CHAPTER 5 POND CULTURE OF FOOD FISH

The intensive pond culture of food fish is mainly conducted in a small man-made water body with an area of several mu to tens of mu for a water depth of 1--2.5 m. Fish in ponds are fed with commercial food and stocked at high density so as to achieve a high and stable production. In the intensive pond fish culture, the Chinese fish farmers have summerized their rich experience accumulated through thousand years of practices into eight main points: (1) water, (2) seeds, (3) feeds, (4) polyculture, (5) high density, (6) rotation, (7) prevention and (8) management. These eight principles for intensive pond fish culture are a scientific summary in theory, which has effectively accelerated the development of pond fish culture in China. In 1983, the total freshwater fishery production in China was 1,840,700 tons, of which 1,428,100 tons were produced from cultivation. The total area for fresh water cultivation was 46,239,700 mu, of which pond area was 14,473,900 mu occupying 31.3% of the total cultivated area, the total fish production from ponds was 1,030,000 tons, making up 72.1% of the total fresh water fishery production from cultivation. It shows that pond fish culture plays a very important role in fresh water fisheries in China.

Rearing period: The rearing period of pond fish farming refers to the production period of raising fish from fingerlings to the edible-sized through a fattening process. In China, the rearing period of fish farming in most places is determined by local climate, culturing methods and demands of the market. Based on the experience, Silver carp and Bighead increase greatly in weight in the second year. By using proper culture techniques, Silver carp and Bighead fingerlings with a body weight of 50 g can reach 1--1.5kg in next autumn; Black carp and Grass carp have a rapid growth in the third year; the 2-year-old Grass carp fingerlings with a body weight of 0.5--1kg can grow up to 3.5--5kg in the following year. Therefore, the rearing period of Chinese carp in Jiangsu, Zhejiang, Hunan and Hubei Provinces now is two years for Silver carp, Bighead, Wuchang fish, Tilapia and Crucian carp, and 3 years for Black carp and Grass carp. Such traditional fish rearing period is much longer than that in livestock and poultry production, so the economic benefit is comparatively lower. Now productive experiments and reforms are conducted to shorten the fish rearing period in different places.

1. Requirements and renovations of ponds

Whether pond conditions are good or not greatly affects the fish growth and fish yields. If the conditions are favourable, the yield may be doubled or even several times higher than that of ponds with unfavourable conditons. Therefore, the good conditons of fish ponds should be ensured so as to achieve high and stable yields.

1) Requirements of Grow-out pond

A fish pond is not only for fish to live in, but also for natural organisms to propagate. All the measures for increasing fish yields can only function through ponds. Certain requirements of grow-out ponds must be satisfied; otherwise, they will have great influence on fish output and managerial operation.

(1) Moderate area

The pond area of 7--10 mu is considered to be an optimal size for intensive culture of food fish. In such a pond, the fish have a larger space for activities and the water is often convected through the wind action, resulting in an increase of D.O.C. and an adjustment of water temperature. Besides, the decomposition of manure and the propagation of plankton can be promoted.

(2) Water supply and water quality

Fish can not live without water; water supply must be adequate. The pond should be filled with fresh water at regular intervals so as to adjust water depth, control water quality and alleviate the serious surfacing of fish. The best thing to do is to take the water from rivers, lakes, reservoirs and other large water bodies as a water source for fish ponds, because in natural water bodies, D.O.C., pH value, water quality and water temperature are more stable and suitable for fish to grow. On the contrary, the waste water discharged from factories and mines usually contains harmful chemical substance, which is not suitable for fish farming.

(3) Depth of the pond

The effective water depth for pond fish culture varies from place to place in accordance with geography, climate, species and culturing method. In order to fully utilize the water body and expand the capacity of the pond, within limits the depth should be as great as possible. An increase in water depth increases the volume of water and would prevent abrupt changes in water temperature and help to stabilize the water quality. Thus, it is beneficial to the fish growth and the polyculture of various species can be realized. In Xihu village in the suburbs of Henyang, Hunan Province and Helei village in the suburbs of Wuxi, Jiangsu Province, the common depth of ponds practised is around 3 meters. But the water is kept shallow in winter whereas deep in summer so that the pond dykes can be used for fodder crop production in winter and spring. Water depth is gradually increased over 2 m when summer comes. In general, the optimum water depth is about 2 meters all the year round.

(4) Bottom soil

Loamy soil is the best bottom for fish ponds because such a bottom has the moderate ability to maintain water level and fertility. Pond water may not easily get too turbid. The bottom silt won't be too thick. Besides, it is good for natural organisms to grow and it is convenient for operation and management. Clay soil is good for maintaining water level and water fertility by virtue of its low permeability, but pond water easily becomes turbid because of the minute soil particles. The bottom silt is often too thick and has a strong absorbability. A lot of nutritive salts are absorbed in pond silt and they can not be used by plankton. Therefore, clay soil is not favourable for the propagation of natural organisms and not convenient for operation and management either.

(5) Shape of the pond and its environment

It is preferable to have fish ponds in rectangular shape and pond dykes in good trim. Pond bottom should be flat with an even depth, thus, it is convenient for rearing management and netting operation. Planting mulberry trees and crops on pond dykes can not only produce food for fish, but it also can protect the pond dikes from rain wash.

There must be no big trees and buildings around fish ponds for fear that they should shade the sunlight and block the blowing of the winds. It's beneficial to the raising of water temperature of fish ponds and the growth of plankton and the improvement of D.O. conditions.

2) Pond renovations

Good pond conditions are regarded as one of the key factors in achieving high and stable yields. You can judge whether the pond conditons are good or not according to the pond depth of a pond, the area of pond water, the height of pond dikes, with or without inlet and outlet, etc. In the past, the traditional fish ponds in Jiangsu and Zhejiang were small, shallow with low dikes and poor water quality. Such ponds are subject to natural disasters and with low productivity, for instance, in early 1950s, the yield of grow-out ponds was only 300 kg/mu, which was considered as the high-yield ponds at that time. With the development of fish farming, the experience of fish farmers proves that the traditional fish ponds are not suitable for achieving high and stable yields. The old ponds should therefore, undergo the following four renovations:

(1) Small ponds are combined to form a larger pond

There is a saying by fish farmers “The broader a water body is, the larger the size of fish in it.” Big ponds could provide larger space for fish to live in and satisfy their ecological requirements.

All the environmental conditions are also comparatively stable in a larger pond. In the past, people believed that the optimum area of grow-out ponds was 4--5 mu. Through consecutive practices it is found that in small ponds the quality is difficult to control and the oxygen consumption rate is high and fish easily come to surface under the condition of polyculture of various species at high density; in larger ponds, the surface is wider, so is the area the wind blows over. It is apparent that wind drift causes surface waves and increases the D.O.C. in water bodies. Thus, the old ponds of 4--5mu are now combined to form a large pond of 7--10mu, even 15 mu as grow-out ponds.

(2) Shallow ponds are dug up to become deeper ponds

Practices have proved that the depth of ponds directly affects the yield of food fish. When the water depth is kept between 2--2.5m, the increase of water volume in per-unit area creates a favourable condition for conducting polyculture of different species at high density. In deeper ponds, water will not be turbid during the harvest, still keeping fresh and clear. In addition, water temperature and quality are more stable without any sudden change. It's beneficial to the growth of fish. However, beyond the limit the surface water and the bottom water can not easily convect. Toxic gases will be produced in the bottom because of low D.O.C. If ponds are too deep, it is not so good for fish to grow and for natural organisms to propagate and the fish yield will be low. The water depth of a fish pond should be agreeable to its area. Normally, the optimum water depth is kept around 2 meters (between 1.7--2.5m) all the year round.

(3) Change inaccessible and stagnant water ponds into free-exchanging ponds

In order to facilitate irrigation and drainage systems and adjust water quality, it is suggested that the stagnant water pond, which is difficult of access and where there are some difficulties in irrigation and drainage should be turned into free exchanging water ponds by combining some fish ponds and connecting the ponds with the water supply. There is a fish farmer's proverb in Wuxi area: “Pond water should be first properly managed if you want culture fish well.” That is to say, we should keep a favourable living environment for fish through management in order to obtain a high fish yield in intensive fish culture.

(4) Change low-dike ponds into high-dike ponds

In order to prevent floods, it is necessary to heighten pond dikes. At the same time, the widened pond dikes could be used for crop plantation.

Through “the four renovations”, the conditions for fish farming are totally changed and the fish output can be increased step be step in the intensive fish culture. For example, in Zhang-zhuang village Huang-qiao township, Wu County of Suzhou city, which is a traditional fish farming place, the average yield was 200--250 kg/mu during 20 years after liberation. Since 1973, they have renovated their old fish ponds in every winter resulting in average yield of 500 kg/mu in 1979, 700 kg/mu in 1982 and 750 kg/mu in 1983. In Helei Fishery village in the suburbs of Wuxi, they have combined 345 smaller ponds into 172 larger ponds since 1975. Now, the yield is up to 1000 kg/mu.

At present, the general criteria for grow-out ponds with a high and stable yield in China are as follows:

1. Moderate area of about 10 mu,

2. Water depth of 2--2.5 m,

3. Good water supply and easy to fill or drain,

4. Ponds are in good order with high and wide dikes, free from floods and easy for operation. Some space of pond dikes may be left for crop production and animal farming. (See Figure 5--1)

2. Stocking and Polyculture of Fingerlings

1) Stocking of Fingerlings

The stocking of fingerlings is an important link in a chain of food fish culture because fingerlings are the material basis of fish farming. The production of food fish culture demands that fingerlings should be of a complete variety of species, in adequate quantities, in appropriate sizes and of no injury and diseases.

(1) Pond clearing

One or two years' culture of food fish often deposits a layer of silt and organic matters on the bottom of grow-out pond and various harmful bacteria and wild fish exist in the water body. All these things are harmful to the growth of fish. Fish ponds should therefore, be cleared once a year so as to improve the living environment for fish.

Pond clearing is normally done in winter. Part of silt is removed out of ponds. Not only it can improve the living environment for fish and enlarge the capacity of the pond but it also can provide a lot of manure of good quality for agriculture. After removing the excessive silt, the pond bottom is open to the air for sunning and freezing, and then drugs can be used to eradicate all the wild fish, pathogen and parasites, etc.

After pond clearing, fresh water and manure can be introduced about one week before the stocking of fingerlings.

(2) Manure application and water filling

Manure application is to enrich nutrition elements and to promote the proper proliferation of natural food organisms in the pond water. This is one of the important measures to solve the problem of food supply and to increase fish yields. After pond clearing, base manure should be applied as early and adequately as possible so that it can propagate enough natural food for fish to feed on at early stage. The general 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. Manure also could be mixed with pond silt so that it can keep the water fertile a little longer.

After the application of base manure, the pond can be filled with fresh water in several times, the initial water filling is about 1 meter. When pond water becomes fertile, fresh water could be added in again. And the pond could also be filled with fresh water in line with the raising of the temperature and the increase of fish body.

(3) Selection of fingerlings

The selection of good quality fingerlings is one of the important links in ensuring high fish yields. The large-sized fingelrings of good quality have many merits--strong adaptability, high survival rate, fast growth, short culture period, high marketing rate and economic returns, etc.

The criteria for selecting and purchasing fingerlings are as follows: Physique: Choose strong and healthy fingerlings without abnormal shape but with plump muscles at the dorsal and peduncle part. Fingerlings should have complete scales and fin rays and smooth skin with bright colour. If not, they are of poor quality.

Size: The fingerlings of the same age should be of uniform size without much difference in either body length or body weight. If the size of fingerlings are not the same and much different from the standardized sizes (Table 5--1), they are also of poor quality.

Movement: Healthy fingerlings will jump violently in hand while poor fingerlings will not; when healthy fingerlings are put in a plate, they jump unceasingly without opening their gill covers, but 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, but those swimming slowly or lonely are of poor quality.

(4) Disinfection of fingerlings before stocking

Disinfection of fingerlings should be conducted before stocking. (It's mentioned in Chapter 6 Fish diseases).

(5) Stocking time

Fingerlings should be stocked as early as possible, which is regarded as one of the experience in achieving high fish yields. In Changjiang River basin, fingerlings are usually stocked in early February when the air and water temperature are comparatively low in a year. At that time, fish are weak in moving so that they will not be easily injured during the process of netting and stocking. The occurence of fish diseases and mortality could be minimized. In the meantime, the earlier stocking means earlier feeding & a longer growth period.

The stocking of fingerlings should be carried out in fine days, but not in rainy, snowy or cold days so as to avoid the possible frostbite of fingerlings during netting and transportation.

Table 5--1 Body Length and Body Weight of Yearlings

2) Polyculture

Polyculture of various fish species developed by Chinese fish farmers through years of practice is one of prominant farming techniques in Chinese fresh water fish farming. Polyculture in China has a long history resulting in higher output and better efficiency in comparison with other countries. It dates back to the Tang Dynasty (618--907). Since then, polyculture of Grass carp, Black carp, Silver carp and Bighead has been conducted. During the whole process of fish farming, polyculture is performed at every rearing stage such as brood fish, fingerling and food fish production. But more species of fish are introduced in polyculture system with the development of fish culture. Now grow-out ponds might be stocked with a mixture of 8--19 species in different combination of sizes and ages.

(1) Advantages of polyculture

(i) To fully utilize natural food organisms in pond water

There are three kinds of natural organisms ie. 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, such as Silver carp and Bighead feeding on plankton; Grass carp, Parabramis pekinensis and Megalobrama on grasses; Black carp on snails and other benthos; Common carp and Crucian carp on benthos and some organic detritus; Mud carp and the Xenocypris on organic detritus and benthic algae and tilapia on all the food because they are omnivora. When they are mixed together, the natural food organisms in fish ponds can be fully utilized and the potential of production can be brought into full play.

(ii) To fully utilize the available space in the water body of ponds

The major cultivated carps have different habitats in the ponds. Compared with monoculture, polyculture can increase the stocking amount per unit area and increase fish output therewith.

(iii) To utilize the beneficial interactions between compatible species cultured in the same pond.

Under reasonable polyculture, all the fish are of mutual benefit. Thus, the production of each species would be increased.

Grass carp, Black carp, Common carp and Wuchang fish are regarded as “feeds-eater” or “Food-feeder” while Silver carp, Bighead and Tilapia are known as “plankton-feeder”. When monoculture of Grass carp is carried out the pond water easily get fertile because Grass carp eat more and discharge more. Unfortunately it's unsuitable for Grass carp to live in because they prefer clear water; nevertheless, Silver carp and Bighead can utilize the natural organisms propagated by the manure of Grass carp when they are mixed in one pond. Therefore, the fertility of pond water can be decreased by filteration of Silver carp and Bighead, which is more conducive for Grass carp to grow. Through the beneficial interactions between different species, one kind of food can be double utilized. It's said in production that “One Grass carp can provide enough food for three Silver carp through proliferating natural organisms.”, which shows the significance of polyculture.

(iv) To raise the utilization rate of artificial feeds

Different species in various sizes will feed on different sizes of feeds. During feeding and food intake of fish, it's unavoidable to scatter some feeds in the water. A part of feeds might be wasted when monoculture of one species or one size is practised. However, polyculture of different species and sizes can maximize the utilization of artificial feeds. It also improves water quality for a better growth of fish.

(2) Stocking density in reason

The reasonable stocking density refers to the optimum stocking rate of fingerlings with which the maximum yields of marketable-sized fish can be achieved through people's efforts and the full utilization of objective conditions.

(i) Importance of reasonable stocking density

Stocking density, known as per-unit stocking amount or stocking rate refers to the quantity of fry or fingerlings stocked in per unit water body. It is usually expressed by number of tails or weight of fish per mu. In high intensive fish farming system (such as industrialized fish farming, fish farming in flowing water, fish culture in net cage), stocking density is expressed by number of tails or weight of fish per unit area or water volume (m² or m³) because of high stocking density and high utilization rate of water body.

The stocking density must be reasonable, because stocking density is inversely proportional to the quality of marketable fish under the same pond conditions and culturing measures. The fish in stocking density beyond the limit cannot reach the marketable size and moreover, fish yields cannot be raised. If the stocking density is too low, the per unit area production is comparatively low, though fish grow faster and reach a larger marketable size and higher plumpness. Under certain conditions, the reasonable stocking density can ensure the desirable size and good quality of fish products. (See Table 5--2)

(ii) Factors determining stocking density

Pond conditions, seed supply, availability of fish species, sizes of fish, feeds and operating techniques, etc. should be taken into consideration in determining how much should be reasonably stocked. The data in the previous year, such as sizes, yields, survival rate, marketing rate, food conversion rate, etc. are taken as factors determining stocking density for the following year. Generally speaking, if fish grow well and the food conversion rate is not higher than the average one and no serious surfacing occurs during the culture period and all the species of fish reach the marketable size at the end of production, the stocking density could be considered as optimum one. On the contrary, some species of fish can not reach the marketable size and the food conversion rate is high, which shows the stocking density is too high. It should be reduced to some extent. stocking density varies correspondingly with the development of production, science and technology. Therefore, the stocking density should be determined by local conditions in order to obtain bumper harvest.

(iii) Correlation between stocking density and D.O.C.

Apart from feeds and water space, water quality (D.O.C. in particular) is the major factor affecting the stocking density. The D.O.C. in pond water is closely related to the growth and survival rate of fish. Under certain water temperature, the oxygen demand of fish varies with different species, ages and sizes. Chinese carps can sustain their normal lives when dissolved oxygen content is above 3mg/l. The optimum D.O.C. is about 5.5mg/l. The respiratory rate of cyprinids increases when D.O.C. is below 2mg/L. If it continues lowering, the fish will come to the surface gasping for air. Asphyxiation will occur when D.O.C. is lower than 0.7-l.Omg/l. The limits of dissolved oxygen content for the major cultured fish is shown in Table 5-3.

Table 5--3. The D.O.C. Requirements of the Major Cultured Fish

Unit: mg/l

Dissolved oxygen content is not only related to the respiration of fish, but also directly affects their feeding amount. Under the normal conditions, the higher the dissolved oxygen content the greater the feeding amount of fish, the lower the food conversion rate, the faster the growth of fish. They are in direct proportion. In China, an experiment on Grass carp shows that the food conversion rate declined 4.2 times while the body weight increment increased by 9.8 times when dissolved oxygen content rose from 2.73mg/L to 5.56mg/L. The similar experiment was also conducted outside China on rainbow trout. It also shows that the higher the dissolved oxygen content, the better the growth of fish, the lower the food conversion rate. (See Table 5--4)

Table 5--4. The Correlation between D.O.C. and Food Conversion Rate on Rainbow trout

It is clear that the dissolved oxygen content is closely related to the respiration, ingestion, growth and survival of fish. In static fish ponds, dissolved oxygen mainly results from photosynthesis of phytoplankton and the diffusion of the air against water surface. The former is more important than the latter. In daytime, D.O.C. in the upper layer of water bodies usually becomes saturated when photosynthesis is high. Nevertheless, the saturated oxygen will easily escape from the water surface into air.

In a fish pond, the respiration of fish itself is not the leading factor in oxygen consumption. The oxygen consumption of fish is only 5--15% of the total consumption round the clock. The oxygen consumption of natural food organisms, e.g. zooplankton occupies less than 4.5% even at the higher biomass; benthos, 0.2%; the oxidative decomposition of manure applied and pond silt, about 8%; and the decomposition of artificial food and fish faeces, about 32%; microbacteria (including the respiration of phytoplankton) and decomposed or suspended organic matters, about 50%, which are the biggest oxygen consumers. Therefore, how to increase the D.O.C. by various measures is the guarantee of high and stable fish yields.

(iv) Models and examples of polyculture at high density

China has a vast territory. Because of the different history of fish culture, climate, food sources and consuming habits, the different fish farming systems which are practical in line with the local conditions have been formed in various places through years of practices. Even in one place, there exist different polyculture farming systems with different fish yields. Generally speaking, five to six, even eight to nine species of fish are polycultured in one grow-out pond. However, among the species cultured, one or two species are taken as major species which are stocked more in number and weight while the rest are considered as minor species. Because the minor species are full of variety, their rearing can result in high yields, low cost, high economic return; therefore, both the major and the minor species play an important role in total output.

The factors determining what kinds of fish are taken as major species and their stocking ratio depend on the availability of fingerlings, feeds and manures, farming techniques, pond conditions and market demand. In view of the fact that the real fish farming conditions differ from place to place, it is difficult so far to work out a standardized stocking model. Here are some practical examples for reference, which are not perfect. However, under a certain rearing management, a desirable result and economic return could be obtained by following these models.

Table 5--5. Stocking model using aquatic grasses as the main food

3. Harvesting and Stocking in Rotation

1) Significance

Harvesting and stocking in rotation is a procedure whereby fingerlings of different sizes are stocked into the pond at the same time. With the growth of fish, the pond becomes overcrowded. Consequently, marketable-sized fish are caught in batches and are replaced by an appropriate amount of smaller fish to maintain an optimal stocking density during the whole culturing period so as to increase fish yields of 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 water bodies was wasted at the initial stage of rearing period and the growth of fish was retarded at the late stage, but this has been changed by using harvesting and stocking in rotation.

In a nutshell, rotary harvesting and stocking is an operating procedure, that is “to stock fingerlings of different sizes at the same time, harvest by stages, catch the edible-sized and leave or restock the smaller ones.”

2) Advantages

(1) To balance the carrying capacity of ponds in accordance with the growth of fish

At present, the measure of high density stocking is commonly practised to obtain high yields. In high yield fish ponds, the amount of stocking is up to 150 kg/mu; some 250–300 kg/mu or even higher, with 8–10 different species in more than 10 different sizes. The pond becomes more crowded when the fish are fattened after a period of time. As a result, the space fish occupies shrinks correspondingly. With a limited living space the growth of fish will be in turn impaired. What is more, the growth of fish is gradually getting slow when they reach a certain body weight. Based on the observation in Guangdong, the body weight of Silver carp could be increased by 0.4–0.6kg/ind/mth. When the total weight of fish is less than 30–40kg/mu. If the total weight is over 30–40kg/mu, the body weight increment of Silver carp is only 0.025–0.15 kg/ind/mth. It is similar to Bighead when the total weight is less than 125–160kg/mu, they grow faster. If it is over that total weight, the growth of Bighead will be retarded. This method can decrease the density in time and maintain a reasonable rate of the pond carrying capacity so as to fully utilize the availability of water bodies and feeds.

Table 5--6 STOCKING MODEL USING TERRESTRIAL GRASSES AS THE MAIN FOOD

Table 5--7 STOCKING MODEL APPLYING ORGANIC MANURES

Table 5--8 STOCKING MODEL USING AQUATIC & TERRESTRIAL GRASSES AS THE MAIN FOOD

Table 5--9 STOCKING MODEL USING TERRESTRIAL GRASSES AS THE MAIN FOOD

Table 5--10 STOCKING MODEL USING GREEN MANURE, ANIMAL MANURES & DOMESTIC SEWAGE

(2) To maximum the utilization of grow-out pond for interfarming fingerlings, which can lay the foundation of maintaining high and stable fish yields It is a prerequisite for achieving high yields of food fish farming to rear fingerlings in adequate quantity, complete species, reasonable sizes and good health. With the development of food fish farming, shortage of fingerling supply becomes a major problem because the stocking amount of fingerlings is increasing so the fingerlings cultured only in fry or fingerling nursery ponds can not meet the stocking demand of grow-out ponds. In grow-out ponds, thru the method of harvesting bigger fish and leaving or replenishing smaller ones or interfarming fingerlings the potential of the pond and fish feeds can be fully utilized. Not only the yields in grow-out ponds can be increased but also fingerlings for restocking next year can be basically provided from grow-out ponds. Thus, the high and stable fish yields can be ensured.

(3) To reduce seasonal variation in fresh fish supply to the market and to speed up capital return

During June to Oct., a lot of fresh fish can be monthly harvested for marketing, which hastens capital return and is beneficial to expanding reproduction.

3) Methods (1) To stock fingerlings of different sizes at the same time, to harvest by stages and in groups, to catch the edible-sized and leave the smaller ones. This is the primary method which is more adaptive to the rural area because this method does not need special fingerling storage ponds. Fingerlings of different sizes are stocked at the same time in winter or early spring and harvested by stages and in groups because large fish grow fast and smaller ones grow slow. Rotary harvesting can be conducted 2–3 times every year. The mid-term catchings often occupy 30% of total annual output.

(2) To stock fingerlings several times to harvest by stages, to catch the edible-sized and to supplement the smaller ones

This method is commonly practised in Jiangsu, Zhejiang, Hunan and Guanglong Provinces. The times of harvesting and stocking differ in accordance with the concrete production consitions in different fish farms. Besides, this method needs special nursary ponds to rear fingerlings. The times of harvesting mainly depend on the growing period and the extent of intensive farming. In Hunan and Guangdong Provinces, rotary harvesting can be carried put 6–8 times every year owing to the longer growing period, but 4–5 times in Jiangsu and Zhejiang Provinces. A batch of fingerlings must be restocked soon after the first two or three harvestings. The supplementary quantity of fingerlings is determined by the production target.

4) Caution

Rotary harvesting is usually conducted in summer and autumn. At high water temperature, fish have a high feeding intensity and active movement, so they are not able to tolerate the long period of operation and crowd. Rotary harvesting should be done when it is cool and there occurs no surfacing of fish. On the contrary, catching should not be conducted when it is stifling with upcoming thunder storm or there occurs surfacing of fish. In addition to that, fish should be less fed one day before rotary harvesting so as to avoid mortality of fish which will jump violently at jamming during the operation. And the dirty drifts should be removed before harvesting. When the fish have been rounded up in a net cage formed by two boats, the boats should be moving around the pond. However, the operation must be done quickly and gently as possible. While the boats are moving, the running water can wash away mucus on fish skin and mud on fish gills so as to prevent the fish from asphyxiation caused by overcrowding. The fish which are under the marketable size should be returned to the pond as soon as possible.

During harvesting, fish consume more oxygen because of violent movement and the pond water gets turbid by turning up the silt at the bottom, it is necessary to turn on aerators and fill the pond with fresh water for preventing fish from serious surfacing.

4. Multiple grade conveyor culture and shortening the rearing period

1) Multiple-grade conveyor culture

Multiple-grade conveyor culture is a special farming technique practised by fish farmers in Guangdong Province. This method is different from the rotary harvesting and stocking. The rotational culture is usually carried out in one pond by stages, catching the edible-sized, restocking or eplenishing the smaller ones, whereas multiple-grade conveyer culture is to rear fingerlings of different sizes in separate ponds based on the growth of fish, with which they are transferred in sequence into other ponds. Therefore, ponds are usually divided into five grades. Each grade is for one size of fish. When the marketable-sized fish are harvested, the larger fingerlings from each grade pond are upgraded in sequence into next grade pond for further culture to the desired size. Hence, a number of ponds are needed as fish are upgraded. The reason for this farming technique which can increase fish yields is that the stocking density of fish can be more reasonable during fingerling rearing period. With the same condition and time, more fingerlings of larger size can be produced from every fingerling rearing pond, therefore, the times of culturing in grow-out pond increased and the size of the adults enlarged accordingly, and then fish yields and its economic efficiency are raised. Basic measures in multiple-grade conveyor culture are as follows:

(i) To practise the reasonable polyculture by taking Grass carp and Bighead as the major species

Multiple-grade conveyor culture is based on polyculture. In Guangdong area, Grass carp, Bighead, Silver carp and Mud carp are usually taken as the major species for polyculture. In recent years, many measures are taken, to increase Grass carp output because Grass carp have higher potential yield and economic value. Bighead are regarded as the major species in traditional polyculture because Bighead have a faster growth than Silver carp and can reach a marketable size in shorter period of time so that several batches of marketable-sized fish supply can be guaranteed every year. Besides, Bighead do not jump and they will not be easily injured during netting, therefore, this species is more suitable for such a farming technique which needs more netting and transferring. In addition to the species mentioned above, a small amount of Common carp, Crucian carp, Black carp and Tilapia are polycultured in grow-out ponds are shown in Table 5--11

Table 5-11 Stocking in Grow-out Pond (Grade 5)

Others: Common carp 20 ind/mu. Tilapia 500–1000ind/mu Wuchang fish 50 ing/mu.

In polyculture, it is necessary to avoid the competition between competitors which have the similar feeding habits, such as Silver carp and Bighead, Silver carp and Mud carp, Grass carp and Parabramis pekinensis. At fingerling stage, polyculture of Silver carp, Bighead and Mud carp is hardly practised. At adult stage, the stocking proportional of minor species should be controlled in order to ensure the growth of major species.

(ii) To control the pond carrying capacity by the timely transferring of fish into other ponds

The rationale of multiple-grade conveyor culture is to maintain an optimal carrying capacity of fish ponds by the regular netting and transferring or harvesting. With the optimal carrying capacity, fish will have maximum growth, whereas fish growth will be retarded if the carrying capacity is beyond its limit. In terms of observation, the body weight increment of Bighead is 0.4–0.6 kg/ind/month when the total weight of fish is lower than 30–40 kg per mu. If the total weight of Bighead is over 30–40 kg/mu, the body weight increment is decreased to 0.05–0.3 kg/ind/month. Similarly, Mud carp grow faster when its total weight of fish is lower than 125–160 kg/mu and they grow slow when the total weight of fish is higher than that. Based on the natural conditions and update technical standard of Guangdong, the optimum stocking rate and maximum carrying capacity are shown in Table 5–12.

Table 5–12 Control Figures of Total Weight of Various Species in Multiple grade Conveyor Culture System

Note 1 : Initial stocking amount: This is the minimum stocking density so as not to waste the potential of a water body.

(iii) Reasonable arrangement must be made to dovetail the grades of the ponds

Fish ponds are divided into grades. The proportion of each grade pond area should be well allocated so that fingerlings produced from one grade pond can meet the demand of the next one. Avoiding the restraint of fish growth caused by too many fingerlings in certain grade pond or the disjointed production caused by insufficient number of fingerlings in certain grade pond. In every fish farm, the allocation of the pond area of each grade should be worked out based on the total pond area, number of grades, culturing period of each grade, stocking rate and the target production. From experience, grow-out ponds normally account for 65% of the total area, large-sized fingerling ponds 23%, medium-sized fingerling ponds 7%, small-sized fingerling ponds 3%, and holding ponds 2%.

As for the stocking size, transferring size, stocking density and culturing period of each species, examples of Grass carp, Bighead and Mud carp farming are shown in Table 5–13, 5–14, 5–15.

Table 5--13 Multiple-grade Conveyor Culture System for Grass carp

Table 5--14 Multiple-grade Conveyor Culture System for Bighead

Table 5--15 Multiple-grade Conveyor Culture System for Mud carp

2) Reforming the traditional fish farming system and shortening the fish rearing period

In China, Changjiang River and Pearl River basins and Taihu District in Jiangsu Province are known as the traditional fish farming places. Fish farmers over thousand years of farming practices have developed a traditional fish farming system, which was adaptive to the state of the art in the past and it is still valuable to fish farming now. However, it must be understood that the traditional fish farming system was restricted by the social system, natural conditions, economic structure, farming techniques and other objective factors in history. Now, science and technology are progressing every day and are widely applied in agriculture and animal husbandry. All the fishery scientists have recognized that the traditional fish farming system can not meet the demand of the development of production so far. The old system needs the timely reformation. The traditional one usually takes 2–3 years to rear fry into food fish, even 4 years for Black carp. The old fish farming system has a long culturing period with great demand of seeds for stocking. The stocking rate is high whereas the fish growth is slow; similarly, the longer the period of culture the more the energy is consumed to maintain the basic metabolism of fish, the higher the food coefficient. The old fish farming system has many links in a chain of production and has the susceptibility to natural disasters. The expenditure for maintenance increases whereas return on the investments comes slow. Finally, its economic efficiency is comparatively poor. However, like other productive system, the fish culture system has formed a fixed conservative idea when once it has been set up. Therefore, the old system is still practised in pond fish culture in China even if it's not reasonable. Now it's being renovated.

In practices, it is found that Silver carp and Bighead fingerlings and Common carp summerlings can reach a body weight of more than 0.5kg/ind in Nov. when they are polycultured at a low stocking density in Black carp yearling pond. Similarly, they can also approach that body weight when they live in lakes, reservoirs and rivers as long as there are abundant natural food. Silver carp and Bighead are artificially controlled to reach a body weight of 10–100g/ind. in the traditional fish farming system so that the culturing period is prolonged to two years and even more. When shortening the culturing period the potential of the growth of fish can be put in full play, so it is totally feasible to reform the traditional fish farming system. In recent ten years, experiments on shortening the culturing period are widely carried out in Jiangsu, Zhejiang, Hubei, Liaoning, Beijing, etc. The new methods are applied in some places leading to good results in production. Generally, there are two patterns to shorten the culturing period:

(1) To culture fry into food fish in the same year If the target fish yield is expected to be 350–400 kg/mu, the reared size of Bighead, Silver carp, Common carp and Grass carp is 0.5 kg/ind. At the end of a year and that of Wuchang fish, Carassius carassius and Tilapia are above 125 g/ind., the stocking rate of Silver carp is 150 ind. /mu,Bighead 100 ind./mu, Grass carp 300 ind./mu, Megalobrama amblycephala 120 ind/mu, Carassius carassius 120 ind./mu, Common carp 80 ind/mu and Tilapia 500 ind/mu. They are all summerlings with a body weight of about 0.5g/ind. The total stocking number is about 1400 ind/mu and the total stocking weight is around 600–800 g/mu.

Farming techniques: The pond area is 1–10 mu with a water depth of 1.5–2.5 m. Ponds are drained in mid or late April and cleared thoroughly. The stable manure should be applied at a rate of 1,000kg/mu 15 days before stocking. Then seeds of Wolffia arrhiza are planted in ponds at a rate of 15–20 kg/mu 4–5 days after water filling. Wolffia should be framed at one corner of the pond and let it propagate naturally. After the plantation of Wolffia seeds, it is necessary to turn over the manure and splash water upon Wolffia. When Wolffia and zooplankton propagate in large quantities up to meeting the demand of fingerlings, ponds could be stocked in late May and early June with healthy, uniform-sized summerlings which have undergone several times of conditioning. In order to rear fry into food fish in the same year, measures should be taken to maximize the rapid growth of fish at this stage under the optimal climate. The most important thing is to plant Wolffia well, because Wolffia is the most palatable food for juvenile fish. When summerlings are stocked for one month and a half, it is an optimal season for Wolffia to grow. During this period, the speed of increase of Wolffia will surpass the speed of the consumption of fish ingestion if Wolffia is cultured well. Besides, feeding platforms for fish should be set up in ponds after stocking in order to let fish get used to feeding at a fixed position. It's getting hot in July and it is not suitable for Wolffia to grow. At that time, the body weight of Grass carp will be about 75 g/ind., Wuchang fish about 13g/ind., aquatic and terrestrial grasses can be applied instead of Wolffia. However, some amount of fine feeds should be also supplemented on the feeding platforms. If Wolffia is not cultured, it is necessary to apply more fine feeds or collect wild Wolffia arrhiza and Lemna minor for fish. The desirable sizes of fish at each developmental stage are shown in Table 5–16.

Table 5--16 The Desired Sizes of Various Species at Differental Stages

Based on the desired sizes of each species at different developmental stages, sampling of fish should be regularly conducted so as to learn the growth of fish and to adjust the amount of feeding and manuring. The water colour of fish ponds should be kept oil green or yellowish brown and the transparency is between 25–35 cm. Fill the pond with fresh water to raise the water level by 10–20 cm every 10–15 days to maintain good water quality. After mid July, fish will have a good appetite so sufficient grass and fine feeds should be supplied. Green grasses are usually supplied in the morning and should be consumed before dusk when the water temperature is higher in mid summer. The feeding rate of green grass is 50% of the total body weight of Grass carp and Wuchang fish. Fine feeds are supplied after grass. The feeding amount of fine feeds is about 2% of the total weight of fish in the pond. Food for fish should be fresh and palatable in size. The feeding quantity depends on weather, water quality and fish's appetite. The details of feeding and manuring in a pond with a net yield of 350–400kg/mu are shown in Table 5–17.

Table 5–17 The Details of Feeding and Manuring in a Pond with a Net Fish Yield of 350–400 kg/mu

With sufficient base manure, the additive manure could be applied in small amount at regular intervals according to the fertility of pond water. Generally speaking, organic manure can be applied at a rate of about 25 kg mu^-1 day^-1 or inorganic fertilizer once every 3–6 days by 1–2 kg of ammonium sulphate or ammonium bicarbonate and 0.5–1 kg of calcium superphosphate.

In order to culture fry into food fish in the same year, the stocking density should be low so that the food in the pond is rich, the fish growth is fast and the survival rate is high. The general fish yield is 450 kg/mu. The increase of fish yield is illustrated in Figure 2.

Figure 2: A. Comparison of the amounts of output increment in a pond

B. Body weight increment of different species in the experimental pond Body weight increment

The survival rates of Bighead, Silver carp, Common carp, Wuchang fish and Carassius carassius are all above 90%. The survival rate of Grass carp is generally about 70%. The hemorrhagic disease is the common one for grass carp. So certain measures should be taken to prevent fish diseases in order to raise the survival rate. Every 10 days after mid July, supplementary feeds soaked in 3–5% salt solution should be supplied for 3 consecutive days, and every 15 days, bleaching powder is used to sterilize the feeding platforms at a rate of 0.25 kg/mu, and every 20 days, quicklime emulsion made out of 15 kg of quicklime per mu is spreed all over the pond. Every 20 days, after mid August, medicated food for enteritis is given for one course of treatment (about one week). There are altogether 3 courses during culturing period. In short, the prevention and treatment of fish diseases are the guarantee to obtain a high yield.

(2) To rear fry into food fish in a whole year

The first type mentioned above is to stock summer-lings in late May and to harvest edible-sized fish in early November. Fish ponds are seldom used from November to next May. At the initial stocking stage, fish ponds are not fully utilized because summer-lings are small-sized and the stocking density is low. In order to increase fish yield fish farms might rear fingerlings into edible-sized fish in a whole year if conditions are available. The method is as follows: One set comprises three fish ponds in similar size, with which, food fish can be produced from November to next May and July. In pond A, fish are harvested in early May and then, the pond is drained and cleared to culture Wolffia arrhiza first. In late May, summerlings are stocked in. The stocking density of Silver carp is 750 ind/mu- Bighead 500 ind/mu and Grass carp 900 ind/mu. In pond B & C marketale-sized fish of Silver carp, Bighead and Grass carp are not harvested until early July. After that, the two ponds are stocked with large-sized fingerlings of Silver carp and Bighead and Grass carp from pond A. But the stocking rate of Silver carp and Bighead should be resonable so that they can reach the marketable size in the same year, while the stocking rate of Grass carp could be higher because they are reared into larger-sized fingerlings for next restocking. At the end of the year, the marketable-sized fish in those 3 ponds are harvested, but the three portions of Silver carp and Bighead and one portion of Grass carp in pond A and Grass carp in pond B and C, which are all under the marketable size, are used as seeds for restocking in next May---July. In this way, the total fish yield can be remarketably increased. The stocking details are shown in Table 5–18 and 5–19.

Table 5–18 Stocking and Harvesting in Pond A

Note: The net food fish yield in this pond is only 75–100kg/mu in early May. It can also be used as a spawning pond for Common carp, Crucian carp and Wuchang fish or used for fodder plantation instead of food fish farming.

Table 5–19 Stocking and Harvesting in pond B and C

With this farming method, 5 batches of food fish and 4 batches of large-sized fingerlings can be produced that is, in pond A. one batch of edible-sized fish is produced in May and Silver carp, Bighead and Grass carp fingerlings with a body weight of 75g produced in July are used for the stocking of pond B and C. At the end of the year, Pond A also produces Silver carp, Bighead fingerlings of 250g in body weight and Grass carp fingerlings of 500g in body weight for restocking of three ponds. In the other two ponds, one batch of edible-sized fish is produced in July, and at the end of the year, they will produce another batch of edible-sized fish and some amount of Grass carp fingerlings for restocking next year. Such a farming technique is the combination of mutliple grade conveyor culture in Guangdong and harvesting and restocking in rotation practised in Jiangsu. In order to get the desired size and number of fish from the three ponds and to make them well dovetailed, careful management should be carried out during the whole culturing period. Particularly, Wolffia must be cultured well at the initial stage in the first pond so as to ensure the quality and quantity of fingerlings for restocking. If the other two ponds. The details of feeding and manuring for the three ponds are shown in Table 5–20, 5–21.

Table 5–20 The Details of Feeding and Manuring in Pond A

Remarks:  Before May, the net yield of food fish is 87.35 kg/mu. After the 2nd 10 days in May, the first batch of fingerlings is 72.13 kg/mu; the second batch of fingerlings 404.7 kg/mu. The total production is 564.18 kg/mu.

Table 5–21 The Details of Feeding and Manuring in Pond B & C Unit: kg/mu

Remarks: The net yield of edible sized fish in the first batch is 215 kg/mu and the 2nd batch 397 kg/mu. The total net fish yield is 612 kg/mu.

5. The mangement of rearing food fish

I) Feeding and Manuring

(1) Feeding

In high density polycultured ponds, each fish only gets a small amount of natural food. Therefore, the supplemental feeds and manures are essential to ensure the normal growth of fish and to obtain a high fish yield.

Feeding is the major measure for intensive pond fish culture and it is also the main daily work of the management for grow-out ponds. Normally, the expenditure of feeds account for more than 50% of the total production cost. So the selection and processing of fish feeds and feeding technique will surely affect the output, production cost and economic efficiency of fish farming.

(i) Plan and allocation of fish feeds

In fish farming, it is necessary to work out a feeding plan. First of all, the total amount of food demand for one year production should be figured out based on the target fish production, the expected body weight increment of each species and food conversion rate and monthly demand according to the water temperature and fish growth in each month. Under the climatic conditions in Jiangsu Province, the monthly feed and manure demand is shown in Table 5–22.

Table 5–22 Feeds and manures demanded in Grow-out ponds

Based on the allocation combined with local potentiality the total amount of feeds and manures for a month can be worked out. In turn, it's possible to plan and arrange the one-year fish production and to determine the quantity of various sp. in terms of the local availability of feeds and manure.

(ii) Feeding techniques

If we know the feeding habits of different species, feeds could be supplied in a proper way. On one hand, we must achieve high yield and good quantity of fish and on the other hand we should save feeds as much as possible; therefore feeds should be supplied by the following guidelines:

A. Feeds are supplied according to the status of season change, weather variations, fish growth ingestion and water quality. It's so called “four observations”.

Seasons: Fish have different appetite in seasons. More feeds should be provided during June to October when the air temperature is comparatively high, but less for the other month. Fish have a good appetite and grow rapidly when the air temperature reaches about 28°C. It is a common saying that fish grow skeleton during April to June and they gain weight during July to September (referring to the lunar calendar). This is also the growing season for aquatic weeds, terrestrial grasses and molluscs. The feeding quantity of fish during these months occupies about 80% of the total amount of a year, nevertheless, small amount of food should be supplied shortly after initial stocking in early spring when air temperature is low and fish have a poor appetite, so it is necessary to provide some supplementary feeds in order to increase the feeding efficiency of fish, feeds could be reduced after late autumn when the air temperature is below 10°. The feeding efficiency and feeding ability of fish are gradually declining from late Oct. up to the end of a year when fish usually are harvested. In order to maintain the normal ingestion of fish, feeds given should be easily taken by fish resulting in fattening. Weather: More feeds should be provided in fine days when the D.O.C. is high and fish have a good appetite. On the contrary feeds should be reduced or should not be supplied in a stifling weather when the D.O.C. is low and fish lose their appetite. Feeding should be conducted after foggy weather clears up because of low air pressure.

Fish status: Generally speaking, feeds could be all eaten up in 7–8 hours after feeding if fish have a normal ingestion. Then, a little more food can be added. If feeds can not be all eaten up within 7–8 hours, it shows fish have an abnormal ingestion status and the feeding quantity should be decreased.

Water quality: If the pond water is fertile and fish have a good appetite more foods could be applied. If the pond water is sheer, the feeding quantity should be also in-reased, but the amount of feeds should be decreased when the pond water is over fertile.

B. “Four Fix” means fixed quality, fixed quantity, fixed time and fixed position of feeding and place.

Fixed quality: Feeds should be fresh and palatable with a nutritive value as high as possible. Spoiled foods can not be used in order to prevent the occurence of fish diseases.

Fixed quantity: Fish should be provided with a fixed amount of food every day. With uneven feeding such as sometimes being well fed sometimes being underfed fish will have poor digestion and bad absorption, and slow growth. The daily feeding amount is mainly dependent on the ingestion status of fish.

Fixed time: Feeding is usually done before 10:00 every morning. In order to raise the food utilization rate, fish should be fed under the conditions of high D.O.C.

Fixed position: Feeds such as molluscs, pellet and fine feeds should be supplied at a fixed position (or on a fixed feeding platform), Just for an easier examination. The floating feeds may be supplied in a floating framework.

(2) Manuring

Like fry and fingerling ponds, grow-out ponds need fertilization Heavy base manure should be put into newly dug ponds because these ponds have less or even no silt and it is difficult to get the water fertile. Therefore, in order to improve the quality of pond bottom, organic manures, such as animal manure, green manure and stable manure, are better to be used at a rate of 400–500kg/mu as base manure. After the decomposition and mineralization of base manure for 3–4 days through exposure to the sunlight, the pond can be filled with freshwater.

And fingerlings could be stocked after another 7–8 days.

The rationale of applying additional manure is timely, evenly and frequently, but in small amounts in order to keep the propagation of natural food organisms going for fish.

In short, feeding & manuring are the fundamental prerequisites for high yields of fish. Feeding should be kept even, good and sufficient, the pond water can be kept fertile, active and crisp with manuring and water filling. It can lessen the infection of fish diseases. Thus the high yield in grow-out pond can be obtained.

2) Daily routine of pond management

With the material basis such as ponds, fingerlings, feeds and manures, high and stable fish yield can only be ensured through the daily routine of pond management, which should be done carefully, diligently and unremittingly. The work must be done well during the whole rearing period.

(1) To inspect ponds frequently in order to prevent fish from serious surious surfacing

Pond inspection should be carried out twice or three times every day to examine whether fish surface or not at dawn, to observe the food intake of fish in the afternoon, to know the daily ingestion status of fish at sunset, and also to learn whether fish will gasp for air. In late spring and early summer, when the weather changes rapidly or in hot summer, when the weather changes suddenly pond patrolling should be conducted around midnight to prevent fish from the serious surfacing. Surfacing takes place often. It is due to high water temperature, intense decomposition of organic materials, stopping of photosynthesis of phytoplankton at night, and oxygen consumption of respiration of aquatic life.

(i) Forecast the surfacing of fish

When the weather is hot with high water temperature or the water quality is fertile, surfacing will happen at dawn or around midnight.

Although fish are not infected with any diseases, they lose their appetite suddenly which shows that the pond water lacks dissolved oxygen

When pond water is overfertile with low transparency or when there's a sudden deterioration of water quality caused by weather change, the decomposition of large amount of dead bodies of plankton, not only consumes D.O. in the pond water but produces some toxic substances like hydrogen sulphide (H₂S) ammonia which will cause serious surfacing.

(ii) Diagnosis of the degree of surfacing

The degree of seriousness of fish surfacing is judged by the time when surfacing happens place where surfacing happens, the sequence in which surfacing of various species takes place and the fish response during surfacing.

Surfacing time: Surfacing occuring before dawn is considered slight: With light being gradually available, photosynthesis of phytoplankton which releases oxygen can increase D.O.C. in pond water and can easily alleviate the hypoxic condition. If surfacing begins in the evening around midnight it will become more severe. As hypoxic condition is further aggravated by the respiration of plants all night, the surfacing of fish will be getting more and more serious.

Surfacing place: It is slight when surfacing happens at the centre of a pond. But it is serious when surfacing takes place all over the pond.

Response of fish: Fish scurrying into deeper water when they are scared indicates slight surfacing. If they make no response and appear to be in a coma when frightened, it is serious because the dissolved oxygen in pond water is depleted.

Sequence in which surfacing of various species takes place: It's due to different oxygen consumption rates and asphxiation points that surfacing of various species takes place in sequence, by which we judge the degree of seriousness It is slight when only Wuchang fish surface because they are the least tolerant to low D.O.C.; it is moderate when Silver carp and Bighead come to surface; it is serious when Grass carp and Black carp gasp for air at water surface. And it is severe when Common carp and Crucian carp come to surface. In that case, a mass mortality of fish will happen.

(iii) Remedies for the surfacing of fish

When there is a symptom of hypoxia, timely measures should be taken to increase the dissolved oxygen content of pond water (fresh water is to be introduced immediately into the pond by using water pumps or aerators). As pumps are used, the outlet of pump should be placed as flat as possible in order to keep fresh water coming out in a straight way. All the fish gasping for air will be attracted to the fresh water area where the D.O.C. is higher.

(2) To control water quality according to water color

Water quality is closely related to fish growth and fish yields. In production, water color is taken as an indicator for adjusting water quality. And various measures are taken to control and improve water quality. From experience, water of fish ponds should be kept fertile active and crisp.

Fertile water means that fish ponds have a fast material recycling and a great biomass of plankton, which are abandunt as natural food for fish.

Active water means the colour of pond water is always changeable. There are daily variations and monthly. A common saying of fish farmers, “brown in the morning and green at night”, indicates the alternative appearance of the dominant species of phytoplankton which are in good quality and great amount.

Crisp water means pond water is fertile, but not turbid. And it has a moderate transparency and a high D.O.C.

The change of water color in fish ponds is rather complicated. It is usually divided into four types: fertile water, sterile water, water bloom and deteriorated water. Based on the change of water colour and routine management, water quality could be improved by an effective feeding and manuring, rational application of aerators, timely water filling and ferquent turnover of pond silt.

(3) Aeration and its effects

At present, impeller aerators are commonly used in China. It has three functions: namely, oxygenation, water-stirring and gas exposure.

(i) Rational application of aerator's

The aerator in not required to be turned on for the whole day and it is only used for a period of time at one or two critical moments. Therefore, we should properly arrange the time and time period of operation according to the functions of aerators and diurnal change of pond water in order to maximize its utilization and effects, but with the minimum energy consumption. There are two main purposes to use aerators in stagnant fish ponds, one is to break the metalimnion of water surface in the day so as to avoid the loss of saturated oxygen to the atmosphere and to prevent the depletion of oxygen at bottom water layer. Another is to directly increase the D.O.C. in fish ponds when oxygen declines to its minimum limit.

From experience, it is suggested that aerators be or not be used according to the following guidelines.

Aerators are operated at noon in fine days. The purpose is to break the metalimnion to maximize the utilization of saturated oxygen. The operation time should not be too long. It is enough so long as the pond water is vertically mixed. It shows by experiments that it's long enough to turn on a 3 kilo watts impeller aerator for 30 minutes in a pond of 3–5 mu, but it takes an hour when the operating load is 7–10 mu of water surface.

If it's a cloudy day, aerators should be put on next early morning. The aim is to increase oxygen directly, because the photosynthesis is rather weak in a cloudly day and dissolved oxygen content is low. After one night, the D.O.C in pond water will decline to its minimum limit. So aerators should be put on from 3:00–5:00 a.m. to the sunrise.

Aerators are often put on at midnight in drizzly days before posible surfacing because the D.O.C is rather low.

However, aerators are not put on at duck or in daytime when the weather is drizzly, because photosynthesis is very weak in cloudy and drizzly days, and the dissolved oxygen C. is not over saturated at the upper layer. So there is no point in stirring water at that time.

(ii) Effects of aerators on fish production

When an aerator is rationally used, it can lessen or basically control the degree of seriousness of fish surfacing and it can also prevent the asphyxiation of fish in ponds. In addition, the material recycling in fish ponds can be accelerated. The uneven distribution of D.O.C in pond water improved, and the metabolic intensity of fish can be promoted so as to decrease the food coefficient, to purify the pond water, to stabilize water quality and to avoid the occurence of fish diseases.