Key Technical Points for Rearing Grass Carp Summer Fry and Older-Size Fingerlings
Feeding Habits of Grass Carp Fry
During the fry stage, the food of grass carp consists mainly of zooplankton, initially dominated by small zooplankton such as protozoans, rotifers and Diaphanosoma, and gradually, as they grow, they swallow larger cladocerans such as Moina, Cyclops and Diaptomus.
Feeding Habits of Grass Carp "Black Fry" (Advanced Fry)
After 15-20 days of rearing, when the body length reaches 1.7-3.1 cm, the pharyngeal teeth have already begun to form protrusions and the intestine has lengthened and become coiled; the feeding habit begins to shift to feeding on chironomid larvae, duckweed (Wolffia), three-leaf duckweed (Azolla), the roots of purple-backed duckweed (Spirodela) and the tender shoots and fibrous roots of other aquatic plants. With continued growth and development, they then enter the fingerling stage.
Feeding Habits of Grass Carp Summer Fingerlings
When grass carp reach a total length of 5 cm, they shift to feeding mainly on large aquatic plants. Invertebrates make up only a small part of the diet of grass carp fingerlings from 5-10 cm in total length. Adult grass carp living in natural waters feed mainly on submerged plants and grasses growing in the shallow littoral zones.
Pond Selection
Grass carp fry rearing ponds are 3-5 mu in area, rectangular, oriented east-west, with a length-to-width ratio of 5:4 to 3:2, an average water depth of 1.2-1.5 m, a pond slope ratio of 1:1 to 1.5, a flat pond bottom, loam soil, a silt thickness of 15-20 cm, and convenient water inlet and drainage.
Grass carp fingerling rearing ponds are 6-10 mu in area, rectangular, oriented east-west, with a length-to-width ratio of 5:4 to 3:2, an average water depth of about 1.8-2.5 m, a pond slope ratio of 1:1 to 1.5, a flat pond bottom, loam soil, a silt thickness of 15-20 cm, and convenient water inlet and drainage.
Pond Clearing and Removal of Wild Fish
Quicklime, tea seed cake, croton, bleaching powder and derris root can be used singly or in combination. For ponds with severe fish diseases, bleaching powder or quicklime, or tea seed cake plus a small amount of quicklime, is used to clear the pond, so as to improve water quality; this has a certain effect on raising the survival rate of fry and fingerlings.
1. Pond clearing with quicklime: Drain the pond water to about 10-20 cm, then evenly splash lime slurry over the whole pond, using about 100-150 jin per 1 mu. For ponds where water supply is inconvenient, full-water pond clearing can be used: with a water depth of 1 m, use about 250-300 jin of quicklime per mu.
2. Pond clearing with tea seed cake: With a water depth of 1 m, use 80 jin per 1 mu, applied 10-15 days before the fry are stocked. At low water temperatures the toxicity disappears slowly; at high temperatures it disappears quickly. First break the tea seed cake into chunks, put them in a wooden bucket or water vat, add water and soak; at a water temperature of 25°C, soaking for one day and one night is enough before use. When applying, add a large amount of water and splash evenly over the whole pond.
3. Pond clearing with bleaching powder: At a water depth of 1 m, use 25-30 jin of bleaching powder per mu; dissolve the bleaching powder in water and immediately splash it evenly over the whole pond. The chlorine content of the bleaching powder should be about 30%. Fish can be stocked 4-5 days after pond clearing with bleaching powder. The effect of pond clearing with bleaching powder is related to the fertility of the pond water; the effect is poor in fertile water.
Fry Rearing
1. Pond fertilization to build up water fertility: The purpose of fertilization is to cultivate various types of plankton in the water and produce the first food for the fry after they are stocked.
2-3 days after disinfection and removal of wild fish, apply 500 jin of fermented manure per mu, or 2000 jin of green manure per mu, or 20 jin of urea plus 20 jin of phosphate fertilizer per mu to cultivate plankton; 5-7 days after fertilization, rotifers and plankton reach their peak of reproduction.
Effects of fertilization: After pond fertilization, the dominant groups of plankton that first appear are related to the nature of the fertilizer. With organic fertilizer applied, phytoplankton such as Ochromonas and cryptomonads, and zooplankton such as Urotricha and Cyclidium, first appear in large numbers. With inorganic fertilizer applied, diatoms, Scenedesmus and Halteria become the main groups.
When the amount of fertilizer applied is large, some species of green algae and blue-green algae develop in large numbers; when the amount of fertilizer applied is small, many species of diatoms, such as spindle-shaped diatoms and disc-shaped diatoms, become the dominant species.
4-5 days after fertilization, zooplankton can reach their peak. The zooplankton that first appear in large numbers are protozoans and rotifers, followed by cladocerans, and then by copepods.
If, after pond fertilization, the pond water appears light greenish-brown and algae, protozoans and rotifers in the pond are multiplying in large numbers while cladocerans are just beginning to appear, this is most suitable for the growth of grass carp fry.
2. Timing of stocking fry: Grass carp "water flower" fry should be stocked on the 5th-7th day after fertilization, when rotifers and cladocerans are at their peak.
At stocking, feed 1 cooked egg yolk per 100,000 fry so that they are stocked with full bellies. Starting from the second day, feed soybean milk, grinding 2-3 jin of soybeans into soybean milk per mu of water surface per day, and feed 2-3 times.
3. Fry stocking: Before stocking, the newly hatched fry are first placed in a fry box and fed cooked egg yolk, at a rate of 1 cooked egg yolk per 100,000 fry. Feeding cooked egg yolk should be carried out according to the following steps:
1) Boil the egg yolk a little longer, wrap the egg yolk in 60-mesh nylon bolting cloth, and agitate and rinse it in a basin of water to filter it, without squeezing it with the hands, so that fine particles of egg yolk pass out through the mesh into the water;
2) Use the fingers to sprinkle the egg yolk water evenly into the fry box; after half an hour, egg yolk particles can be seen inside the intestines of the bream fry (commonly known as "flower intestine");
3) After keeping the fry in the fry box for 3-4 hours, release the fry into the pond.
4. Stocking density: The usual stocking density is 150,000-200,000 fry per mu. If the fry are netted and divided into other ponds when they reach a total length of 1.8-2.0 cm, the stocking density can be increased to 200,000-250,000 per mu.
5. Feeding fry: When the fry are still too small, they mainly swallow plankton such as rotifers and small cladocerans in the water, so the method of stocking into fertilized water should be adopted, applying organic fertilizer before the fry are stocked to cultivate food organisms, and then using the soybean milk rearing method; when soybean milk is splashed into the pond, part of it is eaten by the fry and most of it serves to fertilize the water. The feeding amount is generally 4-6 jin of soybeans ground into milk per mu per day, increasing to 8-10 jin after one week, with the amount controlled according to water color; the ratio of water to soybeans is 30:1.
Within 5-7 days after the fry are stocked, feeding is mainly soybean milk; after 7 days, fine flour, rice bran, wheat bran and other fine feeds can be fed. Feed three times a day, at 9 a.m., 1 p.m. and 4 p.m. When grinding soybean milk, the soybeans must be soaked until swollen, and the bean residue must be removed after grinding. When feeding, splash the milk along the pond edge in two circuits, and make the splashed milk fall like raindrops, evenly distributed.
After 10 days of feeding, switch to soybean cake slurry. Break up the soybean cake, soak it in water for 3-4 hours, then grind it into soybean cake slurry with a soybean cake-to-water ratio of 1:10. After 25-30 days of rearing, the fry can reach a size of over 3.3 cm, and the daily feeding amount is 15-20% of the fish body weight.
6. Adding water in stages: Add fresh water to the pond in stages to raise the pond water level, so as to increase the oxygen content in the water, improve water quality, expand the space for fry activity, and accelerate fish growth. This promotes the reproduction of plankton and increases live food.
When the fry are put into the pond, the pond water depth is about 0.6-0.7 m; after 7-15 days of rearing, the pond water depth is increased to 1 m; after 20-30 days of rearing, the pond water depth is increased to 1.2 m; after 40-45 days of rearing, the pond water depth is increased to 1.5 m; after 2 months of rearing, the pond water depth is increased to 1.8 m. Entering autumn and winter, the pond water depth is kept at about 1.8-2 m.
When adding water, filter wild fish and pests with fine netting at the water inlet, and at the same time avoid the water flow directly hitting the pond bottom and stirring the pond water muddy.
7. Water quality regulation: At stocking, the pond water transparency is below 30 cm; afterward, apply additional fertilizer appropriately to control the fertility of the pond water. In summer, the pond water transparency is kept at about 25-30 cm, and in autumn at about 35 cm.
8. Pond patrol management: After the fry are stocked, patrol the pond three times a day, carrying out the "three inspections" and "three attentions".
In the morning, inspect the floating-head condition of the fry and frequently remove frog eggs; in the afternoon, inspect the activity of the fry and frequently remove weeds from the pond banks; in the evening, inspect the water quality and keep records carefully, arranging the next day's feeding, fertilizing and water adding. At all times pay attention to checking for the occurrence of diseases and carry out prevention and treatment promptly.
9. Conditioning and pond division: After 20-25 days of rearing, when the grass carp fry have grown to about 3 cm, they should be divided and reared separately. Before the summer fingerlings are taken out of the pond, they must undergo 2-3 rounds of netting and conditioning. This enables the fingerlings, in the process of being crowded together, to increase their ability to adapt to low oxygen. It makes the fish body "hardened" and sturdy, able to withstand being taken out of the pond and divided for rearing, and prevents the production of large amounts of mucus and feces during transport that would pollute the water quality, thus raising the survival rate during transport.
1) First netting: Gather the summer fry in the net, check their physical condition, then immediately release them back into the pond. Because the fish are delicate, operations must be careful.
2) Second netting: After an interval of 1 day, carry out the second netting and conditioning. After gathering the fish together, let them swim upstream on their own into the net box. After two hours, release them back into the pond. If they are to be reared on the same farm, then after two rounds of netting and crowding in the box, they can be divided for rearing.
3) Third netting: After the second netting, wait another 1 day, then carry out the third netting; place the fingerlings in a net box in a pond with clearer water and "hang" them there for 1 night, and they can be loaded for long-distance transport early the next morning.
Conditioning fry must be done on clear, fine days, in the morning when the fish are not gulping at the surface. The operation of netting and driving the fish must be slow, so as not to injure the fry. When closing the net, the fish must not be allowed to press against the net. After the fish enter the box, the box should be washed immediately once to wash away excrement and dirt sticking to the mesh, so that the water inside the box circulates freely with the outside, preventing fry deaths from lack of oxygen inside the box.
10. Nutritional formulation during the fingerling rearing stage: In view of the fact that grass carp have different protein requirements at different stages of growth and development, the following principles should be followed:
Before the body length reaches 5 cm, the protein content in the feed should reach 40-45%; at a body length of 6-7 cm, the protein in the feed can be reduced to 35-30%; after a body length of 10 cm, the feed protein can be reduced to 28-25%. From the standpoint of production cost and best economic benefit, the feed protein content can be appropriately lowered. Mineral salts (inorganic salts), vitamins and other additives should be added to the feed in appropriate amounts.
11. Feeding technique and feeding amount: At a body length of 5-8 cm, grass carp are at the stage of dietary transition. In addition to feeding Wolffia and small duckweed, feed compound feed of the corresponding particle size, with a daily feeding rate of 10-15%.
12. Pond water disinfection: In May-June, disinfect the pond water once every half month. Commonly used drugs and dosages: bleaching powder 1 mg/L (1.3-1.5 jin/mu); copper sulfate and ferrous sulfate mixture 0.70 mg/L (0.9 jin/mu); trichlorfon 0.50 mg/L (0.6 jin/mu); quicklime 20 mg/L (20-25 jin/mu); the above drugs are used alternately.
13. Disease prevention and treatment: The main diseases during the grass carp fry and fingerling rearing stage are parasitic diseases and "white head and white mouth" disease, such as trichodinosis, fish lice and anchor worms.
Prevention and treatment method: Regularly splash crystalline trichlorfon and copper sulfate over the whole pond to bring the pond water concentration to 0.3 g per cubic meter, then splash once more the next day, and it can be prevented and treated.
14. Physical and chemical indicators of the water body during the fry rearing period:
1) Organic matter oxygen consumption: When the dissolved oxygen in the pond water is not deficient, the higher the organic matter oxygen consumption, the higher the survival rate of grass carp fry. Before the fry are stocked, the organic matter oxygen consumption should be above 30 mg/L to ensure the fertility needed for plankton growth.
After the fry are stocked, they consume large amounts of plankton and continuously deplete organic matter, so fertilization must be continued. Fertilization should be in small amounts but frequent, once every 2-3 days. Part of the splashed soybean milk cultivates zooplankton, part becomes organic fertilizer, and only a small part is directly used by the grass carp fry.
2) Dissolved oxygen: The dissolved oxygen in the water is closely related to the survival of the fry; when the dissolved oxygen in the water drops below 2 mg/L, the fry will show floating-head behavior. The dissolved oxygen in the water body required for normal growth and development of grass carp fry is 4.5 mg/L; when it drops to 1.5 mg/L, the respiration of the fry is inhibited. In production, if grass carp fry show slight floating-head behavior from before dawn to 1-2 hours after sunrise, this indicates the water is too fertile and feeding and fertilization should be stopped.
3) pH value: When quicklime is used to clear the pond, the pH value and hardness of the pond water are both relatively high, and the rotifers all die, but the resting eggs of rotifers are unharmed; only when the pH value drops below 10 can the resting eggs of rotifers hatch in large numbers. 8-10 days after pond clearing, the efficacy of the lime disappears, the pH drops below 9, phytoplankton begins to multiply in large numbers, and the dissolved oxygen in the pond water rises, often reaching saturation. At this time, rotifers appear in large numbers, and the fry should be stocked promptly.
Rearing of One-Year-Old Grass Carp Fingerlings
The process of rearing summer fry of 2.5-3.3 cm body length into fingerlings of 20-25 cm body length is called fingerling rearing, and the process can be divided into 2 rearing stages:
First-level rearing stage: Rearing summer fry of 2.5-3.3 cm body length into summer fry of 7-9 cm is called the first-level rearing stage.
Second-level rearing stage: Rearing summer fry of 7-9 cm body length into fingerlings of 20-25 cm length is called the second-level rearing stage.
1. First-level rearing stage: Rearing grass carp "fire flakes" into summer fry of 2-3 cun (7-9 cm total length), stocking 30,000-50,000 fire flakes of 3 cm total length per mu. A single-species rearing method is adopted.
1) Artificial feeding: For first-level rearing of 3 cm grass carp fire flakes, soybean cake slurry and crushed compound feed must be fed. To make soybean cake slurry, use 1 jin of dry soybean cake per 10,000 fry, soak it for 4-5 hours, then grind it into slurry with a ratio of 4 parts water to 1 part soybean cake. When feeding, choose one side of a shallow pond, on the pond slope about 30 cm below the water surface, and put out the feed in small heaps, with 2 m between heaps and 2 jin per heap. This is called the "du tan" (heaping on the flat) feeding method, fed once every morning. In the afternoon, feed 10 jin of Wolffia; apply 100 jin of mixed compost per mu per day, which can be increased or decreased according to the fertility of the water; the particle size of the crushed compound feed should be controlled at 0.5-1.0 mm.
2) Adding fresh water: Once a week, add fresh water to a depth of 10-20 cm. After 10-15 days, the grass carp summer fry have reached 7-9 cm and then enter the second-level rearing stage, that is, final rearing.
2. Second-level rearing stage: Rearing summer fry into winter fingerlings or spring fingerlings of 6-9 cun (20-25 cm), stocking 4,000-6,000 summer fingerlings of 7-9 cm total length per mu. During this period a mixed rearing method is adopted, with the pond area relatively large, at 6-10 mu, and the water depth at 1.8-2.5 m.
1) Rearing of the main fish: Grass carp summer fingerlings of 7-9 cm body length can already swallow compound pellet feed. Rearing can be carried out by mainly rearing grass carp, with silver crucian carp as a companion, and silver carp and bighead carp as supplementary.
2) Rearing management: Grass carp summer fry are stocked into clear water; on the second day, 1.5 mm pellet feed is broadcast, adopting the "five fixed" operating method:
Fixed person: Managed by a dedicated person. One person manages 3-5 ponds, with an area of about 30-40 mu.
Fixed time: The number of feedings per day is determined according to temperature changes; the higher the temperature, the more feedings. Feeding begins to be induced at a water temperature of 15°C; at 20-24°C, feed twice a day; at 25-28°C, feed three times a day; above 28°C, feed four times a day, with an interval of 3 hours.
Fixed place: Mechanical feeding with a feeder.
Fixed quality: The feed must be fresh, with a crude protein content of 30%. In the later growth period, the crude protein content of the feed is 28%.
Fixed amount: Formulate the daily feeding rate according to temperature changes.
At a body length of 8-12 cm, which is the later stage after the dietary transition, feeding should combine green feed and fine feed, with a daily feeding rate of compound feed of 7-10%.
At the stage of 12-14 cm body length, mainly feed 2.5 mm pellet compound feed, with aquatic and terrestrial grasses fed in between, and a daily feeding rate of compound feed of 3-6%.
3) Observe frequently, prevent floating-head behavior, prevent fish diseases: During the fingerling rearing stage, patrol the pond in the morning and evening. To prevent oxygen deficiency in the pond water caused by weather changes, fresh water should be added frequently, or an aerator should be turned on to regulate water quality.
Weather changes affect the growth and feeding of bream fingerlings, so feeding should be increased or decreased as appropriate according to weather changes. In the morning the temperature is low and the dissolved oxygen in the water is low, so the feeding amount should be reduced; at noon, when the weather is clear, more should be fed. In the evening, do not let them eat too full. When the dissolved oxygen in the water is below 3 mg/L, the fish only eat but do not gain flesh.
4) Pond water disinfection and medicated feed: From June to September, disinfect the pond water once every half month. Commonly used drugs and their dosages: bleaching powder 1 mg/L (1.3-1.5 jin/mu); copper sulfate and ferrous sulfate mixture 0.70 mg/L (0.9 jin/mu); trichlorfon 0.50 mg/L (0.6 jin/mu); quicklime 20 mg/L (20-25 jin/mu); the above drugs are used alternately.
Medicated feed can be fed once every half month, for 3 days each time, adding 50 g of oxytetracycline to every 100 jin of feed, or selecting sulfonamides, furans and other drugs to make medicated feed, and feeding as appropriate.
Transport of Fry and Fingerlings
In the process of transporting grass carp fry and fingerlings, no matter what method is used, operations must be meticulous, and attention must be paid to the following aspects:
1. The fry must be robust in constitution, with the swim bladder inflated (the "long waist spot"), the mouth parts open, and swimming actively, before they can be loaded for transport. The loading amount for oxygen-filled nylon bag transport is about 80% of that of a transport box.
2. The water must be fresh and non-toxic, with high dissolved oxygen. Water from rivers, lakes, reservoirs and ponds can all be used; colorless, transparent water with low mud content should be chosen. Tap water must be dechlorinated and left to stand for 24 hours before use. In summer, cool water should be used.
3. Before the fingerlings are shipped, the fish body must undergo netting and conditioning to expel feces. Only when the body is firm and the secretion of mucus is reduced can the survival rate be raised.
4. Pay attention to temperature balance; the temperature difference must not exceed ±2°C. After arriving at the destination, the fish should first be put into a box to revive; one hour later, feed the fry cooked egg yolk. Feed one egg yolk per 100,000 fry; after seeing the fry's "flower intestine" (egg yolk swallowed into the intestine), wait another hour before releasing the fish into the pond. For fry in nylon bags and transport boxes, before putting them into the box, first place them in the water so that the internal and external water temperatures are the same. Then open the nylon bag or transport box, pour in pond water, put the bag into the box, and let the fry (fingerlings) swim out on their own.