suitable for fish growth. The water discharged from factories and mines usually contains harmful chemicals, and should be avoided after fish culture.

Depth

The effective water depth varies with geography, climate, species, and culturing method. To fully utilize the pond and maximize yield, the pond should be as deep as possible. An increased volume of water stabilizes both water temperature and water quality and is thus beneficial to fish growth and allows the polyculture of various species. In Xihu, a village in the suburbs of Henyang, Hunan Province, and Helei, a village in the suburbs of Wuxi, Jiangsu Province, the common pond depth is around 3 m. In the winter and the spring, however, the water level is lowered so that the pond dikes can be used for fodder crop production. The average year-round water depth is about 2 m.

Bottom soil

Loamy soil is best at the bottom of a fish pond because it is effective in maintaining water level and fertility. In addition, the water will not become too turbid, the bottom silt will not be too thick, natural organisms will flourish, and operation and management will be relatively easy. Clay soil maintains water level and water fertility because of its low permeability; however, the water easily becomes turbid, the bottom silt is often too thick, and lot of nutritive salts are absorbed and cannot be used by the plankton. Therefore, a clay soil is not favourable for the propagation of natural organisms and not convenient for operation and management.

Pond environment

Rectangular fish ponds with well-formed pond dikes are recommended. The pond bottom should be flat and even to simplify rearing management and netting operations. Planting mulberry trees and crops onto pond dikes not only produces food for the fish but also protects the pond dikes from erosion. Large trees and buildings around fish ponds must be avoided; they would block the sunlight and hinder the winds. These two factors are essential in ensuring adequate water temperature, plankton growth, and DOC.

Pond Renovations

The old traditional fish ponds of Jiangsu and Zhejiang provinces were small and shallow with low dikes and poor water quality. Such ponds are subject to natural disasters and low productivity. In the early 1950s, high-yield grow-out ponds yielded only 300 kg/mu. As fish farming developed, the fish farmers discovered that the traditional fish ponds were unsuitable for achieving high and stable yields. All remaining old ponds should, therefore, undergo the following renovations.

Small ponds to large pond

There is a saying among Chinese fish farmers: “The broader the body of water, the larger the fish”. In the past, farmers believed the optimum area of a grow-out pond to be 4–5 mu. In such ponds, however, water quality is difficult to control and dissolved oxygen is quickly depleted in high-density polyculture. In larger ponds, environmental conditions are comparatively stable. As the wind blows over the larger surface, waves appear and the DOC is increased. Thus, the old, 4–5 mu ponds are now being combined to form large, 7–10 or even 15 mu grow-out ponds.

Shallow pond to deeper pond

Pond depth directly affects fish yield. A water depth between 2 and 2.5 m is favourable for conducting high-density polyculture. In deeper ponds, the water will not be turbid during the harvest. In addition, water temperature and water quality are more stable in deeper ponds. However, if ponds are too deep, the exchange of surface water and bottom water will be limited and toxic gases will accumulate at the bottom of the pond because of a low DOC. These factors would limit fish growth and the propagation of natural organisms, with a consequent reduction in fish yield. The depth of a pond depends on its area and usually, the depth is kept between 1.7 and 2.5 m year-round.

Stagnant-water pond to free-exchanging pond

To facilitate irrigation, drainage, and water-quality regulation, stagnantwater ponds should be converted to free-exchanging ponds by combining and connecting the ponds to a freshwater supply. A favourable environment is essential to obtain high yields in intensive fish culture.

Low-dike pond to high-dike pond

Pond dikes should be high enough to prevent flooding and wide enough to be used for planting crop or raising livestock.

Fig. 5.1. Stable, high-yielding fish ponds in the suburbs of Wuxi, Jiangsu, P.R. China

These four renovations completely change fish-farming conditions and allow the fish output to be increased step by step. For example, in Zhang-zhuang village of Suzhou City which is a traditional fish-farming area from 1949 to 1970, the average annual yield was 200–250 kg/mu. Since 1973, the old fish ponds have been renovated every winter, resulting in average yields of 500, 700, and 750 kg/mu in 1979, 1982 and 1983, respectively. In the Helei fishery village in the suburbs of Wuxi, since 1975, 345 small ponds have been converted into 172 large ponds. The yield is now up to 1000 kg/mu.

The general criteria for a grow-out pond with a high, stable yield are as follows: area of about 10 mu; water depth of 2–2.5 m; good water supply that is easily accessible; and high and wide dikes (Fig. 5.1).

Stocking and Polyculture of Fingerlings

Stocking of Fingerlings

The demands of food fish culture require that grow-out ponds be stocked with fingerlings of a variety of species, in adequate quantities, of appropriate sizes, and with no injuries or diseases.

Pond clearing

After 1 or 2 years of culturing, silt and organic matter accumulate on the bottom of the grow-out pond. This allows the propagation of various harmful bacteria. Fish ponds, therefore, should be cleared once a year.

Pond clearing is normally performed in the winter. Some of the silt is removed from the pond. This will not only improve the living environment of the fish and increase the capacity of the pond but also provide good-quality manure for agriculture. After removing the excessive silt, the pond bottom is open to the air for sunning and freezing. Chemicals can then be used to eradicate all the wild fish, pathogens, parasites, etc. After pond clearing, fresh water and manure are introduced about 1 week before stocking.

Manure application and pond filling

Manure application enriches the nutritional value of the water and promotes the proper proliferation of natural food organisms. It is important to maintain the food supply and maximize fish yields. After pond clearing, a base manure should be applied as early and adequately as possible so that enough natural food is available during the early stages of cultivation. The usual dosage of animal manure, compost, or fermented green manure is 500–1000 kg/mu. Manure is spread evenly on the pond bottom or beside the remaining water and exposed to the sun for several days. The manure could also be mixed with the pond silt. This would keep the water fertile a little while longer.

After the application of a base manure, the pond is filled with fresh water. The initial addition should bring the water level to about 1 m. When this water becomes fertile, more fresh water is added. Alternatively, the pond could be filled with fresh water at a rate dependent on temperature and fish size.

Selection of fingerling

The selection of good-quality fingerlings is important in ensuring high fish yields. Large good-quality fingerlings have many merits: strong adaptability, high survival rate, fast growth, short culture period, high marketing rate and economic returns, etc. There are three criteria for selection and purchasing fingerlings: physique, size, and movement.

Physique — Strong, healthy, normally shaped fingerlings are desirable. The fingerling should have plump muscles at the dorsal and peduncle region. Fingerlings should have complete scales and fin rays and smooth, bright-coloured skin.

Size — Fingerlings of the same age should be of uniform size (length and weight). Standards are listed in Table 5.1.

Movement — Healthy fingerlings will jump violently in the hand; poor finger-lings will not. When healthy fingerlings are put on a plate, they jump constantly without opening their gill covers; poor fingerlings only jump slightly with their gill covers open. When healthy fingerlings are placed in a net cage, they swim actively in groups with their heads downward and caudal fins upward; only their caudal fins can be observed on the water surface. Poor fingerlings swim slowly or alone.

Table 5.1. Standard body weight vs. body length of yearlings.

Disinfection of fingerlings before stocking

Disinfection of fingerlings should be conducted before stocking (see Chapter 6, Fish diseases).

Stocking time

Fingerlings should be stocked as early as possible. In the Changjiang River basin, fingerlings are usually stocked in early February when the air and water temperatures are low. At that time, fish are lethargic and, therefore, will not be easily injured during netting and stocking. The occurrence of disease and the mortality rate are also reduced. Earlier stocking also implies earlier feeding and a longer growth period. Early stocking should not be performed on rainy, snowy, or cold days to avoid the possible frostbite of fingerlings during netting and transportation.

Polyculture

Polyculture is a prominent farming technique in Chinese freshwater fish farming. Polyculture in China dates back to the Tang Dynasty (618–907), and, because of its long history, is more efficient than the polyculture in other countries. It is practiced at every rearing stage of fish farming (brood fish, fingerling, and food fish). High yielding grow-out ponds may now be stocked with a mixture from 8 to 10 species in different combinations of sizes and ages.

Advantages of polyculture

Polyculture facilitates full utilization of the natural food organisms in the pond water. There are three kinds of natural organisms (plankton, benthos, and epiphytic algae) and organic detritus in still-water ponds. Fish production can be greatly increased through polyculture of various species with different feeding habits: silver carp and bighead, which feed on plankton; grass carp, Parabramis pekinensis, and Megalobrama which feed on grasses; black carp, which feeds on snails and other benthos; common carp and crucian carp which prefer benthos and some organic detritus; mud carp and Xenocypris which feed on organic detritus and benthic algae; and the omnivorous tilapia. When these species are mixed, the natural food organisms are fully utilized and production is maximized.

Polyculture also allows the full utilization of the available space in the pond. The major cultivated carps occupy different habitats in the pond. Compared with monoculture, polyculture can increase both the stocking amount per unit area and fish output.

Beneficial interactions between the compatible species cultured in the same pond, become evident in polyculture. Under reasonable polyculture, all species are mutually beneficial. Thus, the production of each species is increased. Grass carp, black carp, common carp and wuchang fish are regarded as “feed eaters” or “food feeders”; silver carp, bighead, and tilapia are known as “plankton feeders”. In grass carp monoculture the pond water easily becomes too fertile; however, this can be counteracted by mixing silver carp and bighead which will feed on the natural organisms propagated by the manure of grass carp. The consequent decrease in pond fertility is beneficial to grass carp growth. Through the beneficial interactions between species, one kind of food can be “doubly utilized”. It is said that in polyculture “one grass carp can provide enough food for three silver carp through proliferating natural organisms.”

Polyculture raises the utilization rate of artificial feeds. Different species of various sizes will consume different sizes of feeds. This reduces feed wastage and improve water quality.

Stocking density

Stocking density also known as per-unit stocking amount or stocking rate, refers to the quantity of fry or fingerlings per unit of water area. It is usually expressed as the number of weight of fish per mu. In intensive fish-farming systems (such as industrialized fish farming, fish farming in flowing water, or fish culture in a net cage), the stocking density is expressed as the number or weight of fish per unit area (square metres) or water volume (cubic metres) because of the high stocking density and high utilization rate.

The stocking density must be reasonable because it is inversely proportional to the quality of marketable fish under the same pond conditions and culturing measures. Excessive stocking densities produce fish below marketable size; therefore, fish yields are not improved. If the stocking density is too low, the per-unit area production is also low, although the fish grow faster and reach larger sizes. A reasonable stocking density can ensure the desirable size and quality of fish products.

Pond conditions, seed supply, species availability, fish sizes, feeds, and operating techniques should all be taken into consideration in determining the stocking density. The data from the previous year (sizes, yields, survival rate, marketing rate, food-conversion rate, etc.) should be used to determine if a change in stocking density is required. If the fish grow well, the food-conversion rate is not higher than the average, no serious surfacing occurs during the culture period, and all species reach the marketable size at the end of production, the stocking density can be considered optimum. If some species of fish are not achieving marketable size and the food-conversion rate is high, the stocking density should be considerably reduced. Optimum stocking densities vary with the level of development of production. Therefore, the stocking density should be determined by local conditions to obtain a good harvest.

Apart from feeds and water space, water quality (DOC in particular) is the major factor affecting stocking density. The DOC in pond water is closely related to the growth and survival of the fish. Oxygen demand varies with species, age, sizes of fish and water temperature. For Chinese carp, DOC should be above 3 mg/L (Table 5.2). The optimum DOC is around 5.5 mg/L. The respiratory rate of cyprinids increases when the DOC is below 2 mg/L. If it continues to drop, the fish will break the surface gasping for air. Asphyxiation occurs from 0.1 to 0.8 mg/L, depending on the species (Table 5.2).

Table 5.2. The DOC requirement (mg/l) of the major cultured fish in China.

Dissolved oxygen content not only affects respiratory rates, but also feeding rates. Under normal conditions, the higher the DOC, the greater the food intake, the lower the food-conversion factor and the faster the growth of the fish (Table 5.3). For grass carp, when the DOC was increased from 2.73 to 5.56 mg/L, the food-conversion rate declined 4.2 times and the body weight increment increased 9.8 times. A similar experiment was conducted outside China on rainbow trout. It also shows that the higher the DOC, the better the growth of the fish and the lower the food-conversion rate (Table 5.3).

Table 5.3. The effects of DOC on the growth and food conversion rate (FCR) of rainbow trout.

It is clear that the DOC is closely related to the respiration, ingestion, growth, and survival of the fish. In static fish ponds, the DOC is mainly dependent on the photosynthesis of phytoplankton and the diffusion of the air against the water surface, the former being more important than the latter. During the day, the upper layer of water usually becomes saturated with oxygen when photosynthesis is high. Nevertheless, the oxygen easily escapes from the water into the air.

In a fish pond, the respiration of the fish is not the leading factor in oxygen consumption, accounting for only 5–15 per cent of the total consumption. The oxygen consumption of natural food organisms (e.g., zooplankton) accounts for less than 4.5 per cent; benthos, 0.2 per cent; the oxidative decomposition of manure applied and pond silt, about 8 per cent; and the decomposition of artificial food and fish feces, about 32 per cent. Microbacteria (including phytoplankton) consume about 50 per cent of the dissolved oxygen.

Because of the different histories of fish culture, climates, food sources, and consumer habits in China, various fish-farming systems, which are practical in respect to local conditions, have been developed. Even in one place, there may be different polyculture farming systems with different fish yields. Usually, five to nine species of fish are polycultured in one grow-out pond. Among the species cultured, however, only one or two species are major species, and dominate the pond in number or weight; the rest are minor species. Because there is a wide variety of minor species, their rearing can result in high yields, low cost, and a high economic return. Both the major and the minor species play important roles in total output.

The selection of the major species and their stocking ratios depend on the availability of fingerlings, feeds and manures, farming techniques; pond conditions, and market demand. Because fish-farming conditions differ from place to place, it is difficult to work out a standardized stocking model. Tables 5.4 to 5.9 give practical examples for reference. With proper rearing management, these models will be productive.

Table 5.4. Stocking model using aquatic macrophytes as the main fish feeds.

^a GC, grass carp; BR, bream (Wuchang fish); SC, silver carp; BH, bighead; CC, common carp: CrC, cruciancarp

Table 5.5. Stocking model using terrestrial grasses as major fish feeds.

Table 5.6. Stocking model using organic manure.

Table 5.7. Stocking model using aquatic and terrestrial grasses as the main fish feeds.

Table 5.8. Stocking model using terrestrial grasses as the main feeds, with grass carp as the major species and a target net yield of 500 kg/mu.

Table 5.9. Stocking model using green manure, animal manure and domestic sewage.

Harvesting and Stocking in Rotation

Harvesting and stocking in rotation is a procedure whereby fingerlings of different sizes are stocked into the pond at the same time. As the fish grow, the pond becomes overcrowded. Consequently, marketable-size fish are caught in batches and are replaced by an appropriate amount of smaller fish to maintain the optimal stocking density during the entire culture period and increase the fish yield per unit area. In the past, fingerlings were stocked at the beginning of a year and harvested at the end of a year. Part of the pond was wasted at the initial stage of rearing and fish growth was retarded at the later stages. This has been changed by harvesting and stocking in rotation. In short, rotary harvesting and stocking is an operating procedure, i.e., “to stock fingerlings of different sizes at the same time, harvest by stages, catch the edible-sized, and leave or re-stock the smaller ones.”

Advantages

This method balances the carrying capacity of the pond with the fish growth. At present, high-density stocking is commonly practiced to obtain high yields. In high-yield fish ponds, the stocking density is usually around 150 kg/mu reaching 250–300 kg/mu or even higher, with 8–10 different species in more than 10 different sizes. As the fish grow, the pond becomes crowded and, correspondingly, the space occupied per fish becomes less. If the living space is limited, fish growth is impaired. In addition, the growth rate decreases as the fish gets larger. Based on observations in Guangdong, the body weight of a silver carp can increase by 0.4–0.6 kg/month when there is less than 30–40 kg/mu of fish. If the fish density is over 30– 40 kg/mu, the monthly body weight increment of a silver carp is only 0.05–0.3 kg. Bighead behaves similarly, with a crucial density of 125–160 kg/mu. Harvesting and stocking in rotation can maintain a reasonable fish density so as to fully utilize the pond and the applied feeds.

Harvesting and stocking in rotation allows the grow-out pond to be used for interfarming fingerlings, and laying the foundation to maintain high, stable fish yields.

To achieve high yields in fish farming, there must be an adequate supply of reasonably sized, healthy fingerlings. As food fish farming develops, fingerling short-ages become a major problem because stocking amounts continue to increase; the fingerlings cultured in fry or fingerling nursery ponds cannot meet the stocking demand of grow-out ponds. By harvesting and stocking in rotation, not only are grow-out pond yields increased but also fingerlings for next-year's stocking can be obtained from the grow-out ponds. Thus, high and stable fish yields are ensured. Harvesting and stocking in rotation also reduces the seasonal variation in fresh fish supply and speeds up capital return. From June to October, fresh fish can be harvested monthly for marketing. This hastens capital return and is beneficial to expanding production.

The primary method of harvesting and stocking in rotations is to harvest them by stages and in groups, catching and edible-sized fish and leaving the smaller ones. This method is more adaptive to rural areas because it does not need special fingerling-storage ponds. Rotary harvesting is conducted 2 or 3 times per year and midterm harvest may account for 30 per cent of the total annual output.

In Jiangsu, Zhejiang, Hunan, and Guangdong provinces, fingerlings are stocked several times and harvested in stages, catching the edible-sized fish and supplementing the pond with smaller ones. The time of harvesting and stocking differ according to the production condition of the fish farm and special nursery ponds are required to rear fingerlings. Harvesting times mainly depend on the growing period and the intensity of farming. In Hunan and Guangdong provinces, rotary harvesting can be performed 6 to 8 times per year because of the long growing period; in Jiangsu and Zhejiang provinces, there are only 4 or 5 harvests per year. Fingerlings must be re-stocked after the first two or three harvests. The extent of this supplementary stocking is determined by the production target.

Harvesting

Rotary harvesting is usually conducted in summer and autumn. At high water temperatures, fish are active and have a high feeding intensity; therefore, they are unable to tolerate a long period of handling and crowding. Rotary harvesting should be performed when it is cool and the fish are not surfacing. In addition, the fish should be fed less the day before harvesting to minimize jumping and the dirty drifts should be removed before harvesting. The fish are caught in a net cage between two boats. The boats should constantly move around the pond to wash away any mucus on the skin or mud on the fish gills to prevent asphyxiation caused by overcrowding. The operation must be done as quickly and gently as possible. Fish that are under the marketable size should be returned to the pond as soon as possible.

During harvesting, fish consume more oxygen because of their violent movement and the pond water becomes turbid as the silt at the bottom gets turned up. After harvesting, aerators should be turned on and fresh water added to the pond.

Multiple-Grade Conveyor Culture

Multiple-grade conveyor culture is a special farming technique practiced by fish farmers in Guangdong province. This method is different from rotary harvesting and stocking: fingerlings of different sizes are reared in separate ponds based on fish growth and are transferred in sequence into other ponds. Ponds are usually divided into five grades: one for each size of fish. When the marketable fish are harvested, fingerlings are transferred in sequence to the next grade of pond for further culturing. This farming technique, which can increase fish yields, allows a more reasonable stocking density during the fingerling-rearing period. Under the same conditions and in the same time, this technique can produce a greater number of larger fingerlings than other techniques. Therefore, the rearing times in the grow-out pond can be increased and larger fish will be produced. Consequently, fish yields and economic efficiency will be improved.

Multiple-grade conveyor culture is based on polyculture. In Guangdong province, grass carp, bighead, silver carp and mud carp are usually the major species for polyculture (Table 5.10), particularly, grass carp and bighead carp. Recently, attempts have been made to increase grass carp output because grass carp have a higher potential yield and economic value. Bighead are the major species in traditional polyculture because they grow faster than silver carp, reaching marketable size sooner; several harvests of marketable bighead can be guaranteed in 1 year. In addition, bighead do not jump during netting, so the chance of injury is minimal.

Table 5.10. Stocking in a grow-out pond (grade 5)

^a Other species are commonly polycultured in grow-out ponds (fish/mu) = common carp, 20; tilapia, 500–1,000; Wuchang fish, 50; black carp, 5–10; snake head (Ophiocephalus), 30–50.

In polyculture, it is necessary to avoid competition between species with similar feeding habits: e.g., silver carp and bighead; silver carp and mud carp; grass carp and wuchang fish. At the fingerling stage, polyculture of silver carp, bighead, and mud carp is rare. At the adult stage, the stocking proportions of the minor species should be strictly controlled to ensure the sufficient growth of the major species.

The rationale of multiple-grade conveyor culture is the maintenance of the optimal carrying capacity by regular netting and transferring or harvesting. At the optimal carrying capacity, fish will show maximum growth, otherwise, fish growth will be retarded. The monthly weight increase of bighead is 0.4–0.6 kg when the total stocking weight of fish is below 30–40 kg/mu. If the total stocking weight of bighead is over 30–40 kg/mu, the body weight increment is reduced to 0.05–0.3 kg/month. Similarly, mud carp grow faster when its total weight is below 125–160 kg/mu; above this density, the growth rate decreases. Based on natural conditions and updated technical standards of Guangdong, optimum stocking rates and carrying capacities are shown in Table 5.11.

Table 5.11. Stocking density and carrying capacities of various species in a multiple-grade conveyor culture system.

It is important to properly set up the pond area of five grades so that fingerlings produced from one grade can meet the demand of the next. To avoid restrained fish growth caused by pond overpopulation or a disjointed production caused by an insufficient number of fingerlings in a certain grade of pond, carrying capacities must be closely monitored. The area alloted to each grade of pond should be based on the total pond area, the number of grades, the culturing period of each grade, the stocking rates, and the target production. Experience has shown that grow-out ponds (grade 5) normally account for 65 per cent of the total area; largesized fingerling (grade 4), 23 per cent; medium-sized fingerling ponds (grade 3), 7 per cent; small-sized fingerling ponds (grade 2), 3 per cent; and holding ponds (grade 1), 2 per cent. Stocking sizes, transfer sizes, stocking densities, and culturing periods of grass carp, bighead, and mud carp are listed in Tables 5.12, 5.13 and 5.14.

Table 5.12. Multiple-grade conveyor culture system for grass carp.

Table 5.13. Multiple-grade conveyor culture system for grass carp

Table 5.14. Multiple-grade conveyor culture system for mud carp

Shortening the Fish-Rearing Period

In China, the Changjiang River and Pearl River basins and the Taihu District in Jiangsu province are the traditional fish-farming areas. The traditional farming techniques developed from over 1000 years of practice are adaptive to the state of the art and are still valuable to fish farming today. The traditional fish-farming system, however, was restricted by the social system, natural conditions, economic structure, farming techniques, and other objective historical factors. Science and technology are now progressing rapidly and being widely applied in agriculture and animal husbandry. Fishery scientists have recognized that the traditional fish-farming system cannot meet present-day demands. The traditional system requires 2 or 3 years to rear fry into food fish; for black carp 4 years. There is a long culturing period and a great demand for stocking seeds. The stocking rate is high, fish growth is slow and the food-conversion factor is high. The traditional fish-farming system has many links in the chain of production and is susceptible to natural disasters. The expenditure for maintenance is high, the return on investment is slow, and economic efficiency is comparatively poor. Like other productive systems, however, the traditional fish-farming system has become a fixed practice, even if it still can be improved.

Silver carp and bighead fingerlings and common carp summerlings can reach a body weight of more than 0.5 kg in November when they are polycultured at a low stocking density in a black carp yearling pond. They can also approach that body weight when they live in lakes, reservoirs, and rivers, as long as there is an abundant natural supply. Silver carp and bighead are artificially controlled to reach a body weight of 10–100 g in the traditional fish-farming system; therefore, the culturing period is prolonged to 2 years or more. Over the past 10 years, experiments on shortening the culturing period have been carried out in Jiangsu, Zhejiang, Hubei, Liaoning, Beijing, etc. The resulting new methods are being applied and leading to better production rates. There are two methods of shortening the culturing period.

May stocking, November harvest

If the target fish yield is 350–400 kg/mu, the harvest size of bighead, silver carp, common carp and grass carp should be around 0.5 kg at the end of a year; that of wuchang fish, Carassius carassius, and tilapia should be above 125 g. The stocking rate of silver carp should be 150 fish/mu; bighead 100 fish/mu; grass carp 300 fish/mu; Megalobrama amblycephala, 120 fish/mu. They are all summerlings with body weights of about 0.5 g. The total stocking rate is about 1400 fish/mu and the total stocking weight is around 600–800 g/mu.

The pond area is 1–10 mu with a water depth of 1.5–2.5 m. Ponds are drained in middle or late April and cleared thoroughly. Stable manure should be applied at a rate of 1000 kg/mu 15 days before stocking. Seeds of Wolffia arrhiza are planted in the ponds 4 or 5 days after filling of water at a rate of 15–20 kg/mu. Wolffia should be framed at one corner of the pond and allowed to propagate naturally. After planting Wolffia, it is necessary to turn the manure and splash water over the seeds. When Wolffia and zooplankton are present in sufficient quantities to meet the demand of fingerlings, ponds can be stocked (late May or early June) with healthy, conditioned, uniform-sized summerlings. To rear fry into food fish in the same year, the rapid growth of fish at this stage must be maximized. It is most important to plant Wolffia well because it is the food most palatable to juvenile fish. When summerlings are stocked for the first 1.5 months, the growth and propagation rates of Wolffia surpass the consumption rate by fish. Feeding platforms should be set up to let the fish get used to feeding at a fixed position. The higher temperatures in July are not suitable to Wolffia growth. At that time, when the body weight of grass carp will be about 75 g and of wuchang fish, about 13 g, aquatic and terrestrial grasses can be applied instead of Wolffia. Some fine feeds should also be supplied on the feeding platforms. If Wolffia is not cultured, it is necessary to apply more fine feeds or to collect wild Wolffia arrhiza or Lemna minor for the fish.

Based on the desired sizes of various species at different development stages (Table 5.15), fish should be regularly sampled to assess their growth and adjust the amount of feeding and manuring. The water colour of the pond should be oil green or yellowish brown with a tranparency of 25–35 cm. Enough fresh water to raise the water level 10–20 cm should be added every 10–15 days to maintain good water quality. By late July, fish will require a substantial supply of grass and fine feeds. Green grasses supplied in the morning should be consumed by dusk, when the water temperature is higher. The consumption rate of green grass is 50 per cent of the total body weight of grass carp and wuchang fish. Fine feeds are supplied after the grass. The amount of fine feeds applied is about 2 per cent of the total weight of fish in the pond. The food should be fresh, and of an appropriate size. Feeding quantities depend on the weather, the water quality, and the appetite of fish. The details of feeding and manuring in a pond with a net yield of 350–400 kg/mu are shown in Table 5.16.

Table 5.15. The desired sizes (g/individual) of various species at different times.

Table 5.16. Feeding and manuring amounts (kg) in a pond with a net fish yield of 350–400 kg/mu.

Note: Values in parentheses are percentages of the total.
^a Equal parts of bean, barley, rapeseed cake, and rice bran or wheat bran.

With sufficient base manure, additional manure can be applied in small amounts at regular intervals according to the fertility of the pond water. Generally, organic manure is applied daily at a rate of about 25 kg/mu. Alternatively inorganic fertilizer is applied once every 3–6 days as ammonium sulphate or ammonium bicarbonate (1–2 kg/mu) and as calcium superphosphate (0.5–1.0 kg/mu).

To culture fry into food fish in the same year, the stocking density should be low so that the food in the pond is plentiful, the fish growth is fast, and the survival rate is high. The usual final fish yield is around 450 kg/mu (Fig. 5.2).

Fig. 5.2. Increase of standing stock (Kg/mu) of fish in pond where summerlings are grown into food fish in the same year.

The survival rates of bighead, silver carp, common carp, wuchang fish, and carassius carassius are all above 90 per cent. The survival rate of grass carp is generally about 70 per cent. Hemorrhagic disease is common in grass carp. Therefore, certain measures should be taken to prevent disease and raise the survival rate. Every 10 days after the middle of July, supplementary feeds soaked in 3–5 per cent salt solution should be supplied for 3 consecutive days every 15 days, bleaching powder should be used to sterilize the feeding platforms at a rate of 0.25 kg/mu every 20 days, quicklime emulsion (15 kg/mu) spread over the pond. Every 20 days after the middle of August, medicated food for enteritis should be given for one course of treatment (about 1 week). There is a total of three courses during a culture period. The prevention and treatment of fish diseases guarantees high yields.

Year-Round Culturing

Fish ponds are seldom used from November to the following May. At the initial stocking stage, fish ponds are not fully utilized because summerlings are small and the stocking density is low. To increase yield, fish farmers should rear fingerlings into food fish over the entire year, provided conditions are appropriate. The method is as follows:

Three fish ponds of similar size are required. In pond A, fish are harvested in early May. Any fish under marketable size are transferred to ponds B or C. Pond A is then drained and cleared to culture Wolffia arrhiza. In late May, the pond is filled and stocked with summerlings. Stocking densities are as follows (fish/mu): silver carp 750; bighead 500; grass carp 900. In ponds B and C, marketable silver carp, bighead and grass carp are harvested in early July and then restocked with large fingerlings of silver carp, bighead and grass carp from pond A. The stocking rates of silver carp and bighead should be low so that they may reach marketable size in the same year; however, the stocking rate of grass carp should be higher because they are reared into larger fingerlings for the next restocking. In November, the marketable-sized fish in all three ponds are harvested. Those fish under marketable size (silver carp, bighead, and grass carp in pond A and grass carp in ponds B and C) are harvested in next May (pond A) or July (ponds B and C). This method can significantly increase the total fish yield (Tables 5.17 and 5.18).

With this farming method, there are five harvests of food fish and four harvests of large fingerlings per year. In pond A, there is one harvest of food fish in May and silver carp, bighead, and grass carp fingerlings with a body weight of 75 g are harvested in July and used to stock ponds B and C. At the end of the year, pond A produces silver carp and bighead fingerlings of 250 g and grass carp fingerlings of 500 g for the restocking of all three ponds. In ponds B and C, one harvest of food fish occurs in July and, at the end of the year, the ponds produce another batch of food fish and some grass carp fingerlings for re-stocking in the following year. This farming technique combines multiple-grade conveyor culture (Guangdong) and harvesting and restocking in rotation (Jiangsu). To achieve the desired yield of fish from the three ponds, careful management throughout the culture period is essential. It is particularly important that Wolffia be properly cultured at the initial stage in pond A to ensure the quality and quantity of fingerlings for re-stocking. The details of feeding and manuring for all three ponds are shown in Tables 5.19 and 5.20.