Freshwater Aquaculture Research and Training Centre (CIFRI), Dhauli
P. O. Kausalyagang
The main purpose of fish selection is to improve the existing and develop new breeds and hybrids and thus increase their productivity. For improving the quality of commercially important fish species, the use is made of their variation in many morphological, physiological and biochemical features. A marked proprotion of this variation is hereditary, the level of which is very high in fish population and helps in fish selection work.
In comparison to the breeding of domestic animals fish culture is a young science. Fish culture in China and India has been in vogue since long. But the domestication of fish and creation of breeds differing from their wild parents in higher productivity traits was actually started only a few centuries ago. With the notable exception of gold fish, ornamental carp and perhaps the common carp, few fish could be considered domesticated even though some strains of trout, for example, are much more adapted to hatchery conditions than their wild counterparts. Other species like the Chinese carps, Indian carps, Tilapia sp. and Channel catfish are becoming domesticated.
Of all culturable carps, only common carp has been bred for sufficiently long time and distinct breeds of this species through selection developed are: In USSR - the Ukrainian carp, Ropsha carp, hybrids of the first generation of the domesticated carp and the Amur Wild carp, the Nivchan carp, the Central Russian carp, the Kazakhistan carp, the Kasnodar carp, the Byelorussian breed and the Parra breed of carp; (Kirpichnikov, 1981) in Israel - ‘Dor-70’ carp (Wohlfarth, Lahman & Hulata, 1980) and in Hungary - the Hungarian strain (Bakos, 1979).
The term ‘mass’ of ‘individual’ selection is used to describe artificial selection and use in subsequent reproduction of individuals having the best phenotypes. The traits used in such a selection depend on specific purpose and may include increased weight or body size, good exterior indices, rate of sexual maturation, the necessary pigmentation, the design pattern of scaling, resistance to unfavourable environmental conditions and to diseases and certain improved physiological or biochemical characteristics easily measurable in fishes. Selection may include interior features such as number of intramuscular bones, size of air bladder, etc.
The efficiency of mass selection is expressed by simple equation (Falconer, 1960).
|R||=||i σ h2 = Sh2 (1)|
|R||=||hereditary improvement of character of one generation;|
|S||=||Selective differential (difference between mean for selected individuals and initial population mean);|
|i||=||intensity of selection i.e. selective differential expressed in mean square deviations;|
|h2||=||heritability of character;|
|σ||=||variability of initial population expressed in terms of standard deviation.|
Now let us consider the possibility of increasing each of the terms in the right part of the above equation, while performing the selection of a fish.
The intensity of selection may be quite substantial in those species which are very fertile. In such case we can choose a small part of a grown population for breeding and rest can be rejected. The number of best specimens left for breeding is called the selection severity coefficient (V)
Where N = initial number of individuals
n = number of selected individuals
Selection severity coefficient in very fertile fish like common carp can be easily brought to 1% (1:100) or even 0.1 (1:1000). Occasionally in selecting Ropsha carp in first year, still more rigid norms of rejection, keeping for breeding purpose only 0.05-0.02% of the population (1:2000 to 1:5000) have been followed (Kirpichnikov, 1967). Such an increase in the selection severity requires a very large number of populations to be cultured. The severity and intensity of selection are functionally correlated (Fig. I). At low values of V (0.01 or less) any further decrease in those characteristics hardly has any effect on the value of selection intensity and does not compensate for the expenditure necessary to grow a large number of fishes prior to the time of selection. If the fertility of fish is low, selection with ratio 1:20 (V = 5%) would be permissible. A further reduction in severity is undesirable as it would be followed by a sudden drop of the value i. With the increased size of initial population and therefore enhanced i may be accompanied by reduction in heritability (h2). It has been observed by many investigators that in young stages of common carp, when grown, under condition of high density, the weight distribution curve becomes markedly asymetric; a few ‘shoot fry’ also called ‘jumpers’ or ‘champions’ appear whose rate of growth greatly surpasses that of their companions of the same age; the food competition being the mean reason for this size difference (Wohlfarth, 1977). In presence of food competition, an increase in severity and intensity of selection above a certain limit will have a bad effect.
The variability of initial population (σ) must be sufficiently high; if V is near zero, even with a very high intensity of selection and considerable heritability (about 0.6–0.8) the efficiency of selection may turn out to be low. In mass selection, only genetic variation is important because any increase in the value of non-heriditary variables due to the increase in environmental variation is meaningless because it will result in a proportional decrease in heritability and will not enhance the efficiency of the selection.
Heritability, in broad sense, is the relation between the whole genetic variance and total variance:
Total variance, in turn, expands into a number of components.
Where σ E is environmental variance and σj is variance of interaction between environment and genotype. The σE2 and σj2 should be reduced and this can be done by creating similar homogenous conditions for growing (to begin with parent maintenance conditions) and reducing food competition which frequently enhances the interactional variance.
Heritability can be increased in many ways:
By crossing unrelated individuals, including crossing between lines, breeding stocks, varieties, subspecies and occasionally species;
By the method of unifying fish growing conditions to selection.
In fish culture, the paratypic variance can be greatly reduced by adopting the following measures, the important ones are:
However, inspite of observing all above-mentioned conditions, the environmental variance (σE2) in common carp and in several other fishes proves to be very considerable. A complete elimination of causes that produce unequal conditions of life for individuals living together in ponds is impossible.
The interaction between the genotype and age in fish is not very important but even then it exists. The early phases of life of fish are greatly influenced by maternal effect. The environmental component of total variance of weight is particularly high at the beginning, but decreases thereafter and heritability of weight and size increases by a factor of 2–3. Genetic differences are more detectable at later stage than during the early phases of life. At later stage fish growth is, however, greatly affected by the rate of gonadal maturation, so when selection is aimed at weight, it should be done at the middle age.
Occasionally, heritability decreases because of heterozygous balance, a phenomenon peculiar to many species of animals and plants. Due to great number of chromosomes in common carp (2n = 104) and many other food fish the probability of heterozygous balance being established in a population appears to be great. Hence, it is the individuals with maximum heterozygosis that will be the most viable and fast growing in a population.
Selection under such conditions very soon becomes inefficient, since the genetic variability proves to be in the main non-additive. As to variability of any character, it is the proportion of additive genetic variability in the total variability of the given character.
σA2 = σG2-σD2-σE2 (6)
Without balanced heterogygosis, the variance of dominance and epistasis (σD2 and σ 2) are small and initially the additive genetic variance does not differ much from the total variance (σG2). If there is substantial heterozygous balance for many polygenes, the difference between h12 and h22 will be very great. A true heritability inspite of considerable genetic variability (σG2) will be near zero, since the variability is not additive.
It is difficult to strive against an established heterozygous balance as the equilibrium of genotype proves to be very stable. The only way to correct, the situation is to perform new, sufficiently remote crossings which may destroy the polygene system and thereby contribute to an increase in heritability. Table-1 shows the phenotypic and genetic parameters for quantitative characters in common carp.
Table-1. Phenotypic and genetic parameters for quantitative character in common carp ( = mean, σ standard deviation), CV = Coefficient of Variation, S.E. = Standard error, Heritability (h2) as estimated from sire (S), dam (D) and family (F) Components or variation
|Traits||σ||CV||h2s ± S.E.||h2D± S.E||No of families||Authors|
|Weight of finger lings||-||-||-||0.10± 0.20||Kirpich- nikov (1972)|
|Body weight||0.25||Smisek (1979)|
|Dry matter||0.15± 0.18||-do-|
|Fat content||0.14± 0.15||-do-|
|N in dry matter||0.15± 0.17||-do-|
|4 month wt. (g)||71||17||23||-||0.48 F||9F||Nagy et al|
|Tolerance to hypoxia||179||50||28||0.15 F||9F||-do-|
|Weight gain(g)||366||81||22||0.47 (b)||17 off/parents||Brody et al. (1981)|
From the above table it is evident that weight heritability in common carp for juveniles is rather low and that for body weight of adults is of medium size and higher for young animals. Selection for weight is always possible, but it does not always give the desired results. In the presence of food competition, more aggressive individuals, who are able to snatch food from others in the pond (Moav & Wohlfarth, 1967) may not necessarily be the best at assimilating food. It is evident that for improving the rate of growth mass selection should be combined with testing for relatives.
Mass selection should be replaced by testing for relatives only in cases when such characteristics as fat content, biochemical composition of meat and degree of boniness are to be known, or if the principal task is to raise the fish production of ponds.
The selection for relatives involves to a great extent the selection for genotypes; the positive characteristics of individuals chosen for subsequent reproduction are known from an analysis of their close relatives. There are two forms of selection for relatives: (a) family selection and (b) evaluation of spawners by progeny testing. As in mass selection, the efficiency of selection for relative is expressed by a simple equation.
R = if. σf hf2 (7)
In this equation intensity of selection (if) equals the difference between mean for selected family and population mean expressed inmean square deviations, of characterises the variability of family means and h2 shows heritability of differences between family eans. In family selection the value σf and selection intensity are reduced than these values in mass selection, but the heritability means can reach very high values if the familities are grown together under similar conditions.
Family selection is generally used when hereditary of selected characteristic is low. This requires farming of several families or offsprings from different pairs or small potential groups under optimum conditions. The member of crossings may be 8–10 or when culture facilities exist, it may be as high as 15 or 20.
The selection intensity (σf) depends to a very great extent, on the number of families. The variability of the family means (σf) is not great and can be increased only at the expenditure of enlarging its genetic component. The heritability of means (hf2) is increased and it approaches when all families are kept under standard conditions. The most important conditions for family selection are:
The simplest method involves comparison of offsprings obtained from different pairs or ‘nests’ of parents (Fig. 2a); in this case the evaluation refers to the combination of individuals. In the simplified diallele crosses (Fig. 2b) which are generally practised, males or females are separately crossed with one or more individuals of the opposite sex. The maturing of each of the males tested with two females provides a sufficiently reliable evaluation of the breeding qualities of these males. A complete diallele cross (2×2, 3×3, 5×5, 10×10, etc.) also enables the fish breeder to select the best individuals belonging to other sex, since the number of offspring increases its proportion to be square number of parents of one sex subjected to examination (Fig. 2c), one has to face the problem of growing large number of offsprings under uniform standard conditions. The progeny testing is also time consuming and requires one or two years.
Testing of spawners of either sex of common carp has been done for selection in USSR (Kirpichnikov, 1966) and in Israel (Moav & Wohlfarth, 1967) that about 70% of the differences between progeny means can be attributed to genetic variance. Thus progeny tests provide an effective tool for identifying genetically superior progenies and parental groups. The conditions and requirements for progeny testing are similar to those used in family selection.
Comparision of the equation R = Sh2 and Rf = Sf, h2f allows one to determine which of the selection methods is better for fish breeding, if Sh2 > Sf, h2f then mass selection is to be preferred over family selection or vice versa. Selection for relatives will be more effective for traits with very low heritability i.e. less than 0.5 and when h2 is higher than 0.5, mass selection is more efficient than the family selection. When h2 = 0.5, family and individual selection are of equal efficiency (Falconer, 1960). With fish, selection should be based on a combination of mass and family selection. Mass selection is only of interest when growth rate is the only trait of economic importance and is highly heritable (Gjedrem, 1963). The response of combined selection is theoretically equal to the sum total response of each of the selection methods used.
Rs = Rf + Rm + R pr, where Rf, Rm RPr refer to the effectiveness of family selection, mass selection and progeny selection.
The first step in combined selection consists of crosses between heterogenous unrelated parents, such crosses are aimed at obtaining a small number of progeny upto 10 in common carp breeding. During the cultivation of these families their reproductive properties are evaluated. These properties include viability, growth rate, the quality of flesh, etc. so that best families can be selected. The second stage includes mass selection in several of the best families. At the third stage parents are examined, using progeny testing; Parents of just one sex where the onset of maturity occurs earlier are tested (males in common carp breeding). This testing is to be completed by the time of onset of maturity of individuals of the other sex.
The information regarding the pattern of inheritance of morphological (both quantitative and qualitative), physiological and biochemical traits, particularly those related to yield capacity can be directly utilized in planning selection work. The later information helps in deciding whether to go for mass selection or selection of relatives and is useful in developing a system of crosses. A good example of such an application of the information can be seen in the purification of the brood stock of the Ropsha common carp from the recessive gene.
In the study on homo-and hetorozygosity in the scaled carp in the gene S, (mirror and scattered scale), the back crossing is carried out thus:
|1)||S (S ?)||nn × ssnn|
|2)||S (s ?)||nn × Ssnn|
In the former case, the scaled and scattered offspring of the heterozygous parents are produced in a 1:1 ratio. In the latter case, the ratio is 3:1 (three scaled per one scattered). The entire offspring of homozygous scaled parents in any of the two crossings have a completely scaled integument. From 1956 to 1964, 469 parents of ‘Ropsha’ carp were examined by this technique, 247 were homozygous. This work enabled complete elimination of the occurrence of the scattered carp as early as in the 5th selected generation (Kirpichnikov, 1971).
Use of genes for marking the breeding stocks is another important promising trend and presently genetic markers have been successfully used for labelling different stocks of common carp e.g., selection of Central Russian carp including the breeding of two stocks differing in the S (pattern of scale) and D (pigmentation of spines and head) loci (Kirpichnikov, 1981) and two strains of common carp in Israel marked with colour recessive gene (Moav and Wohlfarth, 1967). Gene markers are helpful in establishing the origin of certain fish breeds. If the correlation between marker genes and selective trait is significant, the selection involving such markers may be considerably accelerated.
Gynogenesis is a special type of sexual reproduction requiring insemination when nucleus of sperm, which has penetrated the ovum, undergoes inactivation in the egg plasm and development of embryo is controlled exclusively by maternal nucleus. The chromosomes of sperms are eliminated soon after fertilization. The technique of artificial gynogenesis is based on inactivation of sperm by irradiation and diploidization of the female chromosome set by a cold shock. This method has been employed successfully on Cyprinus carpio (Golovinskaya, 1968) and in two Indian major carps viz., Catla catla and Labeo rohita (George John et al., 1984). Also the production of monosex broods for population control has been attempted with grass carp.
The great advantage of this technique is that it constitutes a de facto vegetative reproduction that can maintain and multiply a single superior genotype regardless of its level of heterozygosity. Another advantage is that gynogenetically reproducing populations tend to have only the female sex. This provides means of selecting females with higher genetic tendency for gynogenesis.
In fishes natural gynogenesis is intimately associated with the phenomenon of hybridization, apomxis and polyploidy. Triploid amieotic parthenogenesis androgenesis may occur as a result of back crosses of parthenogenetic diploid females with males of related bisexual species. In case of the cross between triploid gynogenetic females with diploid males, tetraploid gynogenetic individuals may develop. Triploids have been produced by cold shock treatment of the fertilized eggs of common carp (Garvai et al., 1980). These fishes are expected to be sterile. The integgeneric hybrids between grass carp and either common carp or bighead carp appear to be largely triploid (Allen and Stanley, 1981). The population of Carassius auratus glibelio in Japan has been reported to contain tetraploids.
The selection response does, to a large extent, depend on the level of heterogenity of the selected groups. Crosses between unrelated individuals enrich the strain increasing the genetic component of their veriation and thereby facilitating selection. Another result is the disappearance of any harmful effect of inbreeding. Crossing also ensures the preservation and perfection of the reproductive qualities of the breed and allows heterosis to be utilized to its utmost in every generation. Crossing helps in improving the breeding quality of the local breed by making use of the few valuable traits of another beed (improver) and in increasing the viability of the breed by introducing genes responsible for the resistance to environmental factors and diseases. In accordance with the above objectives, following the original crossing, the reproduction, of the hybrid, population is carried out by means of the reproductive, introductory, absorptive or alternate crossing.
This is used when many useful traits are to be combined from both cross breeds or species. This is easily achieved when hybrids are completely fertile but require thorough selection in all hybrid generations. Examples of such crosses are: The Ukrainian and Ropsha common carp breeds, the Hungarian carp, etc.
This is used when it is required to introduce one or several valuable traits of another strain or species into the local highly productive breed. F1 hybrids of two forms are then back crossed many times with individuals of the local breed, whose improvements intended. In this process, the back cross hybrids possessing the desired traits of donor strain are used for subsequent reproduction to improve-the breed. If these traits are determined by dominant clearly manifested genes, the problem of conservation of required characters can be solved relatively easily, otherwise in case of recessive genes or when inheritance is polygenic, the risk of loss of characters is very high.
Similar to introductory cross. A series of back crosses is completed after the initial cross of two strains but hybrids are repeatedly crossed with the individuals of strains used for improvement and not of local strain. Strict measures should be adopted aiming at conserving the most useful traits of absorbed strains, in this case the local one.
This requires intermittent crossing of hybrids with the individuals belonging to the two initial breeds as followed by selection of the necessary combination of traits. After 3 or 4 generations, the alternate cross is replaced by reproducing one, otherwise, it is difficult to stabilize the traits of the new hybrid breed.
F1 hybrids if they possess heterosis can be cultured commercially (Moav & Wohlfarth, 1967). Heterosis appears to depend on two main compensatory mechanisms. The combination of useful dominant genes accumulated by both crossed forms in hybrids (hypothesis or dominance) and the increase in hybrids of the total level of heterozygosity (the hypothesis of overdominance). An increase in the biochemical versality in hybrids occurs in both cases. Heterosis in natural population is generally manifested as a rise in the fitness of hybrids in the elevation of the adaptive value. This is typical of many intraspecific and certain interspecific fish crosses. Maximal care is required in carrying out commercial hybridization, because if F1 hybrids are left in the water bodies, there are chances of contamination of the brood stock of the initial forms. This leads to the deterioration of the economically important strains. An example of such hybridization can be cited in case of domestic common carp, with its living ancestor, the wild carp. Better control of commercial hybridization can be considered by the use of genetic markers, both parents and hybrids may differ in the alleles of the genes responsible for the colour pattern, scale pattern and certain biochemical loci. Some promising important hybrid combinations are: (1) Interbreed hybrids of common carp crosses between ‘Ropsha’ and 'Ukrainian carps, of the scaled and formed, Ukrainian carps, three stocks of the Krasnodar carp. Heterosis manifested in better survival and productivity has been observed in the crosses of Hungarian and Polish carps, and crosses of the Japanese Yamato carp with European mirror carp. (2) Intraspecific hybrids of silver carp of Amur and Chinese origin (3) Hybrids of domestic carp and Amur wild carp (Cyprinus carpio haemalopterus) (4) Intergeneric hybrids of common carp and crucian carp (5) Intergeneric hybrids of silver carp and bighead (6) Intergeneric hybrids of Catla catla and Labeo rohita (Chaudhuri, 1971) and many others species as shown in Table 2
In order to utilize completely the advantages associated with heterogenous crossings, fish breeding should be carried out according to a definite plan, depending on the knowledge of genetics, inbred depression and heterosis in fish crossing. In making crosses, induced breeding of the species followed by artificial fecundation is employed. Also for carrying out genetic experimentation on large scale, an adequate method of marking fish for individual or group observations is to be developed. The best results have been obtained in marking experimental fish by fin clipping or brand marking (Moav et al., 1960) or subcutaneous injections of organic dyes (dichlorotriazine and other compounds) (Zonova and Kirpichnikov, 1971) or by injecting blue, red or yellow flurescent granules into the spine (Smitherman et al., 1983). Under Indian conditions, common carp whon injected with Procian-M blue stain by the author has retained marks for last ten months and still the marks are quite distinct.
The various fish breeding systems followed for the genetic improvement of common carp in different countries are described as follows:
USSR: The method of parallel breeding of two or more groups is used for the selection of Ropsha carp (Kirpichnikov, 1971 1971). In this system two or three groups are concurrently chosen within a breed, without intermingling, allowing inside each a moderate inbreeding and carrying out selection in each generation. For commercial purposes fish from different groups are crossed to avoid close inbreeding (Fig. 4).
(a) Breeding in groups with family selection:
Each year about 20 pairs of common carp are spawned and progeny tested. The tested pairs may include these that excelled in previous years. They may also include a group of full sibs of a superior progeny of the previous year mass spawned with an unrelated male. The faster growing individuals of the best progenies are selected to serve as parents of cross-breed fry. The remaining progenies are cullod. The same process is repeated with new pairs each year (Moav & Wohlfarth, 1967).
As the number of tested combination increases, the possibility of finding a new, best combination becomes small and further improvement requires selection for combining ability within the best parental line. This requirement is met in the recurrent selection (RS) and reciprocal recurrent selection (RRS). The basic feature of the above selection procedures that differentiates them from other selection procedure is that the pure bred parents are selected on the basis of the performance of their cross-bred half-sibs rather than on the basis of their own performance (Fig. 6).
(b) Open gene pool system:
RRS method or any other based on family selection or progeny testing is that it increases, inbreeding in the parent lines. To avoid this, the use of open gene pool system may be made. This system involves the maintenance of genetically marked reserve gene pools for each one of the two parental lines. Each cycle of progeny testing may include a small proportion of the individuals from the reserve gene pools.
If the progeny testing results show that one or more of the gene pool immigrants produced superior cross-bred progeny, pure-bred progeny may be incorporated into the pure-bred parental line. Selection should aim at an increase in combining capacity (Moav & Wohlfarth, 1967). This method has also been followed and extensively used by Soviet fish breeders (Golovinskaya, 1971).
The Hungarian land races of common carp were used for genetic crossing. In each cross only one male and one female were used. The carp spawners were identified by burns or brands, boaring line and individual marks. The progeny populations, were differentiated by collective marks, each group receiving a branded strip 2 cm long on different parts of their body. In planning crossing combination the following aspects were taken into consideration (Fig. 5).
The productive capacity of carp hybrids was established by the evaluation of egg fertility, percentage of survival in the first and second years of life, increase in weight during the second year, food conversion ability, percentage of consumable flesh and fat content of flesh (Bakos, 1979).
The efficiency of mass selection can be enhanced by increasing to the possible extent, the value of three factors: intensity of selection, variability and heritability of the trait to be selected. Of these, increase in heritability is the most important, which can be done by means of crossing, creation of uniform conditions for fish rearing and carrying out selection at the age to be improved.
Family selection and progeny testing in carp breeding prove to be more efficient than mass selection only when the heritability of the character (h2 0.1–0.15). The main difficulty in performing selection for relatives is the requirement of large number of ponds for growing a sufficiently large number of families under identical conditions.
Much benefit may be expected from combined selection in the course of which, mass selection, progeny testing of females, and family selection are performed successively in one generation.
New trends in fish selection such as use of genetic markers, artificial gynogenesis and polyploidy have been described. In Indian major carps, viz. Catla catla and Labeo rohita, the gynogenesis has been successfully carried out for the first time.
Methods of increasing the heterogenity of the selection material and breeding systems, as practised in USSR, Israel and Hungary, have been described.
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Table-2: Showing important hybrids of Indian major carps and Chinese carps produced in India
|Female parent species||Male parent species||Hybrid||Important characteristics||Authority|
|Labeo calbasu||Labeo rohita||Calbasu-rohu||Attained full maturity in 2 years F2 generation produced. Varying characters intermediate between calbasu and rohu.||Chaudhuri (1959 & 1971)|
|L. rohita||L. calbasu||rohu-calbasu||In both hybrids over 94% fertilisation was obtained, their growth rate was far superior to the parent Labeo calbasu.||-Do-|
|Cirrhinus mrigala||L. rohita||mrigal-rohu||Relatively small head, deeper body slender caudal peduncle. D 2/13-14; P 18; V9; A 2/6; C19, L.l. 46–47; ||Naseam Hamza (1971)|
|Labeo rohita||C. mrigala||rohu-mrigal||Intermediate body characters as compared to those of parents. Attains maturity in 2 years.||Chaudhuri (1959 & 1971)|
|Labeo rohita||C. mrigala||rohu-catla||Body characteristics intermediate to parent species. Colouration like catla, small head, mouth terminal, fin rays resembling those of rohu. Broad body of catla, small head of rohu and more flesh (54%) than either of the parents. viz., rohu (48%) & catla (45%). Faster growth than rohu. Full maturity attained in 3 years. Fecundity less than both parental species. F2 generation produced. Slightly better growth rate than the hybrid catla x rohu.||Bhowmick et al (1981)|
Reddy and Verhese (1980).
|Catla catla||Labeo rohita||Catla-rohu||Greater body girth and faster growth than rohu, smaller head than catla and more flesh than both the parents. Primarily plankton feeder but accept artificial feed. Growth slightly faster than rohu but slower than catla. F1 hybrids matured in 2 years. F2 generation produced. Reddy and Venghese (1980) reported growth of hybrid much slower than catla and slightly lower than that of rohu.||Chaudhuri (1971)|
Reddy & Verghese (1980)
|Catla catla||L. calbasu||Catla-calbasu||Faster growth rate than calbasu, smaller head than catla and body girth more than calbasu.||Chaudhuri (1971)|
|L. calbasu||C. catla||Calbasu-catla||Faster growth than calbasu.||-do-|
|C. catla||L. fimbriatus||Catla-fimbriatus||Faster growth than fimbriatus Head smaller than catla and body girth nearer to catla.||-do-|
|C. idella||Hypophthalmichthys molitrix||grass carp-silver carp||Resembled grass carp, irregular large-sized scales; at some places along the lateral line; in some cases active larvicidal and weedicidal tendencies in fry stage. Males matured in 2 years and females in 3 years.||Barrackpore (1980)|
|H. molitrix||C. idella||grass carp-silver carp||Round body of grass carp with smaller scales. Mouth with lower lip protruding as in silver carp. Gill rakers numerous but not fused as in silver carp. An average size of 366 mm/475g in one year. Did not mature. Non-acceptance of weeds.||Singh & Chakraborti (1970)|
|Labeo rohita||Cyprinus carpio||rohu-common carp||Elongated dorsal fin like parent species, but other characteristics intermediate between parent species, Sterile. Survived for several months.||Alikunhi and Chaudhuri (1959)|
|C. mrigala||C. carpio||mrigal-common carp||Survival for several months||Chaudhuri (1971)|
|Catla catla||H. molitrix||catla-silver carp||Body girth comparable to catla, size of head and scales nearer to silver carp and colouration slate grey on dorsal side and silvery on to abdomen||Ibrahim et al (1980)|
|H. molitrix||Catla catla||silver carp-catla||A single specimen, survived for 21 months and attained 497 mm/1.45 kg. Showed roughness of pectoral fin.||Barrackpore (1979)|
|Cyprinus carnio||L. rohita||common carp-rohu||Body characters intermediate between both parent species. Body profile nearer to rohu.||Khan et al (1984) unpublished.|
Fig. 1 Relation between selection severity and Selective intensity (
Fig 2 Progeny testing in fish breeding
Fig 3 Types of Crossings
Fig. 4. Scheme of creating breeding stock of Rapsha hybrid carp
(kirpichnikov, 1967) CC- cultured mirror carp, AWC
Fig 5. Scheme of production of Common carp hybrids showing the process of breeding (Bakos 1979)
Fig. 6. Schematic presentation of a combination of