Using first-hand aquaculture experience from the production front line

toboost fishery development and lift industry returns

Grass carp is one of China's Four Major Chinese Carps, and it is also a major artificially cultured freshwater fish in China. Its main food is grass; the grass carp has a short food chain, tolerates coarse feed, has fairly wide feed sources and low culture costs, and grows fast, with delicious flesh and high economic value. It is a freshwater cultured fish that can bring relatively large economic returns, so its culture has always been taken seriously.

1. Biological characteristics of grass carp

The grass carp belongs to the order Cypriniformes, family Cyprinidae, subfamily Leuciscinae, genus Ctenopharyngodon. Common names for grass carp include: caobang, huan, youhuan, caohuan, baihuan, grass carp, caogen, hunzi and so on. Together with black carp, bighead carp, silver carp, common carp and crucian carp, the grass carp is known as one of China's six major freshwater fishes, among which grass carp, silver carp, bighead carp and black carp are the "Four Major Chinese Carps."

Grass carphas a flat head, relatively wide spacing between the eyes, an arc-shaped mouth, a subterminal mouth, a shorter lower jaw, an upper jaw slightly longer than the lower jaw, and no barbels.

The body is cylindrical, the belly is rounded, and the scales on the body surface are round and large.

The fish body color is mostly tea-yellow, the scale edges are grayish-black, the back is greenish-green, and the belly is grayish-white.

Compared with common carp and silver carp, the scales of grass carp are large and round, each scale has a black edge, and the lateral line is relatively straight.

Among the fin rays, the pelvic fins and pectoral fins are orange-yellow, the caudal fin and dorsal fin are gray, and the dorsal fin is relatively short.

The tail of the grass carp is laterally compressed, and the caudal fin is relatively deeply forked.

The pharynx bears 2 rows of pharyngeal teeth, shaped like comb teeth, mainly used for cutting grass. Grass carp has no true teeth and no chewing function; after ingesting feed it cannot stir and grind the feed, but swallows the ingested feed directly into the esophagus.

Grass carp has very strong activity and is one of the typical herbivorous fishes. Under pond culture conditions, grass carp mainly inhabits the middle layer of the water; it is lively by nature, moves swiftly and swims very fast. However, it must move to the upper layer to feed. The main food of grass carp is higher aquatic plants, such as duckweed, ryegrass, water hyacinth, water spinach, Potamogeton malaianus, Sudan grass, hornwort, Hydrilla verticillata, floating duckweed, napier grass and Themeda grass, etc.

In addition, grass carp has a very large food intake; the daily intake can reach as much as 60%-70% of its body weight. But because grass carp has no stomach and belongs to the stomachless fishes, it cannot produce enzymes to digest cellulose, giving it a very low digestion rate for grass, and therefore it produces a very large amount of feces and easily fertilizes the water. The ingested feed is mainly digested by digestive enzymes secreted by the liver, pancreas and intestine.

The intestine of grass carp is 3 times its body length. Grass carp generally begins to ingest feed before the intestine is emptied, and under starvation conditions its intake begins to increase; however, if the starvation period is too long, appetite is instead suppressed, affecting the growth of grass carp.

In nature, grass carp likes to live in clear waters, usually active in flowing water with lush aquatic plants. In artificial culture, it is not suited to fertilized water but rather to relatively lean water.

At the fry stage, grass carp in ponds mainly feed on zooplankton, such as chironomid larvae, cladocerans and insects, and also feed on commercial feed provided. Only when the fingerlings gradually develop into adult fish can they mainly feed on grass. Therefore, the best food at the fingerling stage is duckweed in the pond and artificial feed, after which they gradually change from feeding on tender grass to larger plant foods.

Grass carp has relatively strong adaptability to water temperature and can survive in water from 0.5-38°C. The suitable water temperature is 20-32°C, and the most suitable water temperature is 27-30°C. At 27-30°C the food intake is greatest; when the water temperature is below 20°C the food intake decreases, and below 5°C it may stop feeding. When the water temperature is below 0.5 (freezing to death) or above 40°C (heatstroke), it begins to die or is near death.

Grass carp has a certain tolerance to low oxygen. When the dissolved oxygen in water is 5 mg/L it can grow and develop normally; when the dissolved oxygen is 1.6 mg/L respiration is inhibited.

2. Feeding patterns of grass carp

During the day, grass carp mostly eats less in the morning than in the afternoon, and eats the most after dusk. In fact, manyfishspeciesalllovetofeedatduskandeat，the reason is temporarily unknown. For grass carp fed pelleted feed, when feeding begins the fish compete for food very intensely; after 30-40 minutes the intake slows, and after 1 hour very little feed is given and eaten, indicating that feeding is basically over.

Grass carp begins to defecate about 2 hours after feeding. Therefore, the second meal is better fed 3 hours after the first meal; otherwise, because grass carp is greedy, it can eat while excreting and excrete while eating. If the interval between meals is too short, a lot of feed will be wasted, the feed conversion ratio will increase, and fish-culture returns will be directly affected.

In the high-temperature season from July to September, the water temperature is 22-30°C, which is the suitable water temperature for the vigorous metabolism of grass carp. In this season, grass carp begins to eat at 6-7 in the morning and also likes to feed at night.When intensively culturing grass carp,feeding can begin at 6 or 7 in the morning, with feed given once every 3-4 hours; feeding 3-6 times per day is relatively reasonable.

In spring and late autumn the water temperature is relatively low, about 15-22°C; feeding twice a day, once at 9-10 in the morning and once at 4-5 in the afternoon, is relatively reasonable. Under such water temperature conditions grass carp mostly does not eat at night; possibly not eating at night is related to the drop in water temperature.

The relationship between grass carp food intake and dissolved oxygen in water. When dissolved oxygen is less than 3 mg/L, grass carp generally stops eating any feed; at 3-4 mg/L the food intake decreases markedly, about half the normal amount; at 5-8 mg/L the food intake is in a normal state, with little difference.

3. Nutritional requirements of grass carp

According to reports, the protein, amino acids and so on required during the growth of grass carp are in a constantly changing state. Usually, from fry to fingerling, the compound feed should suitably contain 30%-35% protein, while from fingerling to adult fish, the protein level contained in the feed should be somewhat reduced and controlled at 25%-28%. Because grass carp is a herbivorous fish, too high protein easily causes liver and gallbladder and fatty diseases.

Ensure the feed is rich in comprehensive nutrition; it is usually required to contain 40% carbohydrates, 20% crude protein, 8% cellulose and 4% crude fat. Green feed mainly refers to water grass, dryland grass, napier grass, ryegrass and Sudan grass, etc., and it must be ensured to be fresh so that grass carp can digest it smoothly. Reasonably supplying nutrients can ensure it obtains enough nutrition, thereby maintaining vigorous growth and ultimately increasing yield.

4. Growth rate of grass carp

Grass carp is a large economic fish that grows fast. Growth is fastest at 2-3 years of age, so a grass carp culture cycle of 2 years is the most cost-effective. After sexual maturity at 4 years of age, the growth rate slows markedly; therefore, raising grass carp too large is not cost-effective.

At 1-3 years of age, female and male individuals have similar growth rates. After 4 years of age, females grow faster than males.

Usually, after 2 years of artificial rearing, the body weight can reach 3-5 kg. Under natural conditions, the maximum weight of an individual grass carp can reach more than 50 kg.

5. Species combination and stocking density for grass carp as the main cultured species

Reasonable combination and mixed culture of fingerlings is not only a decisive factor in increasing production, but also a main way to improve returns. Under relatively good conditions, the higher the stocking density the better is not true; stocking density also has a suitable value. When this value is exceeded, the growth conditions of the stocked fish will deteriorate because too many fish are stocked. In mild cases the growth rate of the fish slows and the goal of increasing production cannot be achieved; in severe cases a large number of fish die, causing major economic losses.

In ponds with grass carp as the main species, the grass carp stocking amount should be kept at about 60-70% of the total stocking amount. In ponds with better conditions, 500-600 fish can be stocked per mu, and the maximum should not exceed 1000 fish, with 200-300 silver carp, 50-100 bighead carp, 80-100 common carp or 200-300 crucian carp as companions, and other fish species may also be appropriately combined.

When grass carp is cultured in ponds, it often produces a relatively large amount of feces, which leads to excessive water fertility and easily causes blue-green algae or severe eutrophication in the water body, making survival unsuitable. For this reason, silver carp must be polycultured in the pond to purify the water quality, for mutual benefit. Many peopledislike silver carp for its low value and refuse to polyculture it, not knowing thatsilver carp is the nemesis of blue-green algae, and it simply must be polycultured.

It is said that in ponds with relatively abundant feed, one grass carp can be matched with two silver carp; if feed sources are relatively scarce, then one grass carp can be matched with one silver carp or even two grass carp with one silver carp.

6. Feeding management

An important part of grass carp feeding management is the management of feed placement. Because grass carp is greedy, it can eat while excreting and excrete while eating; feed only to 70-80% fullness as far as possible, but do not feed too little or too much, avoiding overfullness or excessive hunger, and keep feeding evenly over the long term. If frequentlyoverfull or excessively hungryor "bingedrinkbingeeat"iteasilycausestheintestinaldisease enteritis。

For feeding pelleted feed, it is best to use a feeder, so that the feed is scattered evenly and distributed over a wide area, avoiding pond fish gathering to compete for food, which leaves small-sized and weak fish unable to get food or get enough. Using extruded feed can alleviate this situation.

According to experience, the frequency of feeding pelleted feed is: from March to April, three times a day, namely 8:30 in the morning, 11:30 at noon and 3 in the afternoon; from May to September, four times a day, namely 8 in the morning, 11 at noon, 3:30 and 5 in the afternoon, each feeding for about 30 minutes, with the feeding amount such that 70-80% of the grass carp are full and most of the fish disperse.

For intensive pond culture of grass carp, using compound feed as the main part and green feed as the supplement isthe best model.The compound feed can be hard pelleted feed or floating extruded feed, and the feed is required to be nutritionally comprehensive; green feed includes alfalfa, alfalfa, ryegrass, Sudan grass, dryland grass, napier grass and so on, and the forage is required to be fresh and tender so that grass carp can digest it.

The amount of grass feed should be flexibly controlled according to the weather, the feeding situation of the fish and the water quality, and it is best for it to be finished before dusk each day. From midnight to early morning, grass carp generally stops feeding, possibly related to dissolved oxygen, but in the early night it likes to eat grass foods and feed. In the hot summer season, grass carp should as far as possible not be given night grass, so as to avoid nighttime energy digestion intensifying oxygen consumption and hypoxia; attention must be paid to this.

7. Water quality regulation

The water quality for grass carp as the main cultured species is very important. If conditions permit, new water should often be added to keep the water quality fresh and sanitary, so as to increase water vitality, increase dissolved oxygen, raise the reproduction rate of plankton, meet the feed needs of silver carp and bighead carp, and stimulate the rapid growth of grass carp.

Equip and frequently run aerators to increase oxygen, so as to keep the water quality fresh and dissolved oxygen normal.

Every half month or every month, splash 10-20 kg of quicklime per mu to clarify the water quality. Or apply water quality improvers to maintain excellent water quality over the long term.

Water transparency is best kept at 25-30 cm.

8. Disease prevention and control

Grass carp itself has many diseases, and intensive ponds are even more prone to outbreaks of epidemic diseases. These diseases can cause large numbers of adult fish and fingerlings to die, reducing culture yield. Usually the prevalence of some grass carp diseases brings great economic losses to farmers.

The relatively common diseases in grass carp culture mainly include: enteritis, red-skin disease, hemorrhagic disease, gill rot, septicemia, liver and gallbladder disease, etc. Usually these diseases do not occur singly; the occurrence of one disease is usually accompanied by two or more complications. Moreover, the occurrence of these diseases usually increases in probability as the temperature rises.

In the process of large-scale grass carp culture, these diseases must be prevented and controlled, especially in the relatively hot summer and autumn seasons.

Prevention and control measures

1. Fish diseases should focus on prevention, which is also the key to intensive grass carp culture. Therefore, throughout the whole culture process, most effort should be put into fish disease prevention and control, strictly controlling the disinfection at every stage.

2. The main reasons grass carp is prone to illness are water quality deterioration and aging, moldy feed, and seasonal outbreaks of bacteria and parasites, etc. Because among fishes the grass carp likes feed and has a large food intake, the most effective treatment method is of course the method of internal administration plus external application.

3. In the process of feeding concentrated feed, try to feed green feed once a day or once every certain period, to satisfy the grass-eating nature of grass carp and avoid the occurrence of liver and gallbladder disease. If there is not enough green feed, feed can be appropriately stopped for 1 day every 7-10 days, to reduce the burden on the fish liver and gallbladder and lower the risk of liver and gallbladder disease.

4. Injection of vaccines can be used to prevent and control the occurrence of fish diseases.

5. Limit appetite: grass carp is greedy; as long as the environment is suitable, its appetite is basically unrestrained. The hot-water summer and autumn seasons are the peak growth period for grass carp and also the time of greatest food intake. In the high-temperature season, especially, the food intake of grass carp must be limited; generally feeding to 70-80% fullness is enough, and the remaining residue should be taken out every day.

6. Pond water disinfection: from May to September, it is best to disinfect the pond water once every half month each month. Commonly used drugs and their dosages: bleaching powder 1 g/cubic meter of water, copper sulfate and ferrous sulfate mixture 0.7 g/cubic meter of water, trichlorfon 0.5-1 g/cubic meter of water, quicklime 20 kg/cubic meter of water, etc. Using the above drugs alternately gives better results.

7. Feeding medicated bait: from May to September, regularly feed medicated bait made from Chinese herbal medicines and immune enhancers for health care, mixing the medicine into the feed, mixing with feed 1-3 times per month, each feeding lasting 2-3 days.

8. For treatment methods of related diseases, treat according to the disease contracted. Due to limited space, please refer to related articles; this article omits them.

(Liu Wenjun)