Most aspects of Chinese carp culture have been discussed elsewhere in this manual or in other more detailed FAO publications (e.g., Woynarovich and Horváth, 1984). Therefore only topics omitted or insufficiently covered in these papers are included below.
The major Chinese carps are all freshwater species. They are native of the big rivers of Southeast Asia, which overflow their banks and flood heavily in the rainy, monsoon season. In sections of the rivers where they cannot overflow their banks because of the geological nature of the landscape, e.g, in mountain passes, the water current is significantly increased. Natural spawning of Chinese carps takes place in these fast-flowing sections.
The rapid water current washes the muddy layer off the bottom, leaving only the hard, stony primary substrate. Under these conditions there is no risk of oxygen deficiency, which could threaten the life of the eggs.
Grass carps spawn in the bottom layers of the river; bighead in the middle layers; and silver carps close to the water surface.
When first spawned, the eggs are very small, with a diameter of about 1 mm. During the process of swelling, water is absorbed into the space between the yolk and the eggshell, causing the egg to swell 50–100 fold. Parallel to this process the specific weight of the egg decreases and it becomes lighter, so that even a slow water flow is enough to carry it suspended in the water and drifting with the current.
Development of the embryo is fairly quick in warm (about 22°C) water, and after 24–36 hours the larvae are hatched. Chinese carp larvae do not attach themselves to a substrate in the same way as the larvae of fish species which spawn in stillwater. Instead they move vertically so that the slowly flowing water carries them to the flood plains, which are rich in nutrients. In these shallow, warm areas, food is abundant, and the Chinese carp larvae feed predominantly on rotifers.
Feeding habits characteristic of the species are formed at 3–9 weeks of age. From this time on, the fishes' nutritional and biological characteristics are as described in the previous papers on Hungarian fish culture practices.
It is almost impossible to simulate natural spawning conditions in fish farm ponds. Therefore, in traditional Chinese fish farms, new young Chinese carps for stocking ponds are not obtained as a result of reproduction on the farm, but by catching wild fry resulting from natural spawning and by transporting them to the fish farms.
1 Based on material contributed by Dr L. Horváth, Agricultural University of Gödöllö, Hungary
Fry are caught when they migrate from the flooded areas to the deeper sections of the river as the water level recedes. They are sorted out by species on the basis of their differing oxygen requirement. Then a long and very careful transportation starts, sometimes lasting for several weeks. During the journey, water is changed frequently. Finally the fish are stocked into ponds, and kept there until they reach market size.
Because of the difficulties implicit in catching and transporting wild fry, simple methods of reproduction have been elaborated by Chinese farmers in special “spawning” ponds. These ponds simulate the conditions of the natural spawning places, and the fish are induced to spawn by administration of various hormone extracts.
Spawning ponds of elliptic or drop-like shape, with through- flowing water, proved to be the most effective. An egg collector is built into the outlet structure. The partially swollen eggs are carried to this collector by the out-flowing water.
Various chemicals can be effectively used to induce the ovulation of Chinese carps, but their effect is not as strong as that of common carp hypophysis. The weaker effect is, however, enough to induce ovulation when the pond conditions simulate the natural spawning environment.
The fertilized eggs collected from the spawning pond are transferred to big concrete tanks, where the water flow keeps them in constant motion. Hatching occurs in these tanks, and larvae are also kept there. After larvae have started to feed, the early fry are stocked into nursing ponds. It is not necessary to build a hatchery when this method of reproduction is used. Except for the construction cost, this is a relatively cheap system, but it does not utilize the full biological reproductive potential of the fish.
The first introductions of Chinese carps to the USSR were destroyed during the second world war. The second introductions began in the fifties in the southern, warm zones of the country. Fry were imported from China but the annual importation was not enough to supply a widespread expansion in farming of these fish, making the elaboration of a propagation method within the country a most urgent objective. To this end the results of Grebilskij and his team on the induced reproduction of sturgeon, which had been worked out in the thirties, were used.
This method was based on the application of gonadotropic hormones from the pituitary gland of other species of fish. The first successful reproduction was achieved in 1961 in Turkmania, when Aliev reported on the results of a large scale propagation. Two doses of hypophysis were given. The first treatment prepared the ovary for ovulation, while the final dose induced the actual ovulation. Fertilization was carried out by the so-called “dry” method. Hatching and larval rearing were done in a special fish hatchery. Eggs were hatched in jars and the larvae were reared in Chinese net cages. But later these were discarded because of high losses, and replaced by plastic larval rearing tanks or hatching jars of 50–200 l volume.
The technique used today in Hungary is a modification of this Soviet method as described below.
As Chinese carps do not reproduce spontaneously in fish farms, their propagation can be carried out only in hatcheries with the help of induced breeding. Under the Hungarian climate, sexual maturity is reached in 5 to 7 years. Males mature 1 or 2 years before females.
The principles for keeping common carp spawners are also valid for Chinese carps, with the provision that the peculiarities of the species have always to be considered and the optimal environment characteristics be provided. Grass carp prefer ponds containing transparent, clean water with rich growth of sub-merged vegetation. From 100 to 200 spawners may be kept per hectare, together with a few bighead and silver carps. About 15 to 20% of their body weight is required daily as food, in the form of grass or leguminous plants.
A favourable environment for silver carp is provided by waters of medium eutrophication. In such waters, the stocking rate should be rather low (50– 150 ind./ha). Large ponds are also good for silver carp, where they are stocked together with common carps ready for marketing.
Bighead carps prefer smaller ponds with thick layers of mud. About 200 specimens may be stocked per hectare. While chemical fertilizers are satisfactory for the fertilization of the silver carp ponds, bighead ponds have to be fertilized with animal manure.
When keeping Chinese carp spawners, water replacement in the ponds is essential. The problem of artificial feeding has not yet been solved. In temperate climates, spawners are ready for propagation by the end of May, when weather conditions are normal. By comparison with common carp, yolk development in Chinese carps is less advanced by autumn. During spring and early summer, considerable quantitative changes take place in the sex cells. Consequently, different feeding is required from that of common carp. Most of the food necessary for development of the yolk has to be given to Chinese carps during spring. That mature fish are ripe for propagation is shown in females by their soft bellies, while males are ripe when the pectoral fins become rough and uneven. When pressed slightly in the vicinity of the genital opening, ripe grass carp males release a thick milt. In the case of grass carp females, unripe specimens may be mistaken for ripe ones when their alimentary tracts are distended with food.
With Chinese carps, the separation of sexes is not necessary as spawning does not take place under pond conditions. When handling and transporting Chinese carps, it must be done very carefully since they are much inclined to jump and are therefore prone to injury. Anaesthetization is therefore absolutely necessary during propagation. For this purpose, a MS 222 solution is used at a concentration of 1:10 000–1:12 000.
To induce spawning of Chinese carps in Hungary, common carp pituitary is used. It is given to spawners in two doses. In the first treatment, one tenth of the total dose is given in one millilitre of physiological solution, while the remaining nine-tenths is injected during the second treatment, the decisive one. Normally 12 to 14 hours (with a minimum of 9 hours) must elapse between the two treatments. The propagation has to be scheduled so that stripping can take place early in the morning. Accordingly, the first pituitary treatment has to be carried out the previous morning, and the second one late in the evening.
The weight of fish, which serves as the basis for calculating the amount of pituitary to be given, is measured during the first treatment. The dosage for Chinese carps is 4–4.5 mg/kg body weight. In the case of spawners with big ovaries, a surplus quantity is needed to secure safe ovulation. This is estimated by the circumference of spawners at their widest point; for example, a spawner of 50 cm circumference receives 5 mg of pituitary per kg body weight, while above 60 cm, 5.5 mg/kg is given. It is not necessary to close the genital opening of Chinese carps.
Males are treated once only, 12 to 24 hours before the planned time of stripping. For this purpose, 2 mg of pituitary extract are used per kg body weight. It is advisable to separate males from females after the pituitary treatment, so that spawning does not start in the ripening tanks; eggs will then be partly lost. Sometimes it happens that eggs ripen after the first treatment. High temperatures (above 25°C), oxygen deficiency or a high first dosage may be responsible for this. Generally, the fertilization of such eggs is poor, because they are over-ripe.
The best water temperature for ripening silver carp and grass carp is 22°–24°C, and that for bighead 23°–26°C. Then ovulation takes place 200 to 220 degree hours after the second treatment or about 9–11 hours. When water temperatures are either lower or higher, the period needed for ovulation is either longer or shorter accordingly.
Ripe eggs must be taken from the ovaries without delay, otherwise they over-ripen within 30–60 min and their fertility decreases. Stripping is carried out under anaesthetization. It is advisable to synchronize stripping and fertilization so that losses resulting from water penetration are reduced to a minimum.
One kilogramme of “dry” eggs is mixed with 10 ml of milt taken from several males. Fertilization is carried out by adding 100 ml of clean, well-oxygenated pond water. A few minutes after fertilization the eggs are rinsed several times with clean water, to remove superfluous sperm and ovarian fluid.
After a couple of rinsings, the eggs begin to swell. The perivitelline space of the eggs of Chinese carps is large and substances causing stickiness are almost absent from their surface. Several minutes after fertilization the eggs may be transferred to the incubation/hatching container.
A quantity equivalent to 50 ml of “dry” eggs may be placed in each hatching jar of 7–9 l volume. Subsequently, eggs swell to 50–100 times their original size, and fill between half and three-quarters of the hatching jar. Fully swollen eggs have very thin shells and they are very sensitive to mechanical damage. Not only are the shells delicate, but in some stages of development, cell groups of the animal pole can fall apart. To avoid damage, incubation has to be carried out in the following manner.
During the first 8–10 hours the water flow must be regulated to 0.2– 0.3 l/min. This quantity of water is just enough to keep the eggs suspended in the jar and moving in a gentle, scarcely visible way. This satisfies the low oxygen demand of eggs in the early stages of segmentation. After 10 to 12 hours of development the oxygen demand of the eggs increases. Simultaneously their sensitivity to mechanical damage decreases, allowing the water flow to be safely increased to 0.7–0.8 1/min.
During this time the cell content of infertile eggs disintegrates, and as a consequence of their changed specific weight they float above the fertile eggs. The layer of infertile eggs must be siphoned off with a rubber pipe to eliminate possible infections. At a water temperature of 22°–24°C, hatching of eggs takes place 24 to 32 hours after fertilization. At this stage the embryo moves violently, making the shell of the egg rupture from the inside. The oxygen requirement of the embryos is high; a further increase of water supply to 0.9–1.2 1/min is necessary.
The hatching of eggs may be promoted by the use of an enzyme. In this method, industrial protein-decomposing enzyme (alkaline protease) is added to the hatching water to give a concentration of 1:10 000. This results in complete hatching within a few minutes. The enzyme not only causes the shells of hatching eggs to dissolve, but also breaks down infertile eggs, thereby protecting the filtering system of the larval containers. By adopting this method, the time of hatching may be reduced and the rotation of hatching jars accelerated.
The eggs of Chinese carps are extremely sensitive to the harmful effects of various microorganisms. In polluted water, the amount of bacteria and fungi may considerably increase. By attacking egg shells, these may lead to diseases of the embryos within the eggs. Bad water quality is indicated by the hatching of under-developed larvae, which accumulate at the bottom of the hatching jars. When examined through a microscope, these larvae show improperly developed hearts. Although such larvae will be capable of swimming after a few hours, their viability decreases during the following weeks. As a result, survival rates during pond rearing will be low. Protection against bacteria and fungi is provided by a formalin treatment.
Formalin is added to the hatching water every 4–5 hours to a final concentration of 1:5 000 or 1:10 000. This treatment may be safely continued during the larval stage. Formalin treatment ends when larvae begin to feed, because this chemical may damage the newly-developed gill tissues.
Generally, larvae are reared in the hatchery for 4 or 5 days at a water temperature of 20°C. During this period, the larvae are kept in 50–100 l jars similar to those used for common carp. Their oxygen requirement is met by water upwelling from the bottom of the jars. As many as 50–100 thousand larvae may be kept in each jar. When feeding begins the larvae are fed with nutrients of appropriate particle size and quality. This prevents their energy reserves from being exhausted. As with common carp, the best feed for Chinese carps is a suspension prepared from boiled eggs. This is given every 2–3 hours. In the event of unfavourable weather conditions, larvae can be held longer in the hatchery on such food for several days. Generally, however, the larvae are transferred to rearing ponds as soon as active feeding has begun.
Woynarovich, E. and L. Horváth. 1984 The artificial propagation of warmwater finfish. A manual for extension. FAO Fish.Tech.Pap., (201): 183 p.