Chapter 4
REARING OF FRY AND FINGERLINGS

Zhu Lingen

Rearing fry and fingerlings involves nurturing 3–4 day-old postlarvae, which have begun to eat food, into fingerlings for pond stocking. Rearing is generally divided into two stages. First, fry are cultured for 18–25 days until they become juvenile fish about 3 cm long; these are known as summerlings. Second, summer-lings are reared for another 3–5 months until they become fingerlings with a body length of 8–20 cm. Most “grow-out” ponds are stocked with such fingerlings and some are stocked with 2-year-old fingerlings.

Fingerlings and especially fry are delicate and small, and their power of movement and ability to feed are weak. Their diet is restricted, they have a low environmental adaptability, and they are vulnerable to predators. In addition, they have a high metabolic rate. Rearing should be carefully managed to maximize survival rate and produce healthy, well-developed fingerlings.

Biology of Fry and Fingerlings

Fish grow quickly during the fry and fingerlings stages and during these stages they have different biological characteristics from adults, especially in terms of feeding habits, growth, and habitat preference.

Food Intake

The changes in feeding organs, feeding patterns, and food composition of fry and fingerlings have already been reviewed. Diet feed stuffs of animal are considered to be of the utmost importance for rearing fry and fingerlings because of their high metabolic rate, fast growth, and food intake. All of these rates decline as the body weight increases. Food intake varies with the type of food and the water temperature. At optimum temperature, the maximum daily food intake of grass carp fingerlings is 49.9 per cent of body weight; for silver carp and bighead 16.8 and 16.4 per cent, respectively. The daily food intake of juvenile grass carp is 32 to 71 per cent of body weight and they show maximum food intake from 0800 to 1600 hrs; this decreases sharply in the evening. The maximum food intakes of silver carp and bighead fingerlings are between 1200 and 2000 hrs, declining after 2000 hrs. Silver carp and bighead stop eating between 2400 hrs and 0600 hrs and the intensity of food intake increases significantly after 0800 hrs.

The retention time of food in the gut by fry and fingerlings is related to water temperature. Rotifera and daphnia (Bosmina longirostris) fed to grass carp fry and fingerlings remain in the gut for 0.5–3.3 h at a water temperature of 20–22°C; when the water temperature reaches 30–32°C, food is digested in less than 1 h. Silver carp digest food in 1.3 h at a water temperature of 22–26°C and within 1 h at a water temperature of 30°C.

Growth Rate

Fry and fingerlings of various species have different growth rates. Even the same species show different growth rates at different developmental stages. Black carp, grass carp, silver carp and bighead fry and fingerlings all have high growth rates. Maximum growth rates occur as fry develop into summerlings; the daily growth rate is 15–25 per cent in length and 30–57 per cent in weight. In the first 10 days after stocking, the body weight of bighead and silver carp fry increase by 5 and 6 times respectively. On average, body weight doubles every 2 days. However, because these fish are small, the absolute increase in weight is rather low. The average daily increase in body weight is between 10 and 20 mg and the average daily increase in body length is 0.7 mm for bighead and 1.2 mm for silver carp. (Table 4.1).

Table 4.1. Growth of silver carp and bighead fry *

* Pond manure: Dacao
  Stocking density = 140,000 fish/mu.

At the fingerling stage, the relative growth rate is 5 or 6 times less than that at the fry stage. Within the rearing period of 100 days, the body weight increases 9 or 10 times. On average, the body weight doubles every 10 days. However, the absolute increase in weight is remarkable (Table 4.2)

Different species of fry collected from rivers and polycultured in a manured pond have different growth rates (Fig. 4.1). Over the first 4 days, the growth of grass carp is fastest; bighead and silver carp are second. However, after the 8th day of rearing, silver carp shows the fastest growth; bighead is second; grass carp is third; and black carp shows the slowest growth. This situation continues until the fry become summer fingerlings. The growth rates of cultivated fish are not only genetically controlled but also closely related to ecological conditions such as nutrition, stocking density, water quality, and temperature.

Table 4.2 Growth of silver carp, bighead and grass carp fingerlings.

Fig. 4.1. Growth curves in 12 days after stocking.

Distribution and Environmental Requirements

Fry are more or less evenly distributed in the pond shortly after stocking. When fry reach a body length of about 1.5 cm, their distribution changes with their feeding habits. Grass carp and black carp begin to move to the middle and bottom layers of the pond and most of them live in the shallow places around the pond dikes where there is more macrozooplankton and benthos. Silver carp and bighead, however, gradually leave the pond banks and move to the central area of the pond, staying in the upper and middle layers.

Fry and fingerlings have much higher metabolic rates than juvenile or adult fish, particularly at the fry stage. For example, the oxygen-consumption rate and energy demand of silver carp fry are 5–10 times greater than those of summer finger-lings and 15–20 times greater than those of 2-year-old fingerlings. Therefore, a high dissolved oxygen content and an abundant food supply are essential for rearing fry and fingerlings. The optimum pH in nurturing ponds is 7.5–9. Fry can tolerate salinities of 4–5 per cent. Growth, is retarded, however, when salinity reaches 3 per cent.

Choice of Ponds and Pond Clearing

Choice of Ponds

Ponds should have the following characteristics: sufficient water supply, good water quality, and convenient irrigation and drainage; a rectangular shape, an area of 1–3 mu (15 mu = 1 ha) for fry, and 2–5 mu for fingerlings, and a water depth of 1.0–1.5 m for fry and 1.2–2.0 m for fingerlings; pond dikes that are solid and watertight, with even bottoms, little silt, and no aquatic weeds; a sunny exposure and plenty of sunlight.

Pond Clearing

Ponds are drained either in the winter or in the early spring. The excess silt and weeds are removed and pond dikes are repaired. At least 7 days before stocking with fry, ponds should be treated with chemicals to eliminate any predatory fish, pathogenic bacteria, parasites or other harmful organisms. This creates an environment conducive to the maximum growth and survival of fry and fingerlings. A small amount of silt must remain at the bottom of the pond to control water fertility and quality.

Quicklime

When clearing a pond, a water depth of 6–10 cm should be maintained. As soon as the quicklime is dissolved, the solution is spread evenly in the pond while it is still hot. The general dosage is 60–75 kg/mu; but increases to 125–150 kg/mu for an undrained pond with a water depth of 1 m. As quicklime absorbs water, it transforms into calcium hydroxide, Ca(OH)₂, raising pH above 11. Quicklime not only kills harmful animals and plants but also eliminates pathogenic bacteria. There are three major advantages of using quicklime for pond clearing. First, quicklime can maintain pH of the pond water at a weak alkalinity, which is conducive to optimum growth. Second, quicklime can release N, P, K, and other nutritional elements that are absorbed by the pond silt, thereby enriching the fertility of pond water. Third, calcium itself is essential to animals and green plants; therefore, by applying quicklime for pond clearing the pond itself is also fertilized.

Tea Cake

Tea cake is the residue of the fruits of Camellia sasangua, or Camellia semiserrata. After the fruit oil is extracted, the byproduct contains saponin (C₃₂H₅₄O₁₈), a hemolytic toxin. At a concentration of 10 ppm, saponin causes fish to die in 11 h. The general dosage is 40–50 kg/mu for a water depth of 1 m. Tea cakes are first crushed into small pieces, which are soaked in the water, and the next day they are evenly spread into the fish ponds.

Bleaching powder

Bleaching powder contains about 30 per cent chlorine. When dissolved in water, it transforms into hypochlorous acid and calcium chloride. The hypochlorous acid immediately releases free oxygen (O), which is toxic to many pathogens and other harmful organisms.

Ca(ClO)₂ + H₂O → HClO + Ca^CI_OH
HClO → HCl + (O)

The usual dosage of bleaching powder is 13.5 kg/mu (20 ppm) for a water depth of 1 m. The bleaching powder is first dissolved in water and then evenly spread into the fish ponds.

Rotenone

Rotenone is extracted from the roots of leguminous plants (Derris uliginosa and Derris elliptica). The extracted solution contains about 25 per cent rotenone. The recommended rotenone concentration for pond clearing is 1.3 kg/mu (2 ppm) for a water depth of 1 m. The rotenone solution is diluted 10–15 times with water and then evenly spread into the fish ponds.

Rearing of Fry

Application of Base Manure

After clearing and before stocking the pond with fry, a base manure should be applied to the pond to propagate a planktonic community (rotifera, nauplii, microzooplankton, etc.). Zooplankton proliferate quickly after manure application. Peak populations occur in the following order: protozoans, rotifers and nauplii, microcladocerans, macrocladocerans, copepods. Pond stocking should be timed to ensure a sufficient supply of palatable, natural food at each growing stage of the fry.

The application time and quantity of base manure applied depend on the type of manure. Decayed manure and compost are applied 3–5 days before stocking at a rate of 150–300 kg/mu. Dacao or green manure is applied 5–10 days before stocking at a rate of 200–300 kg/mu. After stocking, additional manure should be regularly applied to stabilize the fertility of the pond.

Stocking Density

The growth and survival of fry are closely tied to the stocking density. This is because stocking density has a direct effect on food supply, water space, and water quality. The optimum stocking density for silver carp and bighead is 100,000-120,000 fry/mu; for grass carp and black carp 80,000–100,000 fry/mu. Lower stocking rates will decrease yield and increase production costs. Stocking density can be optimized with skillful farming and careful management.

Culturing Methods

Dacao

Dacao is commonly used as a green manure for nurturing fry. A base manure should be applied before stocking, and additional manure should be added according to water fertility and dissolved oxygen content. Dacao is applied at a rate of 150–200 kg/mu once every 4 or 5 days. Pond fertility is relatively unstable under Dacao application; therefore, application times and quantities should vary depending on water quality and weather. During the rearing period, commercial food should be added if poor growth is observed.

Soybean Milk

Soybean milk is made from soybean that has been soaked in water and then ground with some water. Milk yield varies with soaking time. The optimum soaking time is 6–7 h at a water temperature of 25–30°C. When grinding, it is proper to add some water so that 7.5–10 kg of milk can be produced from 0.5 kg of soybean. Bean milk is spread evenly into the fry ponds 2 or 3 times daily. Generally, the milk from 3–4 kg of soybean is sufficient for 1 mu of pond area. After rearing for 5 days, the milk from 5 kg soybean should be applied to a 1-mu area. However, the quantity of milk may be varied according to fry growth and pond fertility. Only a portion of the soybean milk is consumed by the fry; most of it serves as a fertilizer. Soybean milk gives a more stable water fertility than Dacao.

Mixture of Soybean Milk and Manure

In this technique, organic manure (Dacao or animal manure) is applied as the base manure before stocking. During the rearing period, either soybean milk or more organic manure is added to the pond depending on the water fertility. The usual amount of organic manure added is 150–200 kg/mu. This method combines fish feeding and pond fertilization and is easily practised.

Compost

Compost is a mixture of green grass and organic manure. After fermentation and putrefaction of the mixture, it is even used to rear fry. Before stocking, compost is applied once or twice as a base manure at a rate of 500 g/m³ (333 kg/mu) of pond water. After stocking, manure should be applied twice daily. The amount applied depends on the water quality and pond fertility; the usual rate of application is 100–150 g/m³ (60.6–99.9 kg/mu) of pond water. The additional manure, which is evenly spread into the pond, is the liquid part of the fermented compost. Application results in a significant reduction in oxygen consumption and a stabilization in plankton propagation. This is because the compost has already been fermented and putrefied and is applied frequently in small amounts.

Grass paste

Grass paste is prepared with water hyacinth, water lettuce, water peanut, etc., in a high-speed masher. Water peanut contains saponin which is toxic to fish. To decrease the toxicity of saponin, table salt is added to the paste at a rate of 0.2– 0.5 per cent of the wet weight of water peanut. The nutritional elements in grass paste are easily consumed; therefore, grass paste should be applied twice a day, once in the morning and once in the afternoon, at a rate of 25–38 kg/mu, depending on the water fertility. Fry only consume the palatable granules (mesophyll cells, organic detritus) in the grass paste; the rest of the paste is rapidly decomposed by bacteria and fertilizes the pond.

Pond Filling and Pond Management

During the rearing period, filling the pond with water by installment is important to achieve the maximum growth and survival of the fry. During stocking, the optimum water depth is 50 cm. This is because the temperature of shallow water increases easily, accelerating the decomposition of organic manures, promoting the propagation of natural organisms, and fostering the growth of fry. In addition, because the water volume is comparatively small, less feed and manure are used. After rearing for many days, according to fry growth and water quality, fresh water should be let in to raise the water level, improve the water quality, expand the water space, and promote fish growth. The first addition of fresh water should be conducted 6 or 7 days after stocking. Fresh water should then be added once every 4 or 5 days, increasing the water level by 20–30 cm each time until water level reaches a maximum depth.

The condition of the ponds and the behaviour of fish should be observed twice daily. Every morning and afternoon, water colour and fry activities, including surfacing should be observed to determine the amount of feed and manure to be administered and if fresh water should be added. If the fry gasp for air at the water surface in early morning and continue to surface after sunrise, the dissolved oxygen content is too low and fresh water must be added accordingly.

If observed, harmful insects, frog eggs, etc., should be removed. Fry should be examined regularly for infection to prevent the spread of disease.

If, after stocking, the fry swim madly in groups along the pond dike, passage of fry should be obstructed with a net or a fragrant food (such as wine lees) should be used as a lure. Otherwise the fry will be unhealthy and exhibit a high mortality.

Training and Transferring of Summer Fingerlings

When fry are reared into summer fingerlings, they must be transferred into fingerling ponds for further nurturing. Before this the fish should be conditioned so that they are strong enough to tolerate the transferring operation. In addition, because of conditioning, fish do not secrete large amounts of mucus and excreta, which would pollute the water during transportation; therefore, the survival rate is improved. To condition the fish, it is caught in a net and placed in a crowded cage for 5 h before transferring. If summerlings are to be transported a long distance, the fish should be put in a second cage in a clear-water pond overnight. Conditioning of summerlings should be performed on days when the fish are not surfacing and started between 0900 and 1000 hrs. Care must be taken at all stages of conditioning to avoid injuring fish and special attention must be paid to the dissolved oxygen content in the cage to ensure the survival of fish.

Rearing of Fingerlings (from Summerling to Yearling)

Ponds and Base Manure

The requirements of fingerling ponds have been mentioned previously and the clearing of fingerling ponds is the same as that of fry ponds. The organic base manure should be applied 5 or 6 days before stocking at a rate of 200–400 kg/mu to provide the fish with an abundant supply of natural food.

Stocking Densities and Polyculture Ratios

It is best to select well-developed fingerlings of uniform size with bright colouration and thick dorsal regions. This type of fingerling is undoubtedly healthy. At the fingerling stage, feeding habits and habitat preferences change. In polyculture, the water body and the various natural foods available in the pond are intended to be fully utilized; however, all the fish prefer to feed on commercial food. Food competition will occur if many species are polycultured in one pond. As a result, the growth of some fish may be adversely affected and fingerling size will be difficult to control. In general, two or three species of fish are polycultured in one pond; there is one major species, the others are minor species.

In polyculture, the stocking ratio of different species should be determined according to the different feeding habits and relationships between the species. For example, with a high stocking density and high commercial feed input, silver carp and bighead polyculture is not practised because silver carp would outcompete bighead for food; this would adversely affect the growth of bighead. If these two species have to be polycultured, the only practical combination is a small proportion of bighead (10–15 per cent or less) mixed with silver carp, the major species. Similarly, grass carp have stronger ability to compete for food than black carp. Therefore, if these two species have to be polycultured, a small proportion of black carp is mixed with grass carp, the major species. Common polyculture combinations are grass carp mixed with silver carp and common carp, and black carp mixed with bighead (Table 4.3).

The stocking density depends mainly on the targeted size of fish for next transferring (Fig. 4.2.). Under good rearing conditions, the stocking density is about 10000 fish/mu.

To produce large fingerlings in a short time, “multiple-grade conveyor culture” is practiced. The stocking density is gradually attenuated and, thus, fish growth is not retarded (Table 4.4).

Rearing of Fingerlings

Feeding as the main approach

Grass Carp — In feeding grass carp, it is important to supply palatable, balanced green fodder at different developmental stages. After stocking, grass carp fingerlings are fed Wolffia arrhiza, which is abundant and nutritionally complete. When grass carp reach a length of 7 cm, Lemna minor or Hydrilla verticillata is supplied. When grass carp reach a length of 8 cm (after 20 days rearing), other aquatic grasses are used (Table 4.5). It should be noted that feed amounts must be carefully controlled during the epidemic season of fish diseases. (The optimum amount of feed can be consumed in 5–8 h.) To prevent grass carp from competing for food with silver carp and bighead, they are fed grasses first. Silver carp and bighead are then fed commercial food.

Table 4.3. Stocking densities, polyculture ratios of summerling and targeted size of fingerling.

* BC, black carp;
BH, bighead;
BR, wuchang bream;
GC grass carp;
SC, silver carp.

Fig. 4.2. Relationship between stocking density and the target transfer size of fish. (It is shown that larger fish can be obtained by reducing the stocking density of fish pond.)

Table 4.4. Stocking models for fingerling rearing of grass carp (GC), silver carp (SC) and bighead (BH) in monoculture.

Table 4.5. Feeding schedule for grass carp yearling.

Black carp — After stocking black carp summerlings, soybean cake paste or soybean dregs are supplied first in a fixed location (Table 4.6). Subsequent feeding will always be in the same place. The daily ration of soybean cake paste for 10,000 fish is 1–2 kg and should be supplied in two feedings; one between 0800 and 0900 hrs and the other between 1400 and 1500 hrs. At the same time, a considerable amount of Wolffia arrhiza is added as a supplement. When black carp reach a length of 10 cm, crushed snails can be offered (Table 4.6): the initial daily feeding rate for 10,000 fish is 35 kg and is gradually increased to 125 kg. All the snails should be supplied and consumed between 0800 and 1600 hrs. When feeds are changed, the two types (old and new) must first be mixed and given to the fish so that the taste of new food will be gradually accepted. The feeds must be fresh and well distributed and the feeding platforms kept clean.

Table 4.6. Feeding schedule for black carp yearling.

Silver carp and bighead — Before rearing silver carp and bighead fingerlings, a base manure is applied to propagate plankton. At the initial stocking stage, silver carp, besides feeding on natural food should be given 1.5–2 kg soybean dregs per 10,000 fish daily. After 1 week of rearing, fish should be given 2–3 kg of wine lees or 0.5–1 kg of soybean cake paste per 10,000 fish daily. The daily amount of bean cake can later be increased to 1.5–2 kg per 10,000 fish. During the culture period, additional manure should be applied according to the fertility of the pond water. Bighead fingerlings are fed in much the same manner as silver carp, but the amounts for bighead should be at least twice those for silver carp.

Common carp and wuchang fish — In a pond with common carp as the major species, bean dregs and other commercial feeds are supplied daily at a rate of 1.5 kg per 10,000 fish. The food is applied to the shallow regions near the bank and amounts should be increased as the fish grow. In a pond with wuchang fish as the major species, bean cake paste is first given twice daily at a rate of 0.5 kg per 10,000 fish. Wolffia arrhiza, Lemna minor, tender grasses, and aquatic plants are then supplied. At the same time, organic manure is applied to increase the amount of natural food.

To raise the efficiency of food utilization and decrease the food-conservasion factor, it is necessary to adhere to the feeding principles and methods, i.e., fixed feeding times and locations. To avoid wastage, food should be supplied on the fixed ground or platforms. These methods also allow the convenient observation of food intake and the elimination of debris. Grasses fed to grass carp and wuchang fish are put in triangular or square bamboo frames. Commercial feeds are usually put on platforms made of planks and frames with an area of 1–2 m². There is one platform for every 5,000–8,000 fingerlings. During the season when fish are vulnerable to disease, platforms should be disinfected regularly. Snails are commonly put on the solid bottom of the pond. The water depth here is about 70 cm.

Manure loading as the main approach

Manure loading is effective in rearing silver carp and bighead. In the 1st month after stocking, organic manure is applied once every 10 days at a rate of 200– 250 kg/mu. In the 2nd month, the same amount of manure is applied once every 7 days. As the water temperature declines, the manure loading rate should also decrease. At that time, commercial food is added daily at a rate of 2–3 kg per 10,000 fish. Summerlings are stocked in a season when the water temperature is increasing; therefore, water quality must be carefully monitored to ensure a sufficient dissolved oxygen content.

Fertilization with barnyard grass

Barnyard grass (Echinochioa crusgalli) grows quickly and is usually planted in unoccupied fingerling ponds. When the grass is mature, enough water is introduced to submerge the grass. The grass then gradually rots and decomposes as a function of both fertilizer and food.

The precise method of barnyard grass fertilization is as follows: First, the ponds are drained in early May. A base manure is necessarily to be applied to sterile pond bottom. Barnyard grass seeds are then sowed at a rate of around 5 kg/mu; they sprout in 2 or 3 days. After 1 month, the barnyard grass can reach a height of 80 cm. Enough water is then introduced to submerge the grass (1.5 m). After the barnyard grass have been submerged for 5–7 days, the pond water is fertile. The usual stocking density of summer fingerlings is 5,000–7,000 fish/mu, with silver carp and bighead as the dominant species (Table 4.7). In such a pond, nitrogen and phosphorus contents are high. However, barnyard grass consumes a lot of oxygen during its putrefaction and, therefore, the dissolved oxygen content must be closely monitored.

Table 4.7. Yield and survival of fingerlings nurtured in the ponds with pre-planted barnyard grass as manure.

^a Origin of the hybrid is unknown.

Pond management

The ponds should be observed every morning and the activity of the fish and the water colour should be noted. If the fish gasp for air for a long time, fresh water should be added. In the afternoon, the feeding status of the fish must be determined so that the feeding schedule for next day can be planned.

The pond area should be kept clean. Any decayed debris should be immediately removed and feeding places should be frequently disinfected. Because of disease, food type, and other factors, fish may have different growth rates. Therefore, fingerlings should be sieved in August or September and cultured separately according to size to ensure a uniform final size.

Rearing of 2-year-old fingerlings

Black carp

From yearling to 2-year-old fingerling, the survival rate of black carp is only 30 per cent. This is due to their heavy food demand and susceptibility to disease. Stocking — The stocking densities and stocking ratios of the various fish species in the pond depend on the desired final sizes and yields (compare Table 4.8 and 4.9).

Table 4.8. Stocking and harvesting model of a 2-year-old black carp fingerling pond.

Table 4.9. Stocking and harvesting model of a 2-year-old black carp fingerling pond.

^a When silver carp and bighead with an initial body length of 12–13 cm reach 0.1–0.15 kg, they are transferred to the grow-out pond.
^b Crucian carp are stocked in May and 3-cm big head are stocked in late July and early August.

Management — At the 2-year-old fingerling stage, black carp stop eating commercial plant food and begin eating animal food (snails, Corbicula sp., etc.). Feeding starts in March. At first, bran, dregs, bean cake, etc., are supplied once every 3 or 4 days at a rate of 3–5 kg/mu. In April, small snails are supplied once every second day at 25 kg/mu. Later, this rate is increased by 5 kg/mu at each feeding. All the food supplied should be consumed within 24 h, otherwise, water quality will deteriorate because of the decaying snails, and the fish may become diseased. In May and June, crushed snails can be given to the fish. Whole snails can be supplied as the fish grow bigger. Water quality must be determined daily and adjusted if necessary. Because black carp at this stage are very weak in adapting to the fertile water, any symptoms of disease must be immediately dealt with and over-feeding must be avoided. Black carp at this stage, also lack the ability to tolerate a low dissolved oxygen content; therefore, this aspect of water quality must be closely monitored (fish surfacing indicates low oxygen content).

Stocking — The stocking density and proportion of grass carp in polyculture are similar to those of black carp. The usual stocking density of grass carp is 800– 1,000 fish/mu and minor species usually include silver carp, bighead, and black carp.

Management — The frequency and quantity of food given to 2-year-old grass carp fingerlings depend upon the growth of the fish, the acceptability of the food and the season. In March, bran, dregs, and other commercial feed are usually applied once every several days at a rate of 2.5–5 kg/mu. In April, duckweed (Lemna minor) is applied. After May, English ryegrass, lettuce leaves, Vallisneria spirolis, and tender terrestrial grasses are supplied. The feeding amount is based mainly on the weather; it is best to apply the food in the morning and have all the food consumed by 1600 hrs. The grasses must be fresh and any leftovers should be removed every afternoon to prevent deterioration of water quality.