Although the common carp propagates in confined waters, its natural environment for propagation is associated with flood plains and inundated terrain, where the fresh oxygen-rich water, rising water level, and inundated grass stimulate spawning. Here, its enemies are few, fry can find ample food, and survival rates are good. The optimal temperature range for common carp spawning is 18°–22°C.
On the other hand, there are many micro enemies in ponds and this poses a threat to successful spawning. It is interesting to note that the common carp does not spawn in ponds where other fishes, especially the carnivorous type, are present. However, some females in brood fish ponds spawn without any stimulation and without even the presence of males, especially during the second half of the spawning season.
The artificially stimulated propagation of common carp occurs in special spawning ponds or Dubisch ponds, as previously described. They usually spawn during the early hours of the morning a day after stocking, if they are fully ripe and if the temperature is within the appropriate range. The propagation instinct of this fish appears to be stronger in tropical and subtropical regions, where they spawn more readily.
The common carp even spawns in very small ponds (25–30 m2) on grass mats or kakabans, which serve as egg collectors. The kakaban with its scattered eggs can then be transferred to hatching ponds.
In lakes, the common carp propagates when heavy rains cause the water level to rise and inundate the grassy areas. It is an age-old custom to catch the common carp while spawning in large groups in the shallow spawning grounds and obtain their sexual products through stripping.
Before the technique of dissolving the sticky layer of eggs was known, common carp eggs mixed with milt used to be distributed on the surface of an artificial nest with the help of a strainer spoon having 2.0–2.5 mm holes (Figure 76). They were then incubated in baskets or spray chambers. Sometimes eggs were distributed on the grassy parts of a freshly inundated pond.
The percentage of success in common carp hypophysation is about 60–70 percent. The results will be much poorer if the brood fish have not been given sufficient quantities of protein-rich food. Ovulation in common carp is total; i.e., all the eggs ovulate at the same time and, therefore, the female can be stripped “empty” in one operation. However, larger females over 7–10 kg in weight often ovulate at intervals and must be stripped over 5–8 hour periods at one-hour intervals.
The common carp can be induce-bred through hypophysation in a very small tank or net box (hapa), where it will scatter its eggs. These eggs adhere to the walls of the tank or hapa in the absence of egg recipients. Adding egg recipients to the tank, such as kakabans or aquatic plants, will ensure better propagation.
Before the technique of suturing the female genital opening came into practice, one had to keep the ward tank under close observation to obtain ovulating females for stripping.
The methods of dissolving the sticky layer, suturing the females, development of first food for the fry, and controlling their enemies have made the commercial propagation of common carp easy and extremely successful, and have enabled it to become the foremost among the cultivated fishes of the world.
Similar propagation technology has been adopted for other European cyprinids, including the carnivorous carp Aspius aspius.
The Chinese major carps are the grass carp (Ctenopharyngodon idella), the silver carp (Hypophthalmichthys molitrix), and the bighead carp (Aristichthys nobilis). They are all river spawners, with non-sticky floating eggs, which swell considerably because of a very large perivitelline space. When it is possible to simulate the riverine environmental conditions, the ripe brood-fish can be made to spawn in tanks and basins through the administration of fish pituitary or human chorionic gonadotropin injection. However, usually only ovulation can be induced in tanks, basins, etc., the sexual products are obtained through stripping and fertilized artificially. The induced spawning of grass carps is the easiest and that of the silver carp the most difficult. Full spontaneous spawning is hardly ever achieved in the case of the Chinese carps. Only about 50–70 percent of the eggs may be released, while the rest (30–50 percent) will be spoiled unless the fish is stripped in time.
The Chinese technology of induced spawning and collection of eggs has already been described in an earlier section. While HCG is normally used for the induced spawning of Chinese carps, a combination of pituitary gland extract and HCG can also be used with good success. Best results are obtained when the brood fish are injected in two equal doses at an interval of about six hours.
To strip the fish and fertilize the eggs artificially in bowls, the introductory and decisive dose injection system is employed. Ten percent of the calculated dose is given first and the decisive dose of 90 percent is given after 18–24 hours. Dosages for Chinese carps are determined according to body weight and the greatest body circumference (Figure 77).
It is important to know the exact time of ovulation, since any delay in stripping may result in overripening of the eggs in the ovary.
The course of ovulation depends on the temperature of the water in which the female is kept. As already explained, the time lag between the decisive injection and ovulation in relation to temperature is expressed in terms of hour-grade. The hour-grade is about 200–220 when the temperature is at the optimum range. The hour-grade decreases at higher than optimal temperatures (20°–26°C) and increases at lower temperatures. Ovulation in Chinese carp usually occurs about 9–11 hours after the decisive injection.
The fertilization of Chinese carp eggs does not pose any problem because of the non-sticking nature of their eggs. It is necessary to avoid a strong current in the incubation device, since it may harm the eggs because of their delicate shell. Subjecting the eggs to tannin treatment would help in avoiding bacterial and fungal attacks.
The larvae hatch in about 24–32 hours after fertilization, at temperatures of 22°–24° C. The hatching can be accelerated with the use of an alkaline protease enzyme.
The larvae of Chinese carp swim vertically during their first day. They can be easily separated from the egg shell, spoiled eggs, and debris by the “swimming out technique”. If the incubator is of such a type that the larvae cannot swim out, they must be removed by siphoning or scooping; otherwise, they may become entangled in the debris and die. During the second day, the larvae rest at the bottom, with only occasional swimming. During this time they develop various organs which enable them to breath, swim, and feed “fish like”.
The micropredators, such as Cyclops, are very harmful to the larvae of Chinese carp and they may kill hundreds of larvae, unless suitable steps are taken to eradicate them or prevent their growth. The larvae fill their air bladder with air during the fourth or fifth day. The protozoans and rotifers form the food of the just-feeding larvae, while they start consuming more and more small crustaceans after about one week.
The fry of Chinese carps are quite hardy, unlike the sensitive fry of the common carp. The fry grow to about 2.5–3.5 cm in length in about 3–4 weeks.
The Indian major carps consist of catla (Catla catla), rohu (Labeo rohita), mrigal (Cirrhinus mrigala), and kalbasu (Labeo calbasu). They spawn usually in inundated areas adjoining river banks and have semi-floating and non-adhesive eggs.
The semi-artificial propagation of major Indian carps is practised in the so-called “bundhs”, which are a special type of ponds where river-like conditions are simulated. They are usually constructed along the slope of an undulating terrain and are provided with proper embankments. They receive considerable quantities of rain water rushing in from their extensive catchment area after heavy showers. The bundhs have outlets for the excess water and shallow areas to serve as spawning grounds.
The bundhs are of two types: (a) the perennial or “wet bundhs”, and (b) the seasonal or “dry bundhs”.
(a) Wet bundh. The wet bundh is either an existing, suitably located, perennial pond or a perennial pond constructed by putting an embankment across the slope of an extensive catchment area. The deep part of the pond retains water throughout the year, while its large shallow marginal area dries up during the dry season. The shallow area becomes inundated with the inrushing water after heavy showers during the rainy season. The brood-fish are either raised in the perennial pond or released from other ponds at the onset of the rainy season. They are stimulated by the rush of large quantities of silt-laden, well oxygenated rain water and spawn freely. To prevent the escape of brood fish, the outlet is covered by bamboo grating or screen fencing.
(b) Dry bundh. A dry bundh is a seasonal shallow pond, much smaller in size than the wet bundh. The recent trend is to construct dry bundhs in suitable terrain. Water accumulates in dry bundhs during early rains, when fully ripe brood fishes from perennial ponds are introduced into them. The spawning is usually triggered when, after a heavy shower, the bundh is flooded with fresh rain water. The dry bundh has a major advantage over the wet bundh in that selective spawning of any one species of major carps to obtain pure seed is feasible.
In both types of bundhs, the fresh, silt-laden and well oxygenated rain water and flooded conditions of the terrain are the most important factors responsible for successful spawning.
For the purpose of hypophysation of Indian major carps, a spawning hapa is first fixed in the pond. The ripe selected brood-fish, one female and two males, are then placed in the hapa after the female has been injected with 2–3 mg of pituitary gland per kg of body weight. After an interval of 6 hours, the female is given a second, higher dose of 5–8 mg/kg body weight and the males are given a low dose of 2–3 mg/kg body weight. Spawning ordinarily occurs within 3 to 6 hours after the second injection.
The fertilized eggs are removed for hatching in the so-called incubation or hatching hapa. The eggs of Indian major carps can easily be incubated in any funnel-type incubator. The incubation time for eggs varies from 15 to 18 hours at temperatures ranging from 27° to 31°C. The above mentioned technique is most successful in the case of rohu and catla.
The oxygen demand of injected Indian major carps is obviously not so high and, therefore, it is possible to induce spawning in the hapa where the exchange of water is poor and no running water is supplied.
The induced spawning of Indian major carps can be arranged in ward tanks with a continuous water supply. They can also be stripped to produce fertilized eggs, as in the case of Chinese major carps.
The channel catfish can be bred by pen spawning as well as by hypophysation.
The pen method of spawning is widely used in channel catfish culture. It could be adapted for other pond spawners and for nest or spawning receptacle spawners as well. The pens are placed in ponds or gently flowing streams. The dimensions of the pens, which are placed in rows, are generally 1.5–2.0 m wide, 2.5–3.5 m long, and 1 m deep. The pens are constructed of planks and non-corrosive wire screen of mesh 2.5–5.0 cm. The lower part of the pen is embedded into the bottom of the pond, while its upper portion extends about 30–50 cm above the level of the water. Then, 45-l milk cans or small drums with one open end are anchored to the bottom and used as spawning receptacles for the channel catfish.
A pair of fish (male and female) is placed in each pen during the spawning season. It is important here to sex the brood-fish reliably. The spawning receptacles should be checked daily, or at least on alternate days. After spawning, either the male fish is left behind to care for the eggs, or the egg mass is collected and hatched in incubation devices. Usually a hatching trough with a shaft to which paddles are attached are used for incubation.
If hatched inside a pen or receptacle, the fry may swim out of the receptacle through the wire mesh into the pond. A new pair of brood-fish can then be placed in the pen to spawn. The favourable temperature range for the spawning of channel catfish is 21°–29°C, the optimum temperature being 26°C.
The channel catfish can be induced to breed even in aquaria by administering prescribed doses of hypophysis extract or HCG. In the case of pituitary extract, a total of 12 mg acetone-dried gland per kg of body weight is given in 3 equal doses, with a time interval of 24–48 hours between the injections. Most of the fish begin spawning within 16 to 24 hours after the last injection. Incubation is generally done in mechanical hatching troughs, where the water is agitated with revolving paddles. The eggs hatch in about 6 days in a temperature of 25°–27°C.
It is possible to advance the spawning of channel catfish by a few weeks by the administration of pituitary extract or HCG.
The fry begin to feed when they are about 3–5 days old. It is very important that suitable feed should be available at this time. A number of artificial starter feeds for channel catfish fry are available in the U.S. market. The young fry should be fed every 2–4 hours around the clock during the first week, after which it suffices to feed them four times a day. The fry which do not learn to feed during the first few days after the absorption of the yolk will not survive.
The fry are usually nursed in troughs or ponds. The nursing trough may be made of wood, metal, fibreglass, or plastic. It is 2.5–3.0 m long, 30 cm deep, and 50 cm wide. Each trough must be supplied with running water and equipped with a drain and a screened stand pipe. About 20 litres of fresh oxygenated water per minute are supplied to each trough.
The fry are either raised up to the fingerling stage in the troughs or are transferred to ponds for further growth. Such ponds measure about 4 000–5 000 m2. Predatory insects are often a problem in fry nursing ponds. The predation by insects can be prevented by not filling the pond until immediately before it is stocked with fry. If the water has been standing in the pond for several days, it should be treated with a non-residual insecticide 2 or 3 days before the pond is stocked. Another old practice is the application of 20–40 l of diesel oil or kerosene per hectare of surface water on non-windy days twice during the week prior to stocking.
The stocking rate in a nursery pond is 100 000–500 000 per hectare. The young fish are fed daily at a rate of about 4–5 percent of their body weight. The size of fingerlings reared in a pond depends on the intensity of the feeding and stocking rate.
European catfish culture practices clearly exemplify the application or adoption of different technologies based on local facilities.
The European catfish spawns both in confined waters and in inundated terrains. Usually they spawn on roots hanging in the water or on water plants. It is a nest spawner. The eggs are adhesive and hang in bunches on the nest.
The brood fish must be raised with abundant food. The European catfish is carnivorous, feeding on trash fishes, frogs, tadpoles, etc. However, it readily accepts pelletted feed as well. During the period the brood fish are being prepared for breeding, they are given a daily diet of trash fishes about 2.5–3.0 times the total weight of the brood fish.
The sexes must be segregated when the temperature of the water is about 12°–15°C, otherwise the males bite the females severely (Figure 78).
Tent-like nests are fixed in small ponds; the ponds should be hard-bottomed. As many pairs of brood fish as there are nests are released into the pond as it is being filled. The fish will spawn after a few days, usually early in the morning. Prior to spawning, the male fish cleans the nest. This is followed by some sex play, with the brood fish swimming around the nest, and this triggers the spawning act. The resultant spawn is guarded by the male fish. For hatching, the nest is dismantled and the egg-laden branches are hung inside a net box (1.0–1.5 mm mesh) fixed near the inflow area of the pond (Figure 79).
The just-feeding fry in the net box are normally fed with plankton. Sometimes they are given specially prepared artificial feed consisting of boiled eggs, fish meat, and flour in a dough-like mixture. This food mixture is smeared on the side of a flower-pot or on a black tray, which is then placed inside the box. It is necessary to remove the food remnants before they start decaying.
The European catfish can be induced to spawn by injection of pituitary gland extract, the dose being 3.5–4.0 mg of acetone-dried hypophysis per kg of body weight. Males are only given about half this dose. The entire dosage is given in only one injection to both the males and females. A tent-like nest is fixed in a small cement tank (4–6 m2) provided with good water supply. One pair of the injected brood fish is then released in the tank. The fish spawn the next day if the temperature is about 23°–25°C. The resultant eggs are hatched in incubation devices, such as hatching boxes, funnel-type incubators, jars or troughs (Figure 80).
After they are brought to the hatchery, the brood fish are tranquillized in a 1:12 500 solution of MS 222 (i.e., 4 g MS 222 in 50 l water). The mouth of both the sexes is stitched with synthetic thread through a small drilled hole, to avoid one biting the other. The thread is loosely bound to allow normal respiration (Figure 81). Both the male and female breeders are then administered the calculated quantity of pituitary extract (3.5–4.0 mg of acetone-dried gland per kg body weight of female and half that dosage for the male) in one injection. Usually they are injected at noon and are stripped the next day early in the morning. The hour-grade for the ovulation of this fish is 430–460. Unlike Chinese carps, there is no danger of fast over-ripening of eggs in this fish.
After stripping, fertilization is effected in a 0.3 percent common salt solution. Since the eggs are very sensitive, it is advisable not to mix them vigorously during fertilization. If the males fail to yield enough milt, one of the males is cut open, half a testis removed and the wound stitched up. (The male will recover and survive.) The half testis removed is then crushed and the milt thus obtained is adequate for fertilization. The eggs become fertilized in about 2–5 minutes, after which the egg mass is placed in suitable incubators for hatching.
After 10–12 hours, the clumped egg mass is treated with a 0.3–0.5 percent solution of alkaline protease enzyme for 2–3 minutes to dissolve the sticky layer of eggs; this will enable the eggs to fall apart and become free. During this treatment, the flow of water is stopped and the egg mass is stirred with a stick to facilitate their separation. The eggs are also treated against fungi with malachite green solution of 5 ppm for 30 to 60 minutes.
When the eggs begin hatching, they are taken out of the incubators and placed in flat trays, where all the eggs will hatch.
The larvae are reared in a rearing box of sieve cloth of about 0.8–1.0 mm mesh. Since the larvae replenish the water around their body by vigorous movements of the tail, there is no danger of death by suffocation (which is rather common among the larvae of other fish).
The just feeding fry are grey in colour and feed on plankton or chopped Tubifex. They also feed actively on specially prepared dough-like food mixture, which is smeared on a flower pot or black tray placed inside the rearing box (Figure 82). It is necessary to provide some hiding place for the fry, otherwise they swim about nervously.
Two or three weeks old fry, which are about 2–3 cm long, are stocked in ponds where there are enough hiding places and smaller fishes to serve as food.
The tench spawns in confined waters, where it scatters its very small sticky eggs on submerged vegetation. The realization of its use as a scavenger fish, having a niche of its own in polyculture operations, has led to its artificial propagation.
The tench becomes sexually ripe when its weight is over 300 g. The sexes are segregated during the spawning period to avoid wild spawning. The segregation task is quite simple, since there is a distinct difference between the pelvic fins of the two sexes. The second ray of the pelvic fin is strong and bent in the males (Figure 83).
The tench respond to induced ovulation when the water temperature is 23°–24°C. Females weighing about 300 g are administered 9–10 mg of acetone-dried pituitary gland extract in one single injection into the abdominal cavity under the pelvic fin. Males of the same size are administered only 6 mg of pituitary gland extract. The two sexes are then put together in the ward basin, where they spawn spontaneously. Splashing water in the ward tank signals the commencement of spawning.
About 5–10 minutes after the commencement of spawning, the females and males are removed from the basin and stripped. One female of 300 g gives 30–50 g of dry eggs and one male 3–5 drops of thick milt. The eggs are sticky. As in the case of the common carp, a common salt solution is used for fertilization and carbamide solution for dissolving the sticky layer of eggs. The eggs are treated with 0.05 percent tannin (5 g tannin in 10 l water) after one and a half hours.
The eggs are incubated in glass containers (Zoug jars) or in acute angled plastic funnels, since the eggs are very small and heavy. The eggs hatch on the third day at a temperature of 22°–23°C. The very small larvae are collected in a funnel-type larva-rearing device with sieve cloth belt made of 150–200 μ meshed sieve cloth material.
The larvae take in air on the fifth or sixth day. Rotifers form the first food of tench fry. The technique of nursing the tench fry is similar to that of the common carp fry.
The pike-perch is an important carnivorous fish both in fish ponds and natural waters. It spawns successfully in confined waters on fibrous materials, nests, or stones, which are cleaned by the male fish prior to spawning. The spawning generally takes place when the water temperature is about 9°–12°C.
In the case of natural waters, artificial nests are fixed on the spawning grounds to kilometre-long ropes, which are checked once every two to three days. The nests covered with eggs are transferred for hatching either to a quiet part of the lake and put in a basket hanging in the water or to a pond which is free of other fish. The eggs can also be incubated in spray chambers, wherein very good results are usually obtained, even achieving a 100 percent hatching rate at times.
For spawning the fish in small hard-bottomed ponds, the brood fish are stocked in a 1:1 sex ratio and a nest is provided for each pair.
The egg-laden nests can be successfully transported over long distances by packing them in moist cotton or moss. The eggs are able to withstand the journey because of their double shell layer. This is the best way to distribute this fish, since the young and adult fish are very sensitive and can hardly survive long-distance transportation.
Induced spawning by hypophysation has not been successful so far. The hormone-treated fish very soon become emaciated and dark in colour; death occurs within a few weeks.
The larvae, which swim about vertically, should be protected from direct sunlight, since it affects their equilibrium and eventually causes death. Likewise, the just-feeding fry must also be kept in the shade since they are blinded in bright light and are unable to locate their prey. Even though just-feeding fry can feed on crustaceans of 0.5 mm size, they grow better on rotifers during the first week of their life. Cannibalism occurs when they are about one month old and when there is a paucity of food.
The giant gourami is a nest-building pond spawner. The male fish guards the nest and aerates the eggs. Its propagation can be induced by putting nest frames and nest material into the pond, where fresh water of adequate depth is provided.
In spite of parental care, the survival rate of eggs under natural conditions is very low because of enemies such as leeches, insect larvae, small crabs, pollution of the water inside the nest, and fluctuations in water level in the pond. Therefore, to achieve better survival rates, the eggs deposited in the nests are collected and hatched under controlled conditions.
The spawning pond for the giant gourami should be about 300–500 m2 in area and about 1 m deep. It is stocked with brood fish at the rate of one per each 7–10 m2 of pond area, the sex ratio being 2 females to 1 male. Introduction of a large number of males will result in their fighting with each other and injuring themselves.
The nest frames are funnel-shaped and are made of split bamboo or sticks. The length of the frame is about 75–90 cm, while the diameter of the funnel at its mouth end is about 30–35 cm. The nest frame is stuck into the earth by its stem on the side of the pond in such a way that it lies nearly horizontal about 15–20 cm below the water surface, with its mouth facing the centre of the pond. The nest frames can also be tied in horizontal position to two sticks stuck in the bottom of the pond (Figure 84). Fibrous nest material is placed inside the frame and scattered around in the water. Ten such nest frames are placed in each pond. When the brood fish are introduced into the pond, they rearrange the nest material and build a round nest inside the cavity of the frame.
The nest frames are inspected regularly for fertilized eggs. As soon as the spawning is over, the nests are detached from the frame and the eggs removed, so that fresh breeders can be introduced to obtain a regular supply of eggs.
For separating the eggs from the nest, the entire nest is gently detached from the frame and placed in a bucket of water. The outer fibres of the nest are then carefully removed. The buoyant eggs will be at the centre of the nest, the fibres of which are removed layer by layer. Each of these layers is immersed for a while under water in a container, whereupon the eggs will float to the surface of the water. These eggs are scooped out with a bowl and quickly transported in a container with clean pond water, for hatching and subsequent rearing of the larvae. The eggs are packed in plastic bags under oxygen pressure when long-distance transportation is involved.
The eggs are yellow in colour and almost transparent. They measure 2.5 mm in diameter. Their yolk is fluid and has an oil globule, which makes them buoyant. The eggs hatch within 48 hours in a water temperature of 28°C.
The hatching of the eggs occurs in tanks or aquaria aerated with a slowly bubbling stream of air. Water plants such as Hydrilla and Ceratophyllum are also placed in the tanks, so that the newly hatched larvae can attach themselves and rest on them.
The newly hatched larvae remain upside down, with their yolk sack pointing upwards. After some time the larvae cling to plants or other objects without much movement. They assume their normal fish-like position on the fifth day after hatching and then commence to feed on minute organisms like Infusoria (ciliates).
The larvae are usually fed with Paramecium culture, which is prepared in the hatchery. Small sized zooplanktons are given to the fry about 10 days after they commence feeding.
The fry develop well in a pond rich in zooplankton, the growth of which is promoted by cowdung manuring. The nursery ponds are stocked with about 200 advanced fry per square metre. They become 6–8 cm long in about 4–6 weeks, their growth rate depending on the quantity and quality of food available.
The traditional practice in grey mullet culture is to capture juveniles of the fish schooling in estuaries and coastal waters about two months after the adults have spawned in the sea.
For artificial propagation, the brood fish are generally obtained from brackishwater fish farms or by fishing them from natural waters during their migration to the spawning grounds. The brood fish are fed with suitable protein-rich feed in small out-door sea water ponds. Prolonged conditioning of the brood fish in salt water of about 32–35 parts per thousand is an essential prerequisite for spawning.
The maturing females are then transferred to small indoor tanks, where the temperature of the water is kept at about 20°–22°C. The gonads of the females are sampled (if necessary) at weekly intervals to start with and daily later as the eggs mature. A polyethylene cannula (small tube) of 0.85 mm diameter is inserted into the oviduct and a sample of the eggs withdrawn. The average diameter of the eggs is an indicator of egg maturation. A female is deemed as ready for hormone injection when the mean diameter of the eggs is at least 0.6 mm.
The dose and sequence of hormone administration depend on the type of hormone available and the state of egg development. If purified salmon gonadotropin is used, the total dose, calculated at the rate of 12–21 mg per kilogramme body weight, is administered in two equal instalments, with an interval of 24–48 hours between them. Common carp pituitary gland has been successfully used in Israel to bring about ovulation in grey mullets. They can also be induced to spawn by administering, in two equal instalments, a dose of 60 000 IU of human chorionic gonadotropin (HCG) per kilogramme of body weight. The injection is given intramuscularly at the base of the dorsal fin. The females spawn in about 12 hours after the second injection. The males produce active spermatozoa without hormone treatment during the spawning season.
Induced spontaneous spawning is generally preferred to stripping and artificial fertilization. Individual females are spawned in 200 l aquaria, into which “running” males are released about 2 hours prior to the expected time of spawning. Excessive drinking of water by the female prior to spawning leads to complete hydration of the eggs. This is characterized by rapid enlargement of the abdomen. Enlargement of the cloaca is an indication of imminent spawning. At this time, the males swim in unison with the females or circle around them. The floating fertilized eggs can be gathered by an egg collector. The spent breeders are removed from the tank or aquarium as soon as the spawning is over.
The eggs may be incubated in funnel-type circular incubators at a density of not more than 400 eggs per litre. Extra oxygen can be provided by aeration. The eggs will hatch in about 50–60 hours at a temperature of 20°–22°C. The newly hatched larvae are stocked in larva-rearing tanks at a density of 10 per litre. By the fourth day, the eyes become pigmented and the larvae are able to feed.
The preparation of fry-rearing tanks and food for the fry must begin at least a week before the spawning. Separate cultures of the algae Dunaliella, Chlorella and Isochrysis are prepared. The fry nursing tanks are stocked with a mixture of the three algal cultures and the rotifer Brachionus plicatilis. The objective is to achieve about 100 million algal cells and 5 000–10 000 rotifers per litre of sea water. The temperature should be in the range of 20°–22°C. Aeration of the tank helps in maintaining the necessary circulation of water and oxygen contents. Controlled addition of freshwater after the first week reduces the salinity of sea water to 24 parts per thousand by the 30th day.
Beginning on the 10th day, the fry diet is supplemented with copepods and brine shrimp nauplii. After the 40th day, large brine shrimps and copepods are given, followed by artificial feed. The fingerlings can be transferred to ponds after they are at least 45 days old.
Details of various chemicals used in fish propagation and young fish rearing operations are given in Appendix II.
The essential aspects of reproductive biology of the fishes dealt with in Chapter 10 are tabulated in Appendix III.
A chronological account of milestones in the artificial propagation of finfish is given in Appendix IV.
Figure 73 The extraction of pituitary gland from fish
Figure 74 Preparation of acetone-dried pituitary gland
Figure 75 Storage of pituitary glands
Figure 76 Spreading fertilized eggs of common carp on artificial nest
Figure 77 Relationship between pituitary dosage and greatest body circumference in the female Chinese carps
|(A) Breeders fed with small live fish|
|(B) When the temperature reaches 12–15°C, separate the sexes|
|(C) When the temperature reaches 20–22°C, prepare tent-nests out of dry roots or pine branches|
Figure 78 Propagation of European catfish (Silurus glanis). Preparatory activities
Figure 79 Propagation of European catfish (Silurus glanis). Induced spawning in earthen ponds by providing nests
Figure 80 Propagation of European catfish (Silurus glanis). Induced spawning by hypophysation in small cement tanks
Figure 81 Propagation of European catfish (Silurus glanis). Inducing ovulation and stripping
Figure 82 Propagation of European catfish (Silurus glanis). Rearing of larvae and fry
Figure 83 Sexual dimorphism in Tinca tinca
Figure 84 Induced propagation of giant gourami (Osphronemus goramy)