Key Technical Points and Daily Management Methods for Grass Carp Farming
The local names for grass carp include hunzi, huanyu, and caoqing. The grass carp has an elongated body, with the trunk slightly subcylindrical and the tail laterally compressed. It has no ventral keel, a medium-sized head, a broad and flat snout, a terminal mouth that is arc-shaped, and a slightly protruding upper jaw. The gill rakers are short and rod-shaped, sparsely arranged. The pharyngeal teeth are comb-like. The scales are quite large and round. The lateral line is slightly curved, extending backward to the middle of the caudal peduncle. The dorsal fin has no hard spines, its origin is opposite to the origin of the pelvic fin, and it is slightly farther from the snout tip than from the caudal fin base. The anal fin has no hard spines, and its origin is closer to the base of the abdomen than to the caudal fin base. The fish body is tea-yellow, the back is bluish gray, the belly is grayish white, the pectoral and caudal fins are grayish yellow, and the other fins are paler. The grass carp looks very similar to the black carp, but the body color differs between the two. The grass carp is tea-yellow with gray, and the paired fins are grayish yellow, while the black carp is bluish black, and its paired fins appear even more bluish black against the white belly.
The grass carp is widely distributed. In China, except for ** and the Qinghai-Tibet Plateau where there is no natural distribution, it is found in the major river systems. Grass carp generally inhabit the middle and lower layers, and sometimes come to the upper layer to feed. They are lively and swim quickly. They are herbivorous. In the fry stage they mainly feed on plankton, and juveniles also eat aquatic insects. Juveniles with a body length of more than 50 mm gradually shift to a herbivorous diet; at a body length of about 100 mm, they can fully adapt to feeding on higher aquatic plants. Adult fish mainly feed on higher aquatic plants, such as Vallisneria, Hydrilla verticillata, Najas minor, Potamogeton, duckweed, and watermeal, which are among their most preferred foods; submerged grassy areas are often fattening grounds for grass carp, and some terrestrial grasses are also favored by grass carp.
The reproductive population of grass carp is mainly 4 to 5 years old, with individuals mainly 650 to 850 mm in body length and 4 to 9 kg in body weight. The smallest female is 4 years old, about 540 mm in body length and about 2.5 kg in body weight; the male is 3 years old, about 550 mm in body length and about 2.4 kg in body weight. The reproductive season is from early May to early June, with the peak period in mid-to-late May.
Artificial Breeding
For grass carp breeding, new fish should be selected from those aged 4 to 5 years or older, weighing more than 5 kg, with a robust constitution, good body shape, fast growth, and no external injuries or diseases, and placed in a broodstock pond of 2 to 4 mu with a water depth of 1.5 to 2.5 m for cultivation. It is appropriate to stock 100 to 125 kg of broodstock per mu, generally 15 to 27 grass carp broodstock of 6 to 8 kg, with a female-to-male ratio of 1:1.5 being best. It is possible to add 5 to 10 silver carp broodstock and 1 to 2 bighead carp broodstock. When there are many snails in the pond, 2 to 3 black carp can also be added. Broodstock should be intensively cultivated, especially in spring cultivation (from March to spawning), which is even more important. Green feed should be the main feed. Properly feed concentrated feed, such as lettuce leaves and cabbage leaves, and also feed sprouted grain or malt. Flush and add new water once every 5 to 7 days, adding 3 to 6 cm each time. One to two weeks before spawning, water should be added every other day to strengthen the flow stimulation for gonadal development and maturation. For mature broodstock, pressing the abdomen lightly allows eggs or semen to be squeezed out from the genital pore. Mature broodstock can be injected with spawning-inducing agents such as pituitary, chorionic gonadotropin, luteinizing hormone-releasing hormone and analogs to promote estrus and spawning. They can spawn naturally after estrus in a spawning pond, or artificial insemination can be used. The collected fertilized eggs can be placed in an incubation container for flowing-water incubation. The incubation density varies with the container, usually 1 million eggs per cubic m. After about 20 to 32 hours (the hatching time varies with water temperature), the membrane is shed. Generally 3 to 5 days after membrane shedding, the yolk sac of the fry disappears and the swim bladder inflates, so they can leave the incubation container and be stocked in the pond.
Fry Cultivation
Fry cultivation refers to the process of cultivating fry into summer fingerlings of about 3 cm after about 20 days of rearing, commonly known as "fatting the pond." The fry cultivation pond area is 1 to 2 mu, pond depth 1.5 to 2 m, water depth 1 to 1.5 m; a rectangular pond shape is preferable; water inlet and drainage should be convenient, and the pond bottom is best with loam soil. The fry pond should be cleared and disinfected. The methods include the following:
1. Quicklime: For a fish pond with a water depth of about 1 m, clearing the pond with water requires about 130 to 160 kg of quicklime per mu. If clearing and disinfecting by the dry method after draining the pond water, only 50 to 80 kg per mu is needed. When the quicklime water has dissolved but not yet cooled, it can be immediately splashed over the whole pond. After 7 to 10 days, fry can be stocked. Besides clearing and disinfecting the pond, quicklime can regulate water quality and act as fertilizer, with more benefits than drawbacks. Therefore, quicklime is commonly used for pond clearing.
2. Bleaching powder: For a fish pond with a water depth of 1 m per mu, about 14 kg of bleaching powder is used. If the water depth is 5 to 10 cm, 3 to 5 kg is needed. Dissolve the bleaching powder in water and splash it over the whole pond on the leeward side. In strong sunlight, the effect is even better. After 5 to 7 days, fry can be stocked.
3. Ammonia water: When the pond water depth is 10 cm, use more than 50 kg per mu. The amount is often related to how long the ammonia water has been stored. During operation, it should be splashed over the whole pond in a leeward place. If splashed under strong sunlight, the effect is greater. Generally, after applying ammonia water, water can be added one day later, and fry can be stocked after 3 to 5 days. The characteristic of this method is that pond clearing and disinfection are fast, and the toxicity disappears quickly.
4. Rotenone: When the water depth is 1 m, about 1.3 kg of rotenone is used per mu of fish pond. The commercially available derris extract is actually an emulsifiable concentrate containing 2.5%, 5%, or 7.5%. During operation, add 10 to 15 times water (effective depth about 2 PPm), dilute, and splash over the whole pond. Fry can be stocked 7 days after pond clearing.
5. Tea seed cake: When the water depth is 1 m per mu, use 35 to 40 kg of tea seed cake. During operation, first break the tea seed cake into small pieces, soak them in a water vat with added water for one day, then take them out, add a large amount of water to dilute, stir evenly, and splash the whole pond together with the residue. The toxicity disappears in about 7 to 10 days.
6. Mixture of sodium pentachlorophenate, MCPA, and prometryn: When the pond water is about 13 to 30 cm, use 1 kg of sodium pentachlorophenate (65% water-soluble original powder), 75 g of MCPA (70% water-soluble powder), and 150 g of prometryn (50% wettable powder) per mu. Mix them, dissolve in water, and splash evenly over the whole pond. Generally, after 10 to 15 days, add water, then apply base fertilizer, and after about 2 to 3 days test the water; when non-toxic, fry can be stocked into the pond.
7. Sodium pentachlorophenate: Use 2 kg per mu of pond (water depth 0.5 m), splash over the whole pond and soak for 5 to 7 days, drain the water, expose to sunlight for 4 to 5 days, then add water.
8. Adding new water: After clearing and disinfecting the fry pond, new water can be added. When adding water, filter it at the inlet with fine mesh silk, and add water to 40 to 50 cm. Base fertilizer should be applied before fry are stocked. Generally, medium fertility is best when fry are stocked. That is, 3 to 5 days before stocking fry, generally apply 200 to 400 kg of livestock manure per mu, or 50 to 100 kg of night soil. If faster fertilization is needed, inorganic fertilizers can be used, generally 5 to 10 kg of ammonia water per mu, or 2.5 to 5 kg of ammonium sulfate, ammonium nitrate, etc. per mu. Through base fertilization, the plankton in the water grows rapidly, and the water color turns tender green or light brown, at which point fry can be stocked for cultivation.
9. Stocking fry: When stocking fry, choose groups with uniform fish size, bright and tender body color, plump and well-proportioned bodies, and active swimming. Generally, stock 100,000 to 120,000 grass carp fry per mu; if the pond area is larger, 200,000 per mu can be stocked. Fry rearing methods vary from place to place, including the soybean milk rearing method, the large grass rearing method, and the pig manure rearing method. In many areas, the soybean rearing method is mostly used. After the fry are stocked in the pond, the first soybean milk feeding should be given 5 to 6 hours later. If stocked in the afternoon, they should be fed in the afternoon. Generally, feed twice a day, at 8 to 9 a.m. and 14 to 15 p.m. No later than 16:00. Within 10 days after the fry are stocked, feed 1.5 to 2 kg of soybeans per mu per day, or 2 to 2.5 kg of soybean cake soaked and ground into milk. After 10 days, adjust according to whether the water quality is rich or lean. If it is rainy and cold, appropriately increase the amount of soybean milk. Soybean milk should be splashed evenly over the whole pond. If combined with fertilization, the amount of soybean cake or soybeans can be reduced. Generally, fertilization can be carried out 4 to 5 days after the fry are stocked.
During fry cultivation, new water should be added regularly. When fry are stocked, the pond water is generally about 50 cm. Four to five days after fry are stocked, 10 cm of water should be added, and afterward new water should be added once every 3 to 4 days, so that by the summer fingerling stage the pond water should reach about 1 m. When adding new water, strictly prevent wild miscellaneous fish from entering the pond. Strengthen pond patrols to facilitate discovering problems and handling them promptly. If the fish pond leaks, it should be blocked in time. Promptly remove fertilizer residue, soybean milk foam on the water surface, and so on, keep the pond water environment clean, and do well in fish disease prevention and control. After 20 to 30 days of cultivation following stocking, the fry generally grow into summer fingerlings of about 3 cm and should be transferred to separate ponds for culture. Before summer fingerlings leave the pond, they should undergo two rounds of net-pulling exercise before being removed from the pond. Generally, about 20 days after the fry are stocked, use a fine-mesh fish net to catch them in the pond, choosing 8 to 9 a.m. on a sunny day, surround the fish in the net, pick out wild miscellaneous fish, and release them back into the original pond after a few minutes. After an interval of one day, use the same method to gather the fish into the net, lift the net, pour all the fish into a net cage, slowly move the net cage so the fish swim against the water for 1 to 2 hours, and then put the fish into the pond. After two rounds of net-pulling exercise, the summer fingerlings are strong and can leave the pond for separate culture.
Fingerling Culture
Grass carp fingerling rearing generally starts in the first year from summer fingerlings to about 13.2 cm one-year-old fingerlings, also called zikou grass carp fingerlings; in the second year they are reared into 23.1 to 26.4 cm or 0.25 to 0.5 kg two-year-old fingerlings, also called laokou grass carp fingerlings. After another year of culture, they can be grown into 2 to 3 kg adult fish.
In recent years there have been many high-yield experiences with grass carp fingerlings. A few examples are given below:
1. Example One
High-yield culture of zikou grass carp fingerlings. The pond area is about 4 mu, the average water depth is about 2.5 m, and drainage and irrigation are convenient. According to conventional methods, 75 kg of quicklime per mu is used to thoroughly clear the pond. About 10 days before summer fingerlings are stocked, 1 ton of fermented cow manure is applied per mu. When summer fingerlings are stocked, the water color is yellow-brown, the transparency is 25 cm, and the water depth is 1.5 m.
For summer fingerling stocking, the main grass carp should be stocked first, with multi-species mixed polyculture and dense stocking, and the timing of silver carp stocking should be controlled. On June 4, 9,300 grass carp summer fingerlings of 3.6 to 4.3 cm were stocked per mu, weighing 6 kg; on June 26, 1,744 crucian carp summer fingerlings of 3.3 cm were stocked, weighing 0.45 kg; on June 30, 1,172 bighead carp of 4.9 cm were stocked, weighing 1.4 kg; on July 1, 2,585 silver carp of 2.9 cm were stocked, weighing 0.60 kg. The total stocking per mu was 14,751 fish, with a total weight of 8.45 kg.
At the year-end harvest, per mu, 1,504 and 1,193 grass carp fingerlings weighing 75 g/fish and 165 g/fish respectively were obtained, weighing 310 kg; 1,167 crucian carp fingerlings of 60 g/fish, weighing 70 kg; 1,110 bighead carp of 70 g/fish, weighing 78 kg; 2,074 silver carp fingerlings of 100 g/fish, weighing 207 kg; the average number of fingerlings stocked per mu was 7,048, and the yield was 665 kg.
After summer grass carp are stocked, feed them watermeal and duckweed. When the summer grass carp grow to 7 cm, switch to chopped tender grass; after 10 cm, directly feed green grass. The feeding amount is determined according to fish growth, feeding, and weather conditions. Feed less on rainy, muggy days and more on clear days. Generally, the feed should be controlled so that concentrated feed is finished within 3 hours after feeding and green feed within 4 to 5 hours. Per mu, 3,260 kg of grass feed and 1,260 kg of concentrated feed were used.
Strengthen water quality management. In the early stage of rearing, add new water once every 2 to 3 days, adding 20 to 30 cm; from mid-July to early October, add water once a day, adding 25 to 30 cm. As the number of water additions increases, keep the water depth above 2.5 m. After mid-October, add water according to water quality conditions. When adding water, generally do so at 6 a.m., and in the middle stage of cultivation at about 1 a.m. Fish disease prevention should be the main focus. In addition to thoroughly clearing the pond, pay attention to regulating water quality after summer fingerlings enter the pond. During fish disease epidemic seasons, splash bleaching powder and quicklime alternately every 7 to 10 days. For different fish diseases, use copper sulfate and ferrous sulfate mixture, rhubarb, trichlorfon, and garlic, etc., mixed with feed.
2. Example Two
A culture pond mainly for grass carp and bream fingerlings has an area of 6.1 mu, a water depth of 2.5 m, and relies on a water pump for irrigation and drainage. The fingerlings stocked per mu were: on July 2, 5,000 grass carp summer fingerlings; on June 11, 6,000 bream summer fingerlings; on June 27, 3,000 crucian carp summer fingerlings; on July 15, 3,000 silver carp and bighead carp summer fingerlings. By the year-end harvest, the average yield per mu was 611.15 kg. Among them, 1,085 grass carp fingerlings of 100 g/fish, weighing 108.6 kg; 5,372 bream fingerlings of 31.25 g/fish, weighing 167.85 kg; 1,946 crucian carp fingerlings of 41.7 g/fish, weighing 81.065 kg; 2,029 silver carp and bighead carp of 122 g/fish, weighing 253.5 kg.