Abstract: Inter-specific hybrids have been produced to increase growth rate, improve productivity through hybrid vigor, transfer desirable traits, reduce unwanted reproduction through production of sterile fish, combine other valuable traits such as good flesh quality, disease resistance and increase environmental tolerances, better food conversion, take advantages of sexual dimorphism and increase harvesting rate in culture systems. Hybrids play a significant role for increase in aquaculture production of several species of freshwater and marine fishes; for example, hybrid catfish in Thailand, hybrid stripped bass in the USA, hybrid tilapia in Israel and hybrid characids in Venezuela. Despite its’ wide-spread use in aquaculture, there have been an impression that hybrids do not hold much attraction for aquaculturist. With the expansion of aquaculture sector and the increased number of species being bred and farmed, there are hybrids that now account for a substantial proportion of national aquaculture production and other hybrids may be emerging through further development. As the domestication of fish species increases, the possibilities to increase production through appropriate hybridization techniques is ongoing with a view to produce new hybrid fishes, especially in culture systems where sterile fish may be preferred because of the concern that fish may escape into the open freshwater, marine and coastal environment. Chromosome-set manipulation (polyploidization) has been combined with hybridization to increase the viability and to improve developmental stability of hybrid fishes. Intentional or accidental hybridization can lead to unexpected and undesirable results in hybrid progeny, such as reduced viability and growth performances, loss of color pattern and flesh quality and also raises risks to maintenance of genetic integrity of species if the hybrids escape to the natural habitat and undergo backcrosses with the parental species. The success of inter-specific hybridization can be variable and depend on the genetic structure, crossing patterns, gamete compatibility and gene flow patterns of the parental species. Appropriate knowledge on the genetic constitution of the broodstock, proper broodstock management and monitoring of the viability and fertility of the progeny of brood fishes is thus very crucial before initiating hybridization experiments. In addition, some non-generic factors such as weather conditions, culture systems, seasons and stresses associated with selecting, collecting, handling, breeding and rearing of broodstock and progeny may greatly influence hybridization success in a wide variety of freshwater and marine fin fishes.
INTRODUCTION
The mating or crossing of two different species is a process called hybridization with the offspring known as hybrids. Hybrids can have some characteristics of both parental species. A hybrid with selected or favored characteristics of each parent is one of the goals of animal husbandry. When a hybrid has characteristics superior to both parents, it is said to have hybrid vigor or positive heterosis, which of course, the ultimate breeding goal. Hybridization occurs widely in fishes under natural conditions (Hubbs, 1955; Schwartz, 1972, 1981) and is observed in fish more commonly than in other vertebrate animal groups (Campton, 1987; Allendorf and Waples, 1996). Several factors have been suggested as contributing to the high incidence of natural hybridization among closely related fish species, including external fertilization, weak behavioral isolating mechanisms, unequal abundance of the two parental species, competition for limited spawning habitat and decreasing habitat complexity (Campton, 1987; Hubbs, 1955). Hybrid zones are defined as areas of narrow regions where genetically distinct populations or species meet, mate and produce hybrids (Barton and Hewitt, 1985). Intra specific hybridization (crossbreeding) is a classical approach for the genetic improvement of livestock animals (Faruque et al., 2002; Das et al., 2003; Mohammed et al., 2005; Askari-Hemmat, 2006; Musa et al., 2006; Mekky et al., 2008; Mirzaei et al., 2009; Ajayi, 2010; Adebambo et al., 2010; Bekele et al., 2010; Razuki and Al-Shaheen, 2011; Farahvash et al., 2011). This method also has shown its potential in aquaculture. Increment of 55 and 22% in growth rate of channel catfish and rainbow trout crossbreeds, respectively, were achieved using this technique (Dunham and Smitherman, 1983; Dunham, 1996a). However no increase in growth rates in Chum salmon crossbreeds when compared with parental strains (Dunham, 1996b). Cross-breeds of different strains of European catfish, Silurus glanis, exhibited higher adaptability under warm water conditions and mixed diet feeding regimes (Bartley et al., 2001). Gjerde and Refstie (1984) investigated the heterosis effect between crosses of five Norwegian strains of Atlantic salmon. They did not find a significant heterosis effect for either growth rate or survival rate. Similarly, Friars et al. (1979) found no heterosis effect for growth rate of Atlantic salmon fry.
Inter-specific hybrids thus, have attracted attention because they can improve productivity through hybrid vigor, transfer desirable traits or produce sterile animals (Chevassus, 1983; Hedgecock, 1987; Longwell, 1987; Menzel, 1987; Rahman et al., 2000, 2005). Hybridization may also be used to combine other valuable traits such as better growth and flesh quality, disease resistance and increase environmental tolerances. In recent years, hybrids of major carps are being successfully produced in public and private hatcheries and are available for farming due to high resistance against unfavorable ecological conditions (Reddy, 2000; Um-E-Kalsoom et al., 2009). Many molecular biologists and fish geneticists realized that the use of inter-specific hybrids in global fisheries production is not well-reported and examined properly. On the other hand, there have still been some controversies in global acceptance for using inter-specific hybrid organisms that have been genetically modified (UNEP, 1994; Bartley and Hallerman, 1995; Hallerman and Kapuscinsky, 1995). This paper focuses on the crossing among different genetically distinct species and rearing of hybrids to understand the potentiality of hybrids in the world’s aquaculture production.
USE OF INTER-SPECIFIC HYBRIDS IN AQUACULTURE PRODUCTION
Inter-specific hybridization has long been practiced in various species of fishes to increase growth rate, improve flesh quality, produce sterile animals, increase disease resistance and environmental tolerance and to improve other quality traits to make fish more profitable (Bartley et al., 2001). Majority of the earlier works on hybridization was conducted for salmon fishes, but these species did not produce hybrids of commercial importance. For this reason, hybrids in these fishes do not draw attention to fish culturists. Due to the increased expansion of fish farming throughout the world, hybrids produced from inter-specific crosses play substantial role for global aquaculture production. The increased use of artificial breeding and in vitro fertilization techniques and increased knowledge of reproductive biology encourage the aquaculturists to produce hybrids in a view to improve the quality traits over their pure parental siblings. Evaluation of some of the important traits and performances that have been improved through hybridization among different species of fishes are described below.
Improved growth performances: Increased growth rate is the most desirable trait for stock improvement in aquaculture. Growth increase may result from dominant variance (Tave, 1986) or from increasing the number of polymorphic loci in an individual. Increased heterozygosity has been implicated in improved growth and other desirable characters in a variety of species such as developmental compatibility (Leary et al., 1983) food conversion efficiency and oxygen metabolism (Danzmann et al., 1985; Koehn and Gaffney, 1984). A hybrid between white bass (Morone chrysops) and the stripped bass (M. saxatilis) is called sunshine bass, exhibits faster growth and has many good culture characteristics than either parents under captive culture system (Smith, 1988). Crosses of the black crappiexwhite crappie (Pomoxis nigromaculatus xP. annularis) stocked in small ponds and impoundments (Hooe et al., 1994), silver carpxbighead carp (Hypophthalmichthys molitrixxAristichthys nobilis (Krasnai, 1987) in polyculture systems and catfish hybrids between the African catfish (Clarias gariepinus) and the Vundu (Heteropneustes longifilis or H. bidorsalis) in intensive concrete tanks (Salami et al., 1993; Nwadukwe, 1995) were reported to grow faster (positive heterosis) than conspecific parents. Improved growth performances were also obtained from crosses of mrigal (Cirrhinus mrigala) and catla (Catla catla) and common carp (Cyprinus carpio) with rohu (Labeo rohita) in pond culture system in India (Khan et al., 1990). Intergeneric hybrids between catla (Catla catla) and fimbriatus (Labeo fimbriatus) were observed to be combine desirable qualities such as the small head of the fimbriatus and the deep body of the catla and exhibited heterosis in terms of meat yield with higher flesh content than either of the parents (Basavaraju et al., 1995). Hybrids between tambaqui (Colossoma macropomum) and pacu (Piaractus brachypomus) in Brazil and Venezuela raceways and ponds grew faster than either parent (Senhorini et al., 1988). Crosses of the green sunfish (Lepomis cyanellus) with bluegill (L. macrochirus) (Tidwell et al., 1992; Will et al., 1994) and crosses of the gilthead sea bream (Sparus aurata) with red sea bream (Pagrus major) also had positive heterosis in growth and other culture characteristics (Hulata, 1995). Several hybrids have been produced in the Mediterranean with the cross between red sea bream and common dentex (Dentex dentex), being especially fast growing in cage culture management (Colombo et al., 1998).
Production of sterile animals: Hybridization often results in offspring that are either sterile or reduced reproductive capacity. Production of sterile animals may be advantageous to diminish unwanted reproduction or to improve growth rate and avoiding energy loss due to prolific breeding. Examination of species karyotype is a good general indication of whether or not hybridization will result in offspring that are sterile. Natural hybrids produced from the cross between grass carp (Ctenopharyngodon idella) and bighead carp (Aristichthys nobilis) are functionally triploids, generally sterile, but with a small proportion being diploid and fertile (Allen Jr. and Wattendorf, 1987). Hybrids between Indian major carps are generally fertile because of similar chromosome numbers (2n = 50). Indian major carps crossed with common carp (4n = 102) results in hybrids that are sterile because they are functionally triploid (Khan et al., 1990; Reddy, 2000). However, crosses of some sturgeon species with different chromosome numbers, as well as most tilapia crosses, produce fertile offspring (Steffens et al., 1990; Earnst et al., 1991; Head et al., 1994; Wohlfarth, 1994). The cross between the black crappie (Pomoxis nigromaculatus) and white crappie (P. annularis) exhibits positive heterosis and is often recommended for stocking in small impoundments because of reduced fertility of the F2 generation that would prevent overpopulation (Hooe et al., 1994). The sunshine bass is generally sterile, but apparently an undetermined percentage of these hybrids are capable of reproduction as evidence of hybrid mating and backcrossing (Avise and van den Avyle, 1984). The red sea breamxgilthead sea bream cross also produced sterile hybrids and this may be an important quality in marine aquaculture due to improved growth rate and good overall performance in cage culture (Hulata, 1995). The tiger trout, a hybrid between brown trout (Salmo trutta) and brook trout (Salvelinus fontinalis) is sterile with poor early survival, but good growth rate and therefore is useful for stocking areas where reproduction is to be very limited (Scheerer and Thorgaard, 1983).
Manipulation of sex-ratio: Production of mono-sex populations in fish is often preferable for aquaculture development. This preference may be due to growth differences between sexes, e.g., male tilapia grow faster than females, whereas female salmonids and sparids grow better than males. A specific sex chromosome may produce a valuable product and monosex populations help reduce unwanted reproduction. Hybridization between some species of tilapias such as Nile tilapia (Oreochromis niloticus) and the blue tilapia (O. aureus) results in the production of predominantly male offspring and reduces unwanted reproduction in grow-out pond culture management (Rosenstein and Hulata, 1994). This cross produces predominantly males because of different sex-determining mechanisms in the two species and the hybrid males had superior growth over pure parental species. Nile tilapia has the XX, XY system with the male being heterogametic, whereas blue tilapia has the ZZ, ZW with the heterogametic genotype being female (Lahav and Lahav, 1990; Wohlfarth, 1994; Verdegem et al., 1997). Similarly, crosses between Nile tilapia (O. niloticus) and Wami tilapia (O. honorum), Nile tilapia and long-finned tilapia (O. macrochir) and Mozambique tilapia (O. mossambicus) and Wami tilapia produce hybrid offspring that are predominantly male with excellent growth and production (Wohlfarth, 1994). The hybrid between striped bass (Morone saxatilis) and yellow bass (M. mississippiensis) produced 100% females with excellent survival and growth in culture system (Wolters and de May, 1996).
Overall improvement: The principal aim of hybridization is to combine desirable traits from different species to increase the overall production or marketability of a cultured species. The major hybrid catfish cultured in Thailand is a cross between African (Clarias gariepinus) and Thai (C. macrocephalus) catfish, which combines fast growth rate of the African catfish with the desirable flesh characters of the Thai catfish (Nwadukwe, 1995). The overall product is improved and the flesh is still acceptable to Thai consumers, although it does not grow as fast as the pure African catfish. The rohuxcatla hybrid grows almost as fast as pure catla, but has the small head of the rohu and is therefore useful in Indian aquaculture (Reddy, 2000). Catlaxfringed-lipped peninsula carp (Labeo fimbriatus) were reported to have small heads of the fringed-lipped peninsula carp and deep body and nearly equal growth rate to the catla; dressing percentage also improved in this hybrid (Basavaraju et al., 1995). The sunshine bass hybrid (white bassxstripped bass) has a suite of advantageous traits including good osmoregulation, high thermal tolerance, resistance to stress and disease, high survival in culture and modified water-bodies and ability to utilize soy beans as a protein source (Smith, 1988; Colombo et al., 1998). The overall growth performances of hybrids (C. catlaxL. rohita) fed on wheat bran was consistently higher followed by rice broken and blood meal (Um-E-Kalsoom et al., 2009).
Among the cultivatable hybrids, red tilapia is more desirable than darker skinned tilapia in Cuba, Venezuela, Thailand, Europe and the United States of America. Most red tilapia are descended from the Nilexblue tilapia cross (Verdegem et al., 1997), but red tilapia also result from the cross of Wami tilapia (O. urolepis hornorum) with Mozambique tilapia (Earnst et al., 1991). It has been reported that red tilapia from Nile tilapiaxMozambique tilapia and Nile tilapiaxWami tilapia are being farmed in central Thailand to Lao PDR for aquaculture purposes (Welcomme, pers. Comm.). The latter cross is also salt tolerant and used for coastal aquaculture in parts of South East Asia (Bhikajee, 1997). Stability of the skin coloration is often a problem in successive generations and studies have been undertaken to understand the genetic mechanisms of color inheritance (Koren et al., 1994; Hussain, 1994).
Hybrids between different species of North American catfish have been researched for more than 30 years. Among the interspecific catfish hybrids, crosses between channel catfish (Ictalurus punctatus) and blue catfish (I. furcatus) exhibits good culture characters of the channel catfish with the ease of harvesting characters of the blue catfish such as better angling and increased seinability (Dunham and Argue, 1998). Once breeding problems are worked out, these hybrids may be useful in culture as they show heterosis for growth rate and are superior to channel catfish in low oxygen tolerance, disease resistance uniformity in body shape, angling vulnerability, seinability and dress-out percentage (Dunham and Argue, 1998). Hybrid produced from the crosses between the muskellunge (Esox masquinongy) and the pike (E. luscious) is sterile and well-adapted to intensive culture systems. However, the hybrid has the similar sport fish characteristics of the pure parental muskellunge, but higher protein requirements than both parental species (Brecka et al., 1995).
Disease resistance and environmental tolerances: Hybridization may be used to improve disease resistance by breeding a higher resistant species with a less resistant one. Dorson et al. (1991) reported that crosses of coho salmon (Oncorhynchus kisutch) with other species, e.g., rainbow trout had increased disease resistance to a variety of salmonid viruses, but other culture characteristics were poor. Viability was increased when hybridization was followed with triploidization and Dorson et al. (1991) stated that the rainbow trout (O. mykiss)xchar (Salvelinus spp.) triploid hybrids had increased resistance to several pathogenic salmonid viruses and early sea water tolerance.
Hybrids may have increased environmental tolerances when one parental species has a wide range of tolerance (e.g., euryhaline species), a specific tolerance (cold tolerance species), or because of increased heterozygosity sometimes being associated with a broad niche (Nelson and Hedgecock, 1980; Noy et al., 1980). Mozambique tilapia and Wami tilapia can reproduce in saline waters; however Nile tilapia has improved culture performance in many aquaculture systems. Hybridization between Mozambique and Nile tilapias yields a red tilapia with salinity tolerance (Lim et al., 1993). Hybrids between Mozambique and Wami tilapia, called the Florida red strains, have high growth rates and can reproduce in salinities of 19 ppt (Earnst et al., 1991). Crosses between Nile tilapia and blue tilapia also resulted in progeny with good salinity tolerances (Lahav and Lahav, 1990; Wohlfarth, 1994). Hybrids also may be used to exploit degraded aquatic environments. Lakes affected by acid rain may not be suitable for native salmonids, but splake, a hybrid between lake trout (Salvelinus namaycush) and brook trout (S. fontinalis) can tolerate reduced pH levels of 4.9-5.4 of acid lakes of Ontario; lake trout reproduce successfully only in waters with pH values above 5.5 (Snucins, 1993). The splake also was shown to have higher survival and growth than both brook and lake trout in lakes with pH in the range of 5.5-7.2 (Inssen et al., 1982).
Hybrid polyploidization: Hybridization combined with chromosome manipulation may increase the viability and developmental stability of hybrid fishes during early life history stages (Wilkins et al., 1995). Polyploid hybrid salmon appear to be better suited for a variety of culture situations than either polyploid or hybrid salmon are on their own. Although many diploid salmonid hybrids are not used for culture, triploidization of the hybrids may confer increased viability, growth and survival (Grey et al., 1993). Triploidization of Atlantic salmon (Salmo salar)xbrown trout (S. trutta) hybrids increased survival and growth rate to a level comparable to Atlantic salmon (Galbreath and Thorgaard, 1995). General disease resistance was improved by triploidizing the cross between rainbow trout and char; rainbow trout and coho salmon triploid hybrids had increased resistance to infectious disease, but the latter hybrids grew more slowly (Dorson et al., 1991). Triploid Pacific salmon hybrids between chum salmon (Oncorhynchus keta) and chinook salmon (O. tshawytscha) have earlier seawater acclimatization times (Seeb et al., 1993).
Experimental hybridization: Laboratory hybridization experiments have been utilized extensively to confirm the probable hybrid nature of certain individuals by demonstrating that two taxa will interbreed when provided with the opportunity to do so, or that gametes from two taxa can be artificially cross-fertilized. Hybrids produced from appropriate cross-fertilization techniques among commercially important fish species have been tested for their growth performance, viability and fertility. Hybrid recently produced experimentally between Sheim (Acanthopagrus latus) and sobiaty (Sparidentex hasta) in Kuwait appears to have good growth, flesh quality and is fertile (Khaled Al-Abdul-Elah Kuwait Institute of Scientific Research, pers. comm.). Hybrids resulting from crossing several sunfish species have been used for the past three decades to improve farm pond fishing. The most desirable hybrids result from crossing the female green sunfish (Lepomis cyanellus) with males from one of three other species. These include the bluegill (L. macrochirus), the redear, or shellcracker (L. microlophus) and the warmouth, or goggleye (L. gulosus). The most commonly used hybrid in the Southeast is the male bluegill (BG)xfemale green sunfish (GS) cross. This BGxGS hybrid has the most desirable set of characteristics which means that the hybrids can outperform their parental species in one or more ways. Rapid and superior growth is one way hybrid sunfish exhibit hybrid vigor. Experimental hybrids between dusky grouper (E. marginatus) and the white grouper (E. aeneus) were evaluated, but all the hybrids died within 10-day post-hatching (Glamuzina et al., 1999). The camouflage grouper (Epinephelus polyphekadion) is more resistant to environmental stress and disease than the marbled grouper (E. fuscoguttatus). Experimental hybrids (marbled grouperxcamouflage grouper) exhibited faster growth performances and increased conversion efficiency (James et al., 1999). Hybrid between the Beluga (Huso huso) and Russian sturgeons (Acipenser gueldenstaedtii) was evaluated and appeared to have a wide salinity tolerance to both fresh and seawater as well as good growth rate (Gorshkova et al., 1996) and these hybrids now are being considered for culture in Russia and Iran (Shilat, Iranian Fisheries Company, pers. Comm.).
Two loach (Misgurnus spp.) are cultured both for food and for ceremonial purposes by Buddhists in Korea (Kim et al., 1994): the mud loach (M. mizolepis) and the cyprinid loach (M. anguillicaudatus). The mud loach grows to a larger size, has a faster growth rate and is more resistant to diseases, while the cyprinid loach has a more desirable body color. These two species of loach was hybridized to combine the fast growth and large size of the mud loach (Misgurnus mizolepis) with the desirable body color of the cyprinid loach (M. anguillicaudatus). Fertilization, hatching, survival and karyology of the hybrids were very similar to the parents (Kim et al., 1995). These hybrids are now being culturing commercially and continued studies are planned to combine other desirable characteristics of the hybrids and their fertility.
Hybrids produced using the eggs of Asian catfish (Clarias batrachus) and African catfish (C. gariepinus) performs as well as either parental control during the alevin stage and better in fry and advanced fry stage, while the reciprocal hybrids are inferior in all performance traits. During the different experiments, this hybrid group showed highest survival from post-larval stage to market size fish (Rahman et al., 1995; Khan et al., 2000). Growth performance was always better than maternal control and in some cases better than or close to paternal control. Preliminary observations of organoleptic testing revealed that the hybrid showed superior taste performance compared to parental groups (Rahman et al., 1995). Further researches are needed to examine other desirable traits of the hybrids and their sterility.
Hybridization between giant catfish (Pangasiodon gigas) and giant pangas (Pangasius sanitwongsei) are now been practicing in Thailand (Pongthana, National Aquaculture Genetics Research Institute, Thailand, pers. Comm.). Both of these catfishes are extraordinarily large, reaching 3 m and 300 kg and the giant catfish is considered as an endangered species whose trade is restricted under the Convention on International Trade in Endangered Species of Wild Flora and Fauna. Hybrids between these two catfish species shows good growth performances and should be used to reduce pressure on the giant catfish so as not to endanger it through excessive catch of brood fish from the wild or through genetic introgression of the two parental species.
Due to the wide geographical distribution of yellow bass (Morone mississippiensis), hybridization tests with stripped bass and comparisons with the sunshine bass have been conducted. The yellow bass hybrid exhibited 65% survival to harvest as compared to 45% for the sunshine bass, but poorer growth rate and condition factor when raised in tanks continuously supplied with pond water (Wolters and de May, 1996). Further research have been undertaken to explore the possibility of combining other desirable traits in the above hybrid progeny.
Unplanned/accidental hybridization: Unplanned and accidental hybridization in hatchery stocks may cause a genetic deterioration into the aquaculture production and open water fisheries. During the production of Indian major carp seeds, different species often are induced to spawn in a common spawning tank thus providing the opportunity for unintentional hybridization (Padhi and Mandal, 1997). Silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) sometimes are hybridized inadvertently because of their similar appearance and because of the shortage of “the correct” species at spawning time due to differences in maturation times between male and female. This hybridization often results in a fish that does not feed efficiently as its gill rakers are intermediate in shape between those of the silver carp that eats phytoplankton and those of the bighead carp that consumes zooplankton. There is much anecdotal evidence of genetic deterioration of carp hatchery stocks in Bangladesh, through inbreeding, negative selection and hybridization (Hussain and Mazid, 2001). Stocks of exotic (i.e., non-indigenous) carps are particularly vulnerable to such degradation, given that the opportunities to go back to wild populations for broodstock replenishment are very limited. Furthermore, anecdotal evidence suggests that hybridization between silver carp and bighead carp is common, at least partly due to a shortage of mature bighead carp males towards the end of the breeding season. Reported aquaculture production of the silver carp in Bangladesh in 2001 was 130,000 t, or 21.7% of freshwater aquaculture production (FAO, 2003), while there was no reported production figure for bighead carp. Bighead carp broodstock are present in many hatcheries, so presumably aquaculture production of bighead carp is present, but not high enough to be reported separately.
Hybridization between silver carp has also been reported to occur fairly frequently in commercial aquaculture hatcheries in Bangladesh. The consequences of hybridization for broodstock purity had recently been investigated. Allelic variation at three microsatellite DNA loci isolated from silver carp routinely distinguished between silver carp and bighead carp. These markers were used in the analysis of samples collected from hatcheries in different regions of Bangladesh. Of 422 hatchery broodstock that were morphologically identified as silver carp, 8.3% had bighead allele(s) at one or more of the three microsatellite loci, while 23.3% of the 236 fish morphologically identified as bighead carp had silver carp allele(s) at one or more loci. The results suggested that while some of these fish might be F1 hybrids, others had more complex genotypes, suggesting further generations of hybridization or introgression between the species in hatcheries, with potentially damaging consequences for the integrity of these stocks and their performance in aquaculture (Mia et al., 2005).
Interspecific hybridization in some carp species has recently been reported in Bangladesh (Hussain and Mazid, 2001). Either out of scientific interest or shortage of adequate hatchery populations (i.e., brood stock), introgressed hybrids are being produced intentionally or unintentionally by private hatchery operators and sold to hatchery and nursery owners. These hybrids are being ultimately stocked knowingly or unknowingly, either in grow-out ponds or in open water bodies like floodplains under the government’s massive carp seed stocking program. There is widespread concern that mass stocking of such hybrids in the floodplains and other related open water might cause a serious genetic introgression problem that could adversely affect aquaculture and inland open water fish production. There is every possibility of segregation of genes with the result that some of the fish carrying the introgressed genes could not be easily distinguished from the pure species (Hussain and Mazid, 2001). Hybrid introgression in major carp species is very likely to have negative consequences as a result of loss of distinct feeding strategies of the pure species, which are the basis of successful polyculture systems (Mair, 1999). If the introgressed hybrids reproduce in natural water bodies or are used as broodstock in hatcheries, they will not be true breeders; therefore, collection of carp seed from the pure species/strains will be difficult.
Hybridization with wild fish is especially prevalent in tilapia ponds connected to natural water bodies that contain indigenous or feral tilapia populations. Such uncontrolled and unintentional hybridization could undermine the performance of cultured stocks and make future use of the contaminated stocks as broodstock questionable. For example, wild three-spotted tilapia (Oreochromis andersonii) invaded Nile tilapia ponds in Mozambique and produced hybrid tilapia that were less marketable than pure Nile tilapia. Inadvertent hybridization at a chinook salmon hatchery was suggested as the probable explanation for the appearance of chinookxcoho salmon hybrids in a California stream (Bartley et al., 1990). The level of unintentional or accidental hybridization has important considerations of aquatic biodiversity and will influence risk assessment on the use of hybrid fishes in aquaculture.
CONCLUSION AND RECOMMENDATIONS
A number of hybridization studies in fishes have been reported (Colombo et al., 1998; Bartley et al., 2001) but certainly not all of the hybrids are contributing to commercial aquaculture production. However, the contributions that hybrid fishes make to global aquaculture production are underestimated. Approximately, 80% of Thai catfish production from hybrids and there is a growing concern that these hybrids may be impacting native catfish (Pongthana, 2001). The tilapia hybrids in Israel are the main tilapia produced, but the 6,691 mt reported were not identified as hybrid (Hulata, 1995). Production of 4,257 mt of hybrid striped bass was reported from the USA, but production of no other fishes was reported, in spite of the fact that red tilapia and other tilapia hybrids are being produced and sold in Florida (Bartley et al., 2001).
Accurate identification of hybrids is important not only for sustainable aquaculture development but also to allow for a better understanding of biodiversity and conservation issues. It would be unfortunate to experience wide-spread loss of pure species in aquaculture as happened with tilapia as a result of wide-spread loss of pure species and subsequent hybridization (Pullin, 1988). It would be a significant cause for concern if hybrid Thai catfish or the hybrid Venezuelan characids pose more of a threat to local species than the pure species. The following points need to be addressed to overcome the above situations as well as to understand the role of hybrids in global aquaculture production:
| • | Genetic stock improvement through inter-generic or inter-specific hybridization of cultured fish species should be initiated under well-designed breeding plans at research institutes and lead central hatcheries under the guidance of fish breeding specialists/biologists |
| • | Data on parental origins and stock identity should be recorded for each hybrid. When crosses are made, the female species should be listed first; random crosses in regards to sex of each parent should also be identified |
| • | As much information as possible should be made available concerning the hybrid. Necessary information includes the stock and sex of each parental species, a comparative evaluation of the reciprocal crosses including a basic description of culture facility of environment and an assessment of the fertility of the hybrids |
| • | Consideration should be given to establishing a recognizable name for established hybrids and those that appear to have good potential for aquaculture and fisheries. The bester and sunshine bass are examples of two accepted names of interspecific hybrids that signify specific hybrids. A number of researchers working on the hybridization of sparids in the Mediterranean have adopted an informal nomenclature where the cross between the genera Dentex and Pagrus was regarded as “Dentagrus”, while the reciprocal cross was named as “Pantex” (Colombo et al., 1998) |
| • | In order to maintain genetic integrity, proper care needs to be undertaken so that the hybrids should not be intermingled and do backcross with their parental siblings (Rahman and Uehara, 2003, 2004; Rahman et al., 2005) |
| • | Genetic stock improvement through inter-generic of inter-specific hybridization of cultured fish species should be initiated under well-designed breeding plans at recognized research institutes and leading central hatcheries under the guidance of fish breeding specialists/biologists. Many private hatchery operators hybridize fish without knowledge of breeding biology and genetics that may cause deterioration of hatchery populations. Therefore government should immediately ban the unplanned/intentional hybridization practices being carried out by the hatchery operators and fish seed multiplication farms |
It should be concluded that hybridization is not only a preferred method of genetic improvement but also a potential tool for stock improvement through transmitting desirable traits in inferior parents. Appropriate evaluation of hybridization depends solely on the genetic structure, crossing patterns, gamete compatibility and gene flow patterns of the parental species. Practical knowledge on the genetic constitution of brood fishes is thus very crucial before initiating hybridization experiments. It can not be ignored that some non-generic factors such as weather conditions, culture systems, seasons and stresses associated with selecting, collecting, handling, breeding and rearing of broodstock and progeny may influence hybridization success to a greater extent (Bartley et al., 2001).