by
H. Chaudhuri
FAO/TA Inland Fishery Biologist (Fish Culture)
Rangoon, Burma
Although the Chinese and the Indian carps have been cultured in ponds for thousands of years, these fishes have not got domesticated, and differ very little from their wild ancestors. But the common carp Cyprinus carpio has been fully domesticated by selective breeding and a large number of strains and varieties have been produced in course of time. This was possible because reproduction in common carp could be controlled. On the other hand, the Indian and Chinese carps do not reproduce in ponds, and this inability to breed under captivity has restricted attempts at selective breeding and hybridization of these fishes.
Success in inducing spawning of Indian carps by injection of fish pituitary hormones was attained in 1957 (Chaudhuri and Alikunhi, 1957) and experiments in hybridization of Indian carps was initiated in 1958 by artificial fertilization of the stripped eggs of an injected female of one species by the sperms of an injected male of another species (Chaudhuri, 1959; 1961). Subsequent attempts were made to hybridize important species of Chinese carps and also the Indian × Chinese carps (Alikunhi et al., 1963).
Successful hybridization of Indian major carps (Family Cyprinidae, sub-family Cyprinini) have been carried out.
2.1.1 Method of crossing
Fully mature female carps are given a preliminary low dose of 2–3 mg of carp pituitary gland per kg body-weight. After an interval of six hours a second injection of a higher dose, 5.8 mg/kg body-weight, is given to each female. The male breeders are also given an injection of a low dose of 2–3 mg/kg body-weight. Depending on the temperature of water, within 4–6 hours after the second injection the female fish is found to be in oozing condition. The females and males are then stripped and the eggs and milt from two different species are mixed in a shallow container. Dry method of stripping is usually followed. Sometimes a little water is mixed and the tray is constantly tilted side ways. After a few minutes pond water is mixed and slowly the excess of milt is washed out. After several changes of water the eggs are transferred into bigger trays filled with fresh water. The eggs are fertilized and the blastomere appears on the animal pole within 45 minutes after stripping at a temperature range of 26°–31°C, and hatching takes place within 15–18 hours.
The cultivated carps of India belong to the three genera, Catla, Labeo and Cirrhina. Crosses and reciprocal crosses between eight species belonging to these three genera were successfully carried out.
2.1.2 Interspecific hybrids
Interspecific hybrids were obtained by crossing four species, Labeo rohita (rohu), Labeo calbasu (calbasu), Labeo bata (bata), and Labeo gonius (gonius) in the following combinations:
| Male parent species | Female parent species | Hybrid | |
| i. | Labeo rohita | Labeo calbasu | rohu calbasu |
| ii. | Labeo calbasu | Labeo rohita | calbasu rohu |
| iii. | Labeo bata | Labeo rohita | bata rohu |
| iv. | Labeo bata | Labeo calbasu | bata calbasu |
| v. | Labeo calbasu | Labeo gonius | calbasu gonius |
In crossing the hybrids rohu calbasu and calbasu rohu over 94 percent fertilization was observed. Hybrids are extremely variable, though the general appearance seems to be more like the paternal species. The growth rates of hybrids of both the reciprocal crosses were far superior to the slower growing maternal species Labeo calbasu.
The embryonic development of the three hybrids bata rohu, bata calbasu, and calbasu gonius was normal but the percentage of hatching was poor. Since Labeo bata is a medium sized carp, the hybrids bata rohu and bata calbasu did not show much promise although initially bata calbasu showed excellent growth.
The hybrid rohu calbasu was observed to have attained maturity in two years. One pair of the hybridswas induced to breed by injection of fish pituitary and the F2 generation was obtained. Artificial fertilization of eggs by stripping was not necessary. The hybrids bred normally after injections of pituitary. Percentage of fertilization was high and several thousand young ones were produced. The hybrids possessed varying characters intermediate between Labeo rohita and Labeo calbasu (Chaudhuri, 1960; Government of India, 1961).
2.1.3 Intergeneric hybrids
The following intergeneric hybrids were produced by crossing six species of fishes belonging to three genera.
| Male parent species | Female parent species | Hybrid | |
| i. | Catla catla | Labeo rohita | catla-rohu |
| ii. | Catla catla | Labeo calbasu | catla-calbasu |
| iii. | Catla catla | Cirrhina mrigala | catla-mrigal |
| iv. | Labeo rohita | Cirrhina mrigala | rohu-mrigal |
| v. | Cirrhina mrigala | Labeo rohita | mrigal-rohu |
| vi. | Cirrhina reba | Labeo rohita | reba-rohu |
| vii. | Cirrhina reba | Labeo calbasu | reba-calbasu |
| viii. | Cirrhina mrigala | Labeo calbasu | mrigal-calbasu |
(i) Catla-rohu hybrid: About 60 percent of the eggs developed and hatching took place after 16 hours. Mortality of hatchlings was high. Growth of the hybrid fry was faster than pure Labeo rohita fry obtained from the same mother. The hybrid characters were intermediate to the parental species. Catla-rohu proved to be an improved variety having a smaller head then Catla catla and wider body than Labeo rohita. Its growth is faster than the latter species, although not as quick as the former. However, it has the advantage over Catla in having a smaller head. Size to size, the quantity of flesh is more in the hybrid than in pure Catla. The hybrid attained full maturity in three years. Although ill-formed testes were observed in a few specimens and the presence of both testis and ovary was observed in a single specimen, a good number of males and females were found to be normal and fully mature. The fecundity however, seemed to be less than the parental species.
(ii) Catla-calbasu: The development of eggs was somewhat abnormal at the beginning. Only 20 percent of the eggs hatched out and only a small percentage of them survived. The growth was satisfactory and the hybrid reached the adult stage.
(iii) Catla-mrigal: Since freely oozing catla male was not available at the time of the experiment, only a few mrigal eggs could be fertilized, of which only a few hatched out. The early growth was satisfactory but all the hatchlings died due to pollution of water while being reared in the laboratory.
(iv) Rohu-mrigal; and (v) mrigal rohu: In crossing reciprocal hybrids rohu-mrigal and mrigal rohu over 90 percent eggs were fertilized. The majority of the body characteristics of the offspring were intermediate to those of the parents. Both the hybrids matured fully in two years.
(vi) Reba-rohu and (vii) reba-calbasu: Twenty percent of the fertilized eggs hatched out but all of them died on the third day.
(viii) Mrigal-calbasu: Sixty percent of the eggs hatched out. The hybrid had a slightly fringed lower lip and two pairs of prominent black barbels resembling that of the maternal parent. Colour of the body was intermediate between the parent species. The hybrids attained maturity in 2–3 years. One eight year old female was later bred by hormone injection.
The intergeneric hybrid catla-rohu attained full maturity in three years. Four year old fish were selected as breeders for the F2 generation. The female received two injections of fish pituitary hormones at an interval of 6 hours and the male a single dose. When released together, they bred within five hours. Percentages of fertilization and hatching were high. Thousands of young hybrids were reared in nursery ponds. Their growth rate, when stocked along with pure Catla catla and Labeo rohita fry was observed to be higher than the latter, but less than the former species. Hybrids were very variable in morphological characters. They grew to adult size but the maturity has not been studied.
2.1.4 Outcrossing of intergeneric hybrids
One fully mature mrigal-calbasu female hybrid was injected with fish pituitary and stripped. The stripped eggs were fertilized by the milt from males of Catla catla, Labeo calbasu and Cirrhina mrigala and the following hybrids were produced:
| Male parent species | Female hybrid | Hybrid | |
| i. | Catla catla | Mrigal-calbasu | Catla-mrigal-calbasu |
| ii. | Labeo calbasu | Mrigal-calbasu | Calbasu-mrigal-calbasu |
| iii. | Cirrhina mrigala | Mrigal-calbasu | Mrigal-mrigal-calbasu |
A good percent of a number of eggs hatched out and the hybrid fry that survived are being reared in ponds. Some males have been observed to have matured in one year.
It was possible to successfully fertilize stripped eggs of Labeo rohita with the milt of Cyprinus carpio (Alikunhi and Chaudhuri, 1959). The eggs of Indian major carps swell to the size of 4 to 6 mm in diameter, are non-adhesive and demersal. The incubation period is 15–18 hours at temperatures of 26°–31°C. The eggs of common carp on the other hand are very small with very little perivitelline space, are adhesive and the incubation period is at least 48 hours at the above temperature range.
Though the percentage of fertilization was satisfactory subsequent mortality of eggs was high. Many larvae died at the time of hatching or soon after hatching. Only a very small percentage of hatchlings could be grown to adult size. The hybrid has elongated dorsal fin like the paternal species but the other characters are intermediate between the parent species. Adults did not mature.
Reciprocal crossing was also tried. Though the eggs of C. carpio could repeatedly be fertilized with the milt of Labeo rohita, the eggs did not hatch out.
An outcross with the female hybrid rohu-calbasu and male Cyprinus carpio was successfully carried. The hybrid is being reared in ponds.
Intergeneric hybridization between the three species Aristichthys nobilis (big head), Hypophthalmichthys molitrix (silver carp) and Ctenopharyngodon idella (grass carp) was attempted. In a few cases the eggs got fertilized but did not hatch out, while in others the hatchlings survived up to a maximum of one week. The crosses were:
| Male parent species | Female parent species | |
| i. | Silver carp | big head |
| ii. | Grass carp | big head |
The following different species of Indian and Chinese carp were crossed but the hatchlings did not survive beyond one week:
| Male parent species | Female parent species | |
| i. | Silver carp | rohu |
| ii. | Silver carp | catla |
| iii. | Grass carp | rohu |
| iv. | Grass carp | mrigal |
| v. | Grass carp | catla |
| vi. | Grass carp | rohu |
| vii. | Silver carp | catla |
| viii. | Big head | catla |
| ix. | Big head | rohu |
Japan has produced several varieties of gold fish (Carassius auratus) and common carp (Cyprinus carpio) by long-time selective breeding; and interspecific and intergeneric hybridization of fish has been started recently. Major part of the work on hybridization in fishes has been carried out on the cyprinid and cobitid fishes. Series of experiments on the hybridization of the cyprinid fishes belonging to the sub-family, Gobiobotiini, have been conducted with a view to elucidate whether or not the success of hybrid development is correlated with the supposed degree of phylogenetic relationship based on morphological studies (Suzuki, 1961, 1962, 1963).
The following crosses yielded hybrids:
| Male parent species | Female parent species | |
| i. | Gnathopogon elongatus elongatus | Pseudorasbora parva |
| ii. | - ditto - | Gnathopogon japonicus |
| iii. | Pseudorasbora parva pumila | Biwia zezera |
| iv. | Pseudogobio esocinus | - ditto - |
| v. | - ditto - | Gnathopogon elongatus elongatus |
| vi. | Pseudorasbora parva pumila | - ditto - |
| vii. | Biwia zezera | - ditto - |
| viii. | Carassius carassius auratus | - ditto - |
| ix. | Pseudogobio esocinus | Pseudorasbora parva |
| x. | Biwia zezera | - ditto - |
| xi. | - ditto - | Gnathopogon japonicus |
Of these, reciprocal crosses have been obtained in all, except the last. The majority of the hybrids could be reared until they reached adult stage. The survival rate was usually similar to those of the controls. The hybrids were typically intermediate, not only in morphological characters, but also in behaviour. The few hybrids that reached adult size were found to be sterile males.
Tang (1965) has reported successful hybridization of the Chinese big head and silver carp. Details of morphological characters, growth rate or behaviour are not available.
During 1966, while conducting fish breeding experiments, the author tried to cross the medium-sized carp Labeo pangusia with Labeo rohita and Cyprinus carpio. Stripped eggs from an injected Labeo pangusia female were fertilized with sperms of Labeo rohita. Similarly another lot of eggs were fertilized with common carp sperms. The cleavages and early development of eggs were normal. The embryo was formed without any observable deformity. But the development stopped suddenly at that stage and no eggs hatched out.
In the Malacca Experimental Station some interesting hybridization work was carried out on Tilapia spp. Hickling (1960) experimented with Tilapia mossambica and demonstrated that when an African male was crossed with a Malayan female the offspring were nearly all males.
In Indonesia several coloured varieties and strains of common carp have been produced by selective breeding.
Successful hybridization and viability of fertilized eggs depend on several cytogenetic factorsmost of which could probably be explained by cytogenetic studies. While conducting hybridization experiments on carps, it has been observed that it is easier to hybridize fishes which are closely related to each other, especially within the same sub-family. Indian carps belong to the sub-family Cyprinini and the Chinese carps come under the sub-family Hypophthalmichthyini. Hybridization within the sub-family Cyprinini was highly successful but when tried between the two sub-families, the desired results were not obtained. Suzuki (1962) succeeds in a number of crosses and reciprocal crosses with several genera and species of Cyprinids, all belonging to the sub-family Gobiobotiini.
Viability of fertilized eggs may also depend on the length of incubation of eggs and size difference of the eggs of parent species. The incubation periods of eggs of the Indian major and medium-sized carps are more or less the same. At the same temperature range, the Chinese carps' eggs also hatch out approximately within the same time. Hatching success was noticed in a majority of the crosses involving Indian and Chinese carps.
In our attempts to hybridize common carp with Indian carps it was observed that the eggs, though fertilized, did not hatch when a female C. carpio was used. The reciprocal cross with a female Indian carp was comparatively better since a small percentage of eggs hatched. The common carp eggs take at least 30 hours more for hatching than the Indian carp eggs. This suggests that female selected from the species having shorter incubation period gives better result.
Similar observations had been made by Buss (1956) in his experiments with Salmonidae. The incubation period of rainbow trout eggs is approximately 10 days shorter than the brook trout or the brown trout eggs. Whenever male rainbow trouts were used to fertilize the eggs of another species the hatchability was nil. In the reverse cross, however, a small percentage of eggs hatched out.
In the hybridization experiment with the common carp and Indian carp the big difference in size of eggs also could be responsible for poorer results. Buss (1956) had also found that the size of the egg was a limiting factor. Lake trout embryos are generally larger than the brook trout embryos. So whenever a brook trout female is crossed with a lake trout male the progeny become crippled in the caudal region because of the confinement of a normally larger embryo in the smaller brook trout eggs.
Morphogenetic disturbance is also sometimes responsible for the high incidence of mortality in developing embryos or hatchlings. The reason for the entire mortality of hatchlings obtained in crossing grass carp and mrigal may probably be attributed to this factor.
Chromosome compatibility is another limiting factor for successful species hybridization. The study of chromosomes as well as cytogenetic studies in the Indian carps have not been undertaken. It is of significance that in the breeding of the interspecific and intergeneric hybrids of the Indian carps the percentage of hatching of the F2 generation eggs was much higher than that of the F1 generation.
In the hybridization of Indian carps all the interspecific and most of the intergeneric hybrids obtained were fertile; and F2 generation of hybrids have been produced by crossing them. Rohu-calbasu, an interspecific hybrid and catla-rohu, an intergeneric hybrid, both were bred successfully by hormone injections. Another intergeneric hybrid produced is mrigal-calbasu. Outcrossing by fertilizing its eggs with the milt of three species of carps belonging to three distinct genera were successful.
Experiments in Japan with Cyprinids gave varying results. Suzuki (1961) observed sterility in artificially produced intergeneric hybrids among bitterlings, but he also obtained an intergeneric hybrid of Gnathopogon elongatus elongatus and Pseudorasbora parva, the eggs of which were fertilized yielding F2 progeny. While he found the males of the intergeneric hybrid of Pseudorasbora parva pumila and Gnathopogon elongatus elongatus, producing abundant spermatozoa, the vast majority of his interspecific and intergeneric hybrids were either sterile males or neuter. According to him, though these hybrids males had normally shaped testes, spermatogenesis was impaired. Observations made on the Indian carps showed that the majority of the interspecific and intergeneric hybrids which grew to adult size attained full maturity although ill-formed gonads were present in a few adult hybrids.
Hickling (1966) in his review on fish hybridization has reviewed the work so far carried out on hybridization of warm-water pond fishes and the production of interspecific and intergeneric hybrids.
In fish culture operations very often the problem of over-population arises due to continuous breeding of some of the cultivated species. Since proper manipulation of stock is highly essential for obtaining higher production of fish in a pond, the fish culturists need to find out ways and means to prevent or control reproduction of these fishes. The problem is acute in countries where various species of Tilapia are cultured. Tilapia mossambica reproduces prolifically and soon overpopulates the ponds, thus resulting in poor growth and poorer production of fish. Various methods namely, mono-sex culture by segregating the males, has been tried with initial success but failed to meet the desired results.
Inducing sterility or mono-sex production by cautious hybridization is an efficient method recently practised for culturing the various Tilapia species. Hickling (1960) got all male offspring in Tilapia mossambica from a cross of an African male (T. hornorum zanzibarica) with a Malayan female, which has considerable significance in tilapia culture.
Since the hybrids are fertile the fish culturist should be cautious enough to remove the parent stock from the spawning ponds to prevent their mating with the offspring. Further experiments in this line in hybridizing various Tilapia species have resulted in the production of one hundred male progeny. Such hybrids were produced from the crosses of the following species:
| Male parent species | Female parent species | |
| i. | Tilapia hornorum | Tilapia mossambica |
| ii. | Tilapia hornorum | Tilapia nilotica |
| iii. | Tilapia nilotica | Tilapia hornorum |
The phenomena of andro and gynogenesis could suitably be applied in Tilapia to produce mono-sex offspring for culture.
Hybridization in nature among the Indian carps, though not very common, does occur. On rare occasions stray specimens of carp are met with, the external characters of which do not agree with any of the known species but are intermediate in characters between two species of carps. Such specimens are in all likelihood the hybrids produced in nature. The birbal fish, very often observed in the bundh type of ponds in the Midnapore and Bankura districts of West Bengal, India, where Indian major carps breed naturally during the monsoon months, (Chaudhuri, 1966), is supposed to be a hybrid of Catla catla and Labeo rohita.
The conditions leading to hybridization of fish in nature are not known. In several experiments conducted by the author mature females and males of different species injected with pituitary failed to breed when put together. On a few occasions the female ovulated but the eggs were not fertilized. Artificial fertilization of eggs with milt by stripping is essential for hybridizing two different species of fishes.
In the bundh type of ponds where hundreds of Indian carps belonging to a number of species breed together in a small spawning area, sex-play between different species has not been observed so far. But occurrence of natural hybrids is fairly common in these ponds. The plausible reason for this seems to be the congestion in the spawning grounds, causing accidental fertilization of eggs by the sperms of another species.
Genetic selection and hybridization of the cultivated species of fishes of Asia have to be undertaken with a view to evolve strains which have quicker growth rates, higher fecundity, earlier maturity, better taste and nutritional qualities, more efficient food conversion, and more resistant to unfavourable environmental conditions, diseases and parasitic infections.
Progeny testing should follow every programme of hybridization. This involves the estimation of breeding worth on the basis of the performance of the offsprings.
Since both fertility as well as induced sterility in hybrids is of vital importance in fish culture, more stress should be given to the study of genetic causes of sterility and reduced fertility in hybrids. Prevention or control of reproduction for proper stock manipulation in the cultivated varieties of fishes which readily reproduce in ponds, is no less important than making the difficult-to-spawn fishes spawn, or, improving the stock by genetic selection and hybridization. Since this aspect of research has received very little attention not only in Asia but also in most of the other parts of the world, it is suggested that this particular aspect of fish culture should receive due attention of all fish culturists.
The hazards of indiscriminate hybridization cannot be overstressed. Fish culturists should be very cautious in their selection and hybridization programmes and see that worthless hybrids are not propagated.
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Buss, K., 1956 Results of species hybridization within the family Salmonidae. Progr.Fish-Cult., 18(4):149–58
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