CHAPTER VI Rearing of Fry and Fingerlings

Rearing fry and fingerlings is to nurture 3 or 4-day-old postlarvae, which start to eat food, into fingerlings for stocking in grow-out ponds. It is generally divided into two stages: at the first stage, fry are cultured for 18–25 days to a body length of 3 cm, usually known as summerlings. At the second stage, summerlings are reared for another 3--5 months to fingerlings with a body length of 8--20 cm. Most of the grow-out ponds are stocked with fingerlings, but sometimes with some two-year-old fingerlings.

At fry and fingerling stage, especially at fry stage, fish are not only delicate and small, but their power of movement and their ability to feed are weak. Their diet is restricted to a number of items and they have very low adaptive power to changes in environmental conditions and have little power to escape from enemies. They, on the other hand, have a high metabolism. Careful management should therefore be performed to raise the survival rate of fry and fingerlings and to produce healthy and desirable-sized fingerlings.

1. Biology of Fry and Fingerlings

Fish have a fast growth during fry and fingerling stage and their biological characteristics are different from that of the adults, especially in feeding habits, growth and habitus.

1) Food intake

The changes of their feeding organs, feeding patterns and food composition of fry and fingerlings are already mentioned in the previous chapters. All in all, animal feedstuff is considered to be of great importance for fry and fingerlings. They have a higher metabolic intensity, faster growth and greater food intake, but all these are relatively declining with the increase of their body weight. The amount of their food intake varies with kinds of food and water temperature, etc. at different developmental stages. Under the optimum temperature, the maximum daily food intake of Grass carp fingerlings comes to 49.9% of its body weight, but only 16.8% and 16.4% for Silver carp and Bighead respectively. It is reported that the daily food intake of juvenile Grass carp is 32--71% and the diurnal variation of food intake is as follows: juvenile Grass carp have a maximum food intake at 8:00 and 16:00 and it increases in the evening while the maximum food intake of Silver carp and Bighead fingerlings is between 12:00--20:00 and it declines after 20:00. Silver carp and Bighead stop eating between 24:00 and 06:00 and the intensity of food intake is rising obviously after 08:00. The retention time of food in the gut of fry and fingerlings is related to the water temperature. It is shown by the experiment that rotifera and Daphnia Bosmina longirostris fed to Grass carp fry and fingerlings remain in the qut for an hour and a half to three hours and twenty minutes at water temperature of 20–22°C. When the water temperature reaches to 30–32°C, foods are digested in less than one hour. The food in Silver carp's gut is digested in one hour and twenty minutes at water temperature of 22–26°C and even within one hour when the water temperature is 30°C.

2) Growth rate

Fry and fingerlings of various species have different growth rates. Even the same species also have different growth rates at different developmental stages. At fry and fingerling stage, Black carp, Grass carp, Silver carp and Bighead all have a high growth rate, but from fry to summerlings, their relative growth rate is at its maximum, which is the peak in their life span. Particularly, the relative growth rate is much higher between 3 and 10 days after stocking and the daily growth rate is 15--25% in length and 30– 57% in weight. Based on the measurement, within 10 days after being stocked in ponds, the body weight of fry would be double 6 times for Silver carp and 5 times for Bighead. On an average, it is double the previous weight every two days. But fish during this period are small in size and therefore, the absolute increase of their body weight is rather low. The average daily increase in body weight is only between 10 and 20 mg and the average daily increase in body length is 0.71 mm for Bighead and 1.2 mm for Silver carp. (see Table 4--1)

Table 4--1 Growth Rate of Silver carp and Bighead Fry

Note: Dacao is used as manure for fish ponds and the stocking density is 140000 ind/mu

At the fingerling stage, the relative growth rate conspicuously decreases, compared with the fry stage. Within the rearing period of 100 days, the body weight of fish is double 9--10 times. On an average, it doubles every 10 days, which is 5--6 times less than that at the fry stage. However, the absolute increase in weight is remarkable. (see Table 4--2)

Table 4--2 Growth Rate of Silver carp, Bighead and Grass carp Fingerlings

Unit: mm & g

In accordance with the observation during the cultivation, the fry of four species collected from rivers and polycultured in a manured pond have different growth rates. after being stocked for 1--4 days, the growth of Grass carp is the fastest; Silver carp and Bighead the second. However, after the 8th day of rearing, Silver carp always come in first; Bighead the second; Grass carp the third and Black carp always the last. This remains until the fry reach to summer fingerlings. The growth curves of the four species of fish are shown in Figure 4--1.

Figure 4--1 Growth curves within the first 12 days after stocking

Apart from heredity, the growth rate of cultivated fish is closely related to the ecological conditions such as nutrition, stocking density, water quality and temperature, etc.

3) 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 follows the change of their feeding habits. Grass carp and Black carp start to move to the middle and bottom layers of water body and most of them live in the shallow places around pond dikes where there are more macro-zooplankton and benthos; whereas Silver carp and Bighead gradually leave pond banks and move to the central area and stay in the upper and middle layers of water body.

Fry and fingerlings have a much higher metabolic rate, particularly at the fry stage. For example, the oxygen consumption rate and energy demand of Silver carp fry is 5--10 times as much as that of summer fingerlings and it is even much higher than that of two-year-old fingerlings. The status of other species is similar to the above; therefore, high dissolved oxygen and abundant food supply should be ensured for rearing fry and fingerlings. The optimum pH value in nurturing ponds is around 7.5--9. Fry can tolerate salinity of 4--5%. Its growth however, is retarded when the salinity reaches 3%.

2. Choice of ponds for Fry and Fingerlings and Pond Clearing

1) Choice of ponds for fry and fingerlings

The conditions to be considered are as follows:

1. With sufficient water supply, good water quality and a convenience for irrigation and drainage.

2. In regular order and with its shape rectanglar and its area moderate (The optimum area of fry pond is 1--3mu and that of fingerling pond 2–5mu) and easy for operation. The water depth is 1--1.5m in fry pond and 1.2-2.0m in fingerling pond.

3. The pond dikes should be solid, leak proof with even bottom, less silt and without aquatic weeds.

4. With a sunny exposure and plenty of sunlight.

2) Pond clearing with drugs

Ponds are drained either in winter or in early spring just for freezing and sunning. Then remove the excessive silt and weeds in ponds and repair pond dikes. Before stocking of fry, ponds should be treated with drugs to eradicate predatory fish, harmful organisms, pathogenic bacteria and parasites, etc. This can create conducive living environment for fry and fingerlings to get the maximum growth and higher survivability. A certain amount of pond silt is however, needed at the bottom so as to control water fertility and quality.

(1) Clearing ponds with quicklime (Cao)

When clearing a pond, a water volume of 6--10cm deep should be maintained in the pond. As soon as quicklime dissolves in water, the solution is spread evenly in the pond while it is still hot. The general dosage is 60--75kg/mu or 125--150 kg/mu for the undrained pond with a water depth of one meter. When quicklime absorbs water, it immediately transforms into calcium hydroxide Ca(OH)₂ which raises pH to above 11. Quicklime for pond clearing can not only kill harmful animals and plants, but also eradicate pathogenic bacteria. Quicklime has both comprehensive and thorough effects for clearing ponds. The advantages of using quicklime for pond clearing are as follows:

1. It can maintain the pH value of the pond water at a weak alkalinity so that it's conducive to the optimum growth of fish.

2. It can release N.P.K. and other nutritional elements which are absorbed by pond silt with the result that it could enrich the fertility of pond water.

3. Calcium itself is an essential nutritive element for animals and green plants, so, to apply quicklime for pond clearing can fertilize pond water.

(2) Clearing ponds with tea cake

Tea cake is the residues of the fruits of Camellia oleosa or C. sasangua or C. semiserrata. It is a by-product after its oil has been extracted. It contains saponin (C₃₂H₅₄O₁₈), a hemolytic toxin, which at a concentration of 10 ppm can cause fish to die in 11 hours. The general dosage is 40–50kg/mu for a water depth of one meter. The tea cakes are first crushed into small pieces, which are soaked in the water and then, they can be evenly spread into fish ponds next day.

(3) Clearing pond with bleaching powder

Bleaching powder contains about 30% chlorine. It dissolves in the water and transforms into hypochlorous acid and calcium chloride. The hypochlorous acid immediately releases new born (O) which can kill pathogen and harmful organisms.

The usual dosage is 13.5kg/mu for a water depth of one meter (at a concentration of 20ppm). Bleaching powder first dissolves in water and then is evenly spread into fish ponds.

(4) Clearing ponds with rotenone

Rotenone is extracted from the roots of leguminous plant Derris uliginosa and Derris elliptica. The solution contains about 25% rotenone. The effective concentration of rotenone for pond clearing is 2 ppm, that is, 1,332g/mu for a water depth of one meter. The rotenone solution is first diluted 10--15 times with water and then evenly spread into ponds.

If the drugs mentioned above are used for pond clearing, all the drugs must be applied 7--10 days before the stocking of fry.

3. Rearing of fry (from fry to summerlings)

1) Application of base manure

After pond clearing and before the stocking of fry, base manure should be applied to the pond so as to propagate the planktonic community (such as rotifera, nauplius and micro-zooplankton, etc.) as proper feeds for fry. After manuring, various zooplanktons proliferate fast in the pond. The peak orders are as follows: protozoa--rotifera and nauplius--micro-cladocera--macro-cladocera-copepoda. Therefore, fry should be timely stocked in ponds so that they can get enough palatable natural food at each growing stage. The applying time and quantity of base manure are dependent on the type of manure. If the decayed manures or compost are used, they are applied 3--5 days before stocking with the amount of 150--300kg/mu. And if Dacao or green manure are used, they are applied with the amount of 200 --300kg/mu, 5--10 days before stocking. After stocking, additional manure should be applied so as to stabilize the fertility of the pond water. At the initial stage (fry are only reared for the first 10 days with a body length of less than 2 cm), fry should be ensured with enough natural food of rotifera and micro-cladocera and their biomass are 20-40mg/l. At the late stage (Fry have been reared for more than 10 days and their body length reaches more than 2 cm.) while applying manure to propagate zooplankton, the biomass of phytoplankton required for culturing Silver carp is 30 mg/l and the commercial food is supplemented for Grass carp and Bighead at the late stage.

2) Stocking density

The growth and survival rates of fry are closely related to stocking density. That is to say, they are affected by food, water space and water quality. The optimum stocking density for Silver carp and Bighead fry is 100,000--120,000 ind/mu, Grass carp and Black carp fry 80,000--100,000 ind/mu. The lower stocking rate of fry will waste water body and increase the production cost. The stocking density can be appropriately increased with skilful farming techniques and careful management.

3) Culturing methods

(1) Culture with Dacao

Dacao is used as green manure for nurturing fry. Base manure should be adequately applied before stocking, but the additional manure is put into fry ponds in accordance with the water fertility and the dissolved oxygen content. Dacao is applied once every 4--5 days, with 150-200kg/mu each time. During the rearing period, commercial food should be supplemented if the poor growth of fry is observed. However, the fertility of pond water is not so stable by Dacao application. Therefore, the applying time and quantity of Dacao vary with the pond water quality and weather.

(2) Culture with soyabean milk

Bean milk is made from soyabean which is soaked in water and then ground with some water. The milk producing rate is related to the soaking time of soyabean. The optimum soaking time is 6--7 hours at water temperature of 25--30°C. When grinding, it is proper to add some water at a standard of 7.5--10kg of milk from 0.5kg of soyabean. Bean milk is spread evenly into fry ponds 2--3 times every day after stocking. Each mu of fry ponds genearlly requires the amount of bean milk which is made from 3--4kg of soyabean. The amount of soyabean should be increased to 5kg after rearing for 5 days. However, the feeding quantity may be increased in accordance with the growth of fry and the fertility of pond water. Bean milk is required to be spread evenly into the pond. Part of bean milk in the water is directly consumed by fry and most of it are used as fertilizers to propagate natural food. The water fertility in fry ponds by using soyabean milk is more stable than that by using Dacao.

(3) Culture with a mixture of soyabean milk and manure

Organic manure of Dacao and other animal manures are applied into the pond as the base manure before stocking. During the rearing period, soyabean milk is supplemented in less amount in this method than that in the previous method. According to the water fertility, the additional manure should be applied instead of bean milk. The general amount of organic manure added is 150--200kg/mu. This method is the combination of feeding fish and manuring the pond for the propagation of plankton. It is easy to be practised.

(4) Culture with compost

Compost is a mixture of green grasses and organic manure. It is also used for rearing fry after fermentation and putrefication. Before stocking, compost as base manure is applied once or twice at a rate of 500g/m³ of pond water. This manure application should be continued twice every day after stocking. However, the amount of additive manure applied depends on the water quality and the fertility of the pond water. The general application amount is 100--150g/m³ of pond water. The additive manure applied is the liquid part of fermented compost, which is evenly spread into fry ponds. The consumption of oxygen is greatly reduced and it stabilizes the propagation of plankton because compost has already been fermented and putrefied and moreover, it is applied more frequently with small amounts each time.

(5) Culture with Grass paste

Grass paste is mashed water hyacinth, water lettuce, water peanut, etc. through high-speed masher. Water peanut contains saponin, a harmful substance, therefore, in its paste we should add some table salt of about 2--5% of its weight in order to decrease the toxicity of saponin. The nutritional elements in grass paste are easily to be consumed, so grass paste should be applied in time every 2–3 days. The method of application of grass paste is twice a day, one in the morning and another in the afternoon, at a rate of 50--75kg/mu/day. But it also depends on the water fertility. Fry only consume the palatable granules (mesophyll cells, organic detritus) in grass paste, while the rest of the paste is rapidly decomposed by bacteria as manure in the pond.

4) Filling the pond at different stages and pond management

During the rearing period, filling the pond with water by instalments is considered as an important measure to raise the growth and the survival rate of fry. The optimum water depth is 50cm when fry are stocked because the temperture of shallow water increases easily and can accelerate the decomposition of organic manures and promote the propagation of natural organisms. So it is favourable for the growth of fry. At the same time, it can also save the amount of feeds and manures because the water volume is comparatively smaller. In accordance with the growth of fry and water quality, fresh water should be let in after rearing for many days in order to raise water level to improve water quality, to expand water space and to promote the growth of fish. The amount and frequency of water filling are dependent on the concrete conditions. Generally speaking, the first filling of fresh water is conducted when fry are stocked for 6--7 days. Then fresh water is frequently filled into fry ponds once every 4--5 days by 20--30cm deep each time, and finally, the water level is increased to its maximum.

The daily management of fry ponds is as follows:

1. The ponds should be patrolled every morning and afternoon for careful observation of the water colour and fry activities including surfacing to determine the amount of feeds and manures to be administered and whether it is necessary to fill the pond with fresh water for next day. If fry gasp for air at the water surface in the early morning and continue to surface even after the sunrise, it shows that dissolved oxygen content is too low, so fresh water must be timely filled into the fry pond.

2. Harmful insects, frog eggs,etc. should be eradicated. Fry should be often examined for disease infection so that they can be cured at an early date.

3. If fry swim madly in groups along the pond dike after stocking, measures should be taken to stop swimming of fry by obstructing the passage of fry with a net or using fragrant food (such as wine lees) to lure them, otherwise it will cause poor health and high mortality of fish.

5) Training and Transferring of summer Fingerlings

When fry are reared into summer fingerlings, their body sizes are much larger. It is necessary to transfer them into fingerling ponds for further nurturing, but conditioning should be conducted before transfer in order to make fish strong enough to tolerate the transferring operation; besides, through conditioning, fish can avoid secreting a large amount of mucus and excreta which will pollute the water quality during the transportation. Therefore, it can raise the survival rate. The method of conditioning is to net the fish into a cage where fish are crowded for 5 hours before transferring. If summerlings are to be transported to a distant place, fish should further be put in a cage for the second time in a clear water pond overnight. Conditioning of summerlings should be performed in fine days without surfacing, usually at 9:00--10:00 a.m.. The conditioning operation is to be conducted carefully for fear that fish should be injured. Special attention must be paid to the dissolved oxygen content in the net-cage in order to ensure the safety of fish.

4. Rearing of Fingerlings (From summerling to yearling)

1. Fingerling ponds and base manure

   The requirements of fingerling ponds have been mentioned above and the clearing of fingerling ponds is the same as that of fry ponds. Generally, the organic base manure should be applied 5--6 days before stocking at a rate of 200–400kg per mu in order to provide the fish with abundant natural food.

2. Stocking density and ratio of fish species in polyculture

   It is better to select fingerlings with complete carriage, bright colouration, thick dorsal part and uniform size. all those fingerlings are considered as the healthy ones. At the fingerling stage, the feeding habits of various species of fish on natural food have obviously changed, so have their habitats. Polyculture is practised to fully utilize the water body and various foods available in ponds; nevertheless, all the fish prefer to feed on the commercial food. Food competition will occur if many species are polycultured in one pond. As a result, the growth of some fish may be affected and the size of fingerlings will be difficult to control. In general, 2--3 species of fish are polycultured in one pond. One species is taken as the major species and others as the minor species.

   In polyculture, stocking ratio of different species should be taken into consideration in accordance with the different feeding habits and the relationships between species such as the relationship between Silver carp and Bighead or between Grass carp and Black carp. Generally, polyculture of Silver carp and Bighead is not practised in one pond under the conditions of high stocking density and high input of commercial feeds, because Silver carp have a comparatively strong ability in competing for food than Bighead. The growth of Bighead will be affected if they are polycultured in one pond. If these two species have to be polycultured in one pond, only a small proportion of Bighead (usually less than 10-15%) are mixed with the major species, Silver carp, while Silver carp are not mixed with Bighead if Bighead are taken as the major species in the pond. Similarly, Grass carp have weaker ability to compete for food than Black carp, so the polyculture of these two species is not usually practised. If it has to be performed, only a small proportion of Black carp is stocked in ponds with Grass carp which is taken as the major species. Black carp may take some natural benthos as their food.

   In production, Grass carp are mostly mixed with Silver carp and Common carp while Black carp are often with Bighead. In this way, it can promote the growth of Grass carp and Black carp and can strengthen their target fish's ability to feed. So long as pond water has been fertilized, Silver carp or Bighead or Common carp can be stocked into the ponds as the minor species.

   The stocking density mainly depends on the target size of fingerlings. The relationship between the stocking density and the transferring size is listed in Table 4--3.

Table 4--3 Stocking Density and Transfer Size of the Target Fish

Under the good rearing conditions, the total stocking density is about 10,000 ind/mu. Examples are shown in Table 4--4.

Table 4--4 Stocking Density and Polyculture Ratio of Summerlings

In order to produce large-sized fingerlings in short time; multiple grade conveyor culture can be practised. The stocking density can be attenuated grade by grade and will be getting more reasonable. Thus, the growth of fish will not be retarded. Examples are shown in Table 4--5 and 4--6.

Table 4--5 Stocking Model for Grass carp

Table 4--6 Stocking Model for Silver carp and Bighead

Note: The stocking density of Bighead is 20% lower than that of Silver carp.

3) Rearing method of fingerlings

(1) Food feeding as the main approach

(i) Feeding for Grass carp

The main point for feeding Grass carp is to supply the palatable and balanced green fodder at different developmental stages. After stocking, Grass carp fingerlings are fed with Wolffia arrhiza, which is abundant, and its nutritional elements are more complete. When Grass carp reach to more than 7 cm in length, Lemna minor or Hydrilla verticillata can be supplied and when Grass carp reach to more than 8 cm in length after 20 days rearing, other aquatic grasses can be used. It should be noticed that the feeding amount must be controlled during the epidemic season of fish diseases. (It is the optimum amount if the feeds can be consumed within 5-8 hours.) In order to prevent Grass carp from competing with Silver carp and Bighead, they are fed first with enough grasses every day and then the commercial food for Silver carp and Bighead, thus, the food intake and the growth of the latter two can be ensured. Examples of feeding for Grass carp are shown in Table 4--7.

Table 4--7 Feeding Schedule for Grass carp yearlings

(ii) Feeding for Black carp

After the stocking of Black carp summerlings, soyabean cake paste or soyabean dreg etc. are supplied first in a fixed feeding place. Afterwards, fish will feed habitually in that fixed place. The daily ration of soyabean cake for 10000 tails is 1–2kg which should be ground into the paste by mixing with some water and supplied twice a day, one between 8 and 9 o'clock in the morning and the other between 2 and 3 o'clock in the afternoon. At the same time, a considerable amount of Wolffia arrhiza is supplemented. When Black carp reach a body length of above 10 cm, the crushed snails can be offered.

The feeding rate is 35 kg/10000 ind/day at the initial stage and it is gradually increased to about 125kg/10000ind/day. It is better to have all the snails supplied and eaten up by fish between 8:00 a.m. and 4:00 p.m. The point which should be noticed is that when we change one food to another for Black carp fingerlings, the two types of food must be mixed at a certain proportion so that the taste of new food can be gradually accepted by fish. Attention should be also paid to the well distributed feeding and the freshness of feeds and the cleanliness of feeding platforms. (see Table 4--8)

Table 4--8 Feeding Schedule for Black carp Yearling

(iii) Feeding for Silver carp and Bighead

Before rearing Silver carp and Bighead fingerlings, base manure is applied to propagate plankton. At the initial stocking stage, Silver carp, besides feeding on the natural food, should be supplemented with 1.5--2kg of bean dreg/10000 ind/day; Supplementary feeds are changed to 2--3kg of wine lees/ 10000 ind/day or 0.5--1 kg of bean cake (ground into paste) after rearing for one week. Then the bean cake amount could be increased to 1.5--2kg/10000 ind/day. During the culturing period; additional manure should be applied according to the fertility of pond water to proliferate plankton. The feeding method for Bighead fingerlings is much the same as that for Silver carp, but the feeding amount for Bighead is double that for Silver carp or even more.

(iv) Feeding for Common carp and Wuchang fish

In a pond with Common carp as the major species, bean dreg and other commercial food are supplied once every day at a rate of 1.5kg/10000/day.

The food is put in at the shallow places near the bank. The feeding amount should be increased with the growth of fish. While in a pond with Wuchang fish as the major species, they are fed first with bean cake paste twice a day at a rate of 0.5kg/10000 ind/day. Then Wolffia arrhiza, Lemna minor, tender grasses and aquatic plants are supplied. At the same time, organic manure is used to increase the biomass of natural food.

In order to raise the efficiency of food utilization and decrease the food coefficiency, besides the feeding amount mentioned above, it is necessary to adhere to the feeding principles and methods, i.e. fixed feeding time and fixed feeding place.

Fixed feeding place: Food should be supplied on the fixed ground or platforms so as to avoid the waste of food. And it is convenient to observing the food intake of fish, eliminating the debris, disinfecting the feeding platforms and fish body during the epidemic seasons of fish diseases. Grasses supplied for Grass carp and Wuchang fish are put in bamboo frames either in triangular or square shape.

Commercial food is usually put on feeding platforms made of planks and frames with an area of 1--2 m². Every 5000--8000 fingerlings have one platform.

Snails, etc. are commonly put on the solid ground without silt at the bottom of the pond. It is about 70cm under the water surface.

(2) Manure loading as the main approach

This method is effectively practised for rearing Silver carp and Bighead with supplementary food. Within the first month after stocking, organic manure is used once every 10 days at a rate of 200--250kg/mu. In the second month, the same amount of manure is applied once every 7 days. When the water temperature is declining, the manure loading amount should be decreased accordingly. At that time, commercial food is supplemented at a rate of 2--3kg/10000 ind/day. The stocking of summer fingerlings is usually conducted in the season when water temperature is getting warmer. Therefore, special attention must be paid to variation of water quality lest the pond should lack D.O.

(3) Fertilizing the pond water with barnyard grass (Echinochioa crusgalli)

Barnyard grass has a fast growth and high production and it is usually planted in fingerling ponds which are not occupied before the stocking of summer fingerlings. When it is grown up, water is introduced to submerge the grass, which is gradually rotten and decomposed under the water. In this way, barnyard grass is used either as fertilizer or food for fish. Methods for its plantation are as follows: First, drain the ponds in early May and prepare the pond bottom. Base manure is applied in sterile pond bottom. Barnyard grass seeds are usually sowed at a rate of about 5kg/mu and they will sprout in 2--3 days. After one month's cultivation, barnyard grass can reach a height of 80cm. Then water is introduced up to 1.5m deep to submerge the grass. When barnyard grass is submerged for 5--7 days, the pond water turns fertile. Summer fingerlings soon after stocking can get a large quantity of plankton and detritus. The general stocking density is 5000--7000 ind/mu with Silver carp and Bighead as dominant species. Based on the measurement, the N, P content in the water is higher. On the other hand, barnyard grass consumes a lot of oxygen during its putrefication and therefore, special attention must be paid to the pond management lest fish should die of insufficient D.O. (see Table 4--9)

Table 4--9 Effects of Barnyard Grass on Fish Growth

4) Pond management

1. Pond patrolling should be done every morning to observe the activity of fish and water colour. If fish gasp for air for a long time, fresh water should be filled in. In the afternoon, it is necessary to examine the feeding status of fish in order to work out the feeding amount for next day.

2. Action should be taken frequently to remove any decayed debris and to disinfect feeding places so as to keep the pond clean.

3. Fresh water is timely added in to improve the water quality. During the whole rearing period, fresh water is regularly added in 4--5 times or even more in accordance with water quality of fish ponds.

4. During the whole rearing period, fish may have different growth rates because of fish diseases, feeds and other factors. So fingerlings should be sieved in Aug.--Sept. and cultured separately according to their sizes in order to ensure uniform size.

5. Prevention and treatment of fish diseases and eradication of enemies should be strictly carried out.

5) Rearing of two-year-old fingerlings

(1) Rearing of two-year-old Black carp fingerlings

During yearling to two-year-old fingerling stage, the survival rate of Black carp is only 30% because their food requirements are great and the fish disease infection occurs often.

(i) Stocking

The stocking density and stocking ratio of various fish species are dependent on the desired sizes and other conditions. Two examples are shown in Table 4--10.

Table 4--10 Stocking and Harvesting Models of Two-year-old Black carp Fingerling Pond

Table 4--11 Stocking and Harvesting Models of Two-year-old Black carp Fingerling Pond Unit:mu

Note:  1. When Silver carp and Bighead with a body length of 12--13cm reach 0.1--0.15kg, they are transferred as seeds for restocking in grow-out ponds.

2. Crucian carp are stocked in May and Bighead of 3 cm are stocked in late July and early August.

(ii) Management

At this rearing stage, the feeding habit of Black carp changes from eating commercial plant food to eating animal food such as snails and Corbicula sp., etc. which are served as their main food. Generally, feeding can be started in March. At first, bran and dregs, bean cake, etc. are supplied once every 3--4 days at a rate of 3--5kg/mu. In Apr. small snails are supplied once every other day at 25kg/mu. Later on, it is increased by 5kg/mu each time. All the food supplied should be consumed within 24 hours. In may--June, crushed snails can be given to the fish, but the food should be eaten up within one day, otherwise, water quality will be deteriorated by the decaying of snails. Consequently, fish diseases will easily arise. Later on, uncrushed snails in suitable size can be supplied as the fish grow.

The daily management work is to adjust water quality and treat fish diseases because Black carp at this stage are very weak in ability to adapt to the fertile water and are easily infected by diseases. So attention should be paid to controlling the feeding quantity and water quality. At the same stage, Black carp are also lacking the ability to tolerate the low D.O.C. therefore, notice should be concentrated on the deterioration of weather and water quality so as to prevent the fish from serious surfacing and high mortality.

(2) Rearing of two-year-old Grass carp fingerlings

(i) Stocking

The stocking density of Grass carp and the proportion of Grass carp in polyculture are similar to that of Black carp. The usual stocking density of Grass carp yearling is 800–1000 ind/mu and mixed with Silvercarp, Bighead and Black carp, etc. The example is shown in Table 4--12.

Table 4--12 Stocking and Harvesting of Two-year-old Grass carp Fingerling Pond

(ii) Management

Food supply for Grass carp should be in accordance with the growth of fish, acceptability of food and the change of seasons. In march, it is usually to apply bran and dregs and other commercial food once every several days at a rate of 2.5–5 kg/mu/day. In April, duckweeds Lemna minor is supplied. After May, English rye grass, lettuce leave, Valli-sneria spirolis and tender terrestrial grasses, etc. However, the feeding amount is mainly based on the weather, it is better to apply food in the morning and to have all the food consumed between 3 and 4 o'clock in the afternoon. Grasses for fish must be fresh without any putrefication and the leftover of grasses should be removed everyday in the afternoon lest they should decay and ferment, resulting in deterioration of the water quality.