The use of inter-specific hybrids in aquaculture and fisheries

Transcription

1 Reviews in Fish Biology and Fisheries 10: , Kluwer Academic Publishers. Printed in the Netherlands. 325 The use of inter-specific hybrids in aquaculture and fisheries D.M. Bartley*, K. Rana & A.J. Immink Fisheries Department, Food and Agriculture Organization of the United Nations; *Address for correspondence, FIRI Fisheries Department, FAO, Viale delle Terme di Caracalla, Rome, Italy ( Accepted 28 August 2000 Contents Abstract Introduction page 325 Use of hybrid finfish in aquaculture and fisheries 326 Increased growth rate 326 Manipulation of sex-ratio Production of sterile animals Overall product improvement Increased environmental and disease tolerances Hybridization and polyploidization New or experimental hybrids Accidental hybridisation Discussion 334 Reporting hybrid production Hybrids as genetically modified organisms Acknowledgements 336 References 336 Key words: genetic improvement, hybridization, stock enhancement Abstract Inter-specific hybrid fishes have been produced for aquaculture and stocking programmes to increase growth rate, transfer desirable traits between species, combine desirable traits of two species into a single group of fishes, reduce unwanted reproduction through production of sterile fish or mono-sex offspring, take advantage of sexual dimorphism, increase harvestability, increase environmental tolerances, and to increase overall hardiness in culture conditions. Hybrids constitute a significant proportion of some countries production for certain taxa; for example, hybrid striped bass in the USA, hybrid clarid catfish in Thailand, hybrid characids in Venezuela, and hybrid tilapia in Israel. Despite its widespread use, there is a general impression that inter-specific hybridization is not a very useful tool for aquaculture. We believe this impression stems from inaccurate reporting of some useful hybrids, limited testing of strains used for hybrids, and from early work on salmonids that did not result in hybrids of commercial advantage. Experimentation with new hybrid fishes is ongoing, especially in marine culture systems where sterile fish may be preferred because of the concern that fish may escape into the marine and coastal environment. Hybridization has been used in tandem with polyploidization to improve developmental stability in hybrid progeny. The results of inter-specific hybridization can be variable and depend on the genetic structure (including the sex) of the parent fish. Inadvertent hybridization and backcrossing can lead to unexpected and undesirable results in hybrid progeny, such as failure to produce sterile fish, loss of color pattern, and reduced

2 326 viability. Hybridization is only one tool to improve aquaculture production and will require knowledge of the genetic structure of the broodstock, good broodstock management and monitoring of the viability and fertility of the progeny. Hybridization does represent a genetic modification wherein genes are moved between different species; implications for biodiversity conservation and regulation of this type of modification are discussed. Introduction Hybridization is the mating of genetically differentiated individuals or groups and may involve crosses within a species (also known as line crossing or strain crossing) or crosses between separate species. This breeding technique is used by aquaculturists in the hope of producing aquatic organisms with specific desirable traits or general improvement in performance. Commonly, the desired goal is to produce offspring that perform better than both parental species (hybrid vigor or positive heterosis). Hybridization also may be used to transfer other desirable characteristics, e.g. disease resistance, from one group or species to another, to combine valuable traits from two species into a single group, e.g. good growth and flesh quality, and to produce sterile individuals. This article focuses on the hybridization between different species of fishes, i.e. inter-specific hybridization, in aquaculture and stocking programmes to understand better the role of hybridization in global fishery production. A review of the literature and our discussions with fishery geneticists revealed that the use of interspecific hybrids is under-reported in many areas of the world. Additionally, there are policy issues with regulatory aspects of using inter-specific hybrids because they represent organisms that have been modified genetically, but fall outside internationally accepted definitions of genetically modified organism (UNEP, 1994; Bartley and Hallerman, 1995; Hallerman and Kapuscinsky, 1995). Use of hybrid finfish in aquaculture and fisheries Hybridization has been used in numerous species of fish to increase growth rate, manipulate sex ratios, produce sterile animals, improve flesh quality, increase disease resistance, improve environmental tolerance, and to improve a variety of other traits to make fish more profitable to raise (Table 1). Much of the early work on finfish hybridization in aquaculture was conducted on salmonids, but in general these species did not produce hybrids of commercial advantage. As a result, there was, or perhaps still is, an impression that hybrids do not hold much attraction for aquaculturists. With the expansion of the 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. The increased use of induced spawning techniques such as hypophysation (the use of pituitary gland extract to induce ovulation) and synthetic hormones, in-vitro fertilisation technologies and increased knowledge of reproductive biology will enable the aquaculturist to overcome many of the behavioural, biological and geographical reproductive isolating mechanisms that prevent fish from hybridizing in nature. The following sections describe some of the traits that have been or can be improved through hybridization. Increased growth rate Improved growth rate is probably the most desirable trait for genetic improvement in aquaculture. Increased growth rate may result from dominance variance (Tave, 1986) or from increasing the number of polymorphic loci in an individual. Increased heterozygosity has been implicated in improved growth and other fitness related traits in a variety of species by influencing characters such as developmental stability (Leary et al., 1983), food conversion efficiency, and oxygen metabolism (Danzmann et al., 1985; Koehn and Gaffney, 1984). The sunshine bass, a cross between white bass (Morone chrysops, MORNONIDAE) and the striped bass (M. saxatilis, MORNONIDAE) grows faster, has better overall culture characteristics than either parental species under commercial culture conditions in tanks, ponds and cages and is the preferred product in the USA (Smith, 1988). Crosses of the silver carp x bighead carp (Hypophthalmichthys molitrix x Aristichthys nobilis, CYPRINIDAE) (Krasnai, 1987) in polyculture systems, black crappie x white crappie (Pomoxis nigromaculatus x P. annularis, CENTRARCHIDAE) stocked in small

3 327 Table 1. Hybrid fishes commonly used in aquaculture and fishery stocking programs Species Hybridized Effect/advantage and comments Reference Cyprinids Grass carp x bighead carp (Ctenopharyngodon idella x Aristichthys nobilis) Hybrids are functional triploids, generally sterile, but with a small proportion being diploid and fertile. Allen and Wattendorf, 1987 Silver carp x bighead carp (Hypophthalmichthys molitrix x Aristichthys nobilis) Hybrids are fertile, with positive heterosis for growth rate, but gill raker deformities are apparent in some hybrids or backcrosses. Pure lines may be lost due to fertility of hybrids when parents and hybrids are grown together. Food and feeding strategy is intermediate to parents. Krasnai, 1987, Shilat Iranian Fisheries Company pers. comm. Indian Major carps [Rohu (Labeo rohita), mrigal (Cirrhinus mrigala), catla (Catla catla), and fringe-lipped peninsular carp (Labeo fimbriatus)] Crosses of common carp (Cyprinus carpio) with rohu (Labeo rohita), mrigal (Cirrhinus mrigala), and catla (Catla catla) Hybrids between Indian Major carps are fertile with characters intermediate to parental lines. The most interesting hybrid is the rohu x catla that is hardy and combines fast growth of catla with desirable head shape of rohu. Catla x fringe-lipped peninsula carp hybrid has desirable head and body shape, improved dressing percentage and growth rate similar to catla. Hybrids are functional triploids that are sterile with good growth and survival in monoculture and with good seinability. A subsititute for fertile common carp in areas where reproduction is unwanted. Hybrids had many deformities and high juvenile and larval mortality. Reddy, 2000 Khan et al., 1990 Trout and salmon Brown trout x brook trout (Salmo trutta x Salvelinus fontinalis) Atlantic salmon x brown trout (Salmo salar x S. trutta) Lake trout x brook trout (Salvelinus namaycush x S. fontinalis) Rainbow trout x char (O. mykiss x Salvelinus sp.) Common name is Tiger trout for this hybrid that is sterile, with low early survival, but then good growth. It was suggested for stocking areas where reproduction is to be limited. Triploidization of this hybrid increased survival and growth rate to a level comparable to Atlantic salmon, but offspring are sterile. Common name is Splake for this hybrid that is fertile, fast growing and tolerant of acid water. Increased disease resistance to salmonid viruses resulted from several crosses. Scheerer and Thorgaard, 1983 Galbreath and Thorgaard, 1997 Snucins, 1993 Dorson et al., 1991 Pacific salmon crosses (Oncorhynchus spp.) Most diploid hybrids are not useful in culture or fisheries, but have potential for disease resistance, sterility, and early seawater tolerance when the diploid hybrids are made triploid. For example, chum salmon x chinook salmon (O. keta x O. tshawytscha) triploid hybrids had early sea-water tolerance.. Crosses of coho salmon (O. kisutch) with other species, e.g. rainbow trout, had increased disease resistance, but other culture characteristics were poor. Scheerer and Thorgaard, 1983; Dorson et al., 1991; Grey et al., 1993; Seeb et al., 1993 Tilapia (most if not all tilapia hybrids appear to be fertile) Nile tilapia x blue tilapia (O. niloticus x O. Hybrids are fertile with increased cold and salinity tolerance. Crosses of some strains yield all-male offspring; aureus) hybrid males had superior growth. Certain strains produce red skin color forms. Reciprocal cross gives 50% males and females. Lahav and Lahav, 1990; Wohlfarth, 1994; Verdegem et al., 1997

4 328 Table 1. Continued Species Hybridized Effect/advantage and comments Reference Nile tilapia x Wami tilapia (O. niloticus x O. urolepis hornorum) Nile tilapia x long-finned tilapia (O. niloticus xo.macrochir) Mossambique tilapia x Wami tilapia (O. mossambicus x O. urolepis hornorum) Mossambique tilapia x Nile tilapia (Oreochromis mossambicus x O. niloticus) Cross yields predominately male offspring with some strains producing red-skinned fish with salt tolerance. Cross yields predominately male offspring, but strain of Nile tilapia important for good fry production. Cross yields predominately male offspring. Hybrids are fertile, however often with slow growth and dark color. Certain strains produce the Florida red tilapia with good growth and salinity tolerance. Hybridization of some strains produced tilapia with salintiy tolerance. Also known as the Taiwan red tilapia, progeny of these hybrids display a variety of different skin colors. Wohlfarth, 1994 Wohlfarth, 1994 Krasnai, 1987; Earnst et al., 1991; Head et al., 1994; Wohlfarth, 1994 Lim et al., 1993 Miscellaneous freshwater fish Black crappie x white crappie (Pomoxis nigromaculatus x P. annularis) Mud loach x cyprinid loach (Misgurnus mizolepis x M. anguillicaudatus) Tambaqui x pacu (Colossoma macropomum x Piaractus brachypoma) Tambaqui x pacu (Colossoma macropomum x Piaractus mesopotamicus) Muskellunge x pike (Esox masquinongy x E. lucius) Green sunfish x bluegill (Lepomis cyanellus x L. macrochirus) Hybrids have positive heterosis for growth. The F 1 s are fertile but the F 2 s have limited recruitment for unknown reasons. Recommended for stocking small impoundments. Hybrids have high hatch and survival rates and are probably fertile. Hybrids have good growth rate and are probably fertile. Hybrids have good early survival and are probably fertile. These hybrids, called the tiger muskellunge, are sterile, cost effective to produce and raise, and are suitable for intensive culture and stocking programmes. Hybrid has good culture characters, growth, and tolerance to low oxygen. They are fertile, but produce skewed sex ratios. Other Lepomis hybrids have similar characteristics. Hooe et al., 1994 Kim et al., 1995 FAO unpublished report Senhorini et al., 1988 Brecka et al., 1995 Tidwell et al., 1992; Will et al., 1994 Miscellaneous marine and diadromous fish Black drum x red drum (Pogonias cromis x Sciaenops ocellatus) Hybrids have faster growth rate to 9 months and slightly lower survival than parents; fertility is unknown. Reciprocal cross yielded 0% fertilization. Henderson-Arzapalo and Maciorowski, 1994 Beluga x sterlet sturgeon (Huso huso x Acipenser ruthenus) Beluga x Russian sturgeon (H. huso x A. güldenstäti) Common name for this cross is the bester. Hybrids are fully fertile with good culture characters, that do not need to be anadromous as Beluga. The reciprocal cross is less desirable. This hybrid is being tested in the former USSR and appears to have tolerance to both fresh and seawater as well as good growth rate; fertility is unknown. Steffens et al., 1990 Gorshkova et al., 1996 White bass x striped bass (Morone chrysops x M. saxatilis) Common name is sunshine bass and the hybrid has many good culture characteristics. It is generally sterile, but can be fertile in some crosses. The reciprocal cross, named Palmetto bass, is not as desirable. Smith, 1988; Hallerman, 1994

5 329 Table 1. Continued Species Hybridized Effect/advantage and comments Reference Striped bass x yellow bass (M. mississippiensis) Gilthead seabream x red seabream (Sparus auratus x Pagrus major) Red seabream x common dentex (P. major x Dentex dentex) Sheim x sobiaty (Acanthopagus latus x Sparidentex hasta) Dusky grouper x white grouper (Epinephelus marginatus x E. aeneus) Marbled grouper x camouflage grouper (Epinephelus fuscoguttatus x E. polyphekadian) Experimental hybrid with good survival, but lower growth rate and condition factor than sunshine bass. This cross yielded 100% female offspring. Cross produced sterile hybrids with improved growth rate and good overall performance in cage culture. Fast growing hybrid with good overall culture qualities in cages. Several other species of sparids have also been produced in the Mediterranean area, but are not widely used in culture. Hybrid recently produced experimentally in Kuwait. It appears to have good growth, flesh quality and is fertile. Experimental hybridization to improve early development performance in parental lines resulted in 100% mortality of controls and hybrids after 10d. Experimental grouper hybrids with increased growth rate and conversion efficiency. Wolters and DeMay, 1996 Hulata, 1995 Colombo et al., 1998 Khaled Al-Abdul-Elah, Kuwait Institute of Scientific Research, pers. comm. Glamuzina et al., 1999 James et al., 1999 Catfish African x Thai catfish (Clarias gariepinus x C. macrocephalus) Excellent hybrid widely used in Thailand that combines the desirable flesh and growth characters of parents. Hybrids are fertile and there is concern on introgression with native Thai catfish. Suresh, 1991 African catfish x Vundu catfish (C. gariepinus x Heterobranchus longifilis) Hybrids and backcrosses are fertile and have better growth rate than parents in intensive culture systems. Salami et al., 1993; Nwadukwe, 1995 African catfish x catfish (unknown common name) (C. gariepinus x H. bidorsalis) Giant catfish x giant pangasius (Pangasiodon gigas x Pangasius sawitsongai) Hybrids have positive heterosis for growth rate in tanks. Salami et al., 1993 Large catfish hybrids with good growth rate. Use of hybrids may reduce pressure on the giant catfish, an endangered species, if demand for the hybrid can replace demand on the pure giant catfish. Mekong River Commission unpublished report Channel catfish x blue catfish (Ictaluras punctatus x I. furcatus) Hybrids have superior growth in ponds and high density culture, good disease resistance, increased tolerance to low oxygen levels, good dressing percentage, good body shape, and ease of fishing and seining. Problems remain with breeding parental lines. Dunham, 1987; Dunham et al., 1990; Dunham and Argue, 1998 ponds and impoundments (Hooe et al., 1994) and catfish hybrids between the African catfish (Clarias gariepinus, CLARIIDAE), and the Vundu (Heterobranchus longifilis or H. bisorsalis, CLARIIDAE) in intensive concrete tanks (Salami et al., 1993; Nwadukwe, 1995) were reported to grow faster (positive heterosis) than parental lines. Good growth and improved general performance in culture systems also were obtained from crosses of common carp (Cyprinus carpio, CYPRINIDAE), with rohu (Labeo rohita, CYPRINIDAE), mrigal (Cirrhinus mrigala, CYPRINIDAE), and catla (Catla catla, CYPRIN- IDAE) in small ponds in India (Khan et al., 1990) Progeny from crosses of tambaqui (Colossoma macropomum, CHARACIDAE) with the pacu (Piaractus brachypoma and P. mesopotamicus, CHARA- CIDAE) grew faster than parental species in Brazil and Venezuelan raceways and ponds (Senhorini et al.,

6 , FAO unpublished report). Crosses of the green sunfish (Lepomis cyanellus, CENTRARCHIDAE) with bluegill (L. macrochirus, CENTRARCHIDAE) (Tidwell et al., 1992; Will et al., 1994), and crosses of the gilthead seabream (Sparus auratus, SPAR- IDAE) with red seabream (Pagrus major,sparidae) reared in Israel (Hulata, 1995) also had positive heterosis for growth and other culture characteristics. Several hybrids in the family SPARIDAE have been produced in the Mediterranean with the cross between red seabream and common dentex (Dentex dentex, SPARIDAE), being especially fast growing in Mediterranean cage culture (Colombo et al., 1998). Manipulation of sex-ratio In aquaculture it often is preferable to produce monosex populations of fish. 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 may produce a valuable product, such as caviar; and monosex populations help reduce unwanted reproduction that would result from mixed-sex populations, e.g. tilapia overpopulation and stunting. Hybridization between some species of tilapias (CICHLIDAE) such as Nile tilapia (Oreochromis niloticus) and the blue tilapia (O. aureus) results in the production of predominantly male offspring and reduces unwanted natural reproduction in growout ponds (Rosenstein and Hulata, 1993). This cross produces predominately males because of different sex-determining mechanisms in the two species: Nile tilapia has the XX, XY system with the male being heterogametic, whereas blue tilapia has ZZ ZW with the heterogametic genotype being female (Lahav and Lahav, 1990; Wohlfarth, 1994). Other tilapia crosses producing predominately male offspring are Nile tilapia with Wami tilapia (O. honorum) or with the longfin tilapia (O. macrochir) and the Mossambique tilapia (O. mossambicus) crossed with the Wami tilapia (Wohlfarth, 1994). The cross between striped bass and yellow bass (M. mississipiensis, MORONIDAE) produced 100% females in production trials (Wolters and DeMay, 1996), however the mechanism producing such a biased sex-ratio is not known. Production of sterile animals Hybridization between species often results in offspring that are sterile or with diminished reproductive capacity due to problems with gonad development and chromosome pairing. Similar to manipulation of sex-ratio, the production of sterile animals may be advantageous to reduce unwanted reproduction or to improve growth rate by avoiding channelling energy into reproduction. Examination of species karyotype is a good general indication of whether or not hybridization will result in sterile offspring. Hybrids between Indian Major carps are generally fertile because of similar chromosome numbers (2N = 50). Indian Major carps (CYPRINIDAE) crossed with common carp (4N = 102) results in hybrids that are sterile because they are functionally triploid (Khan, 1990; Reddy, 2000). Another natural triploid results from the cross between grass carp (Ctenopharyngodon idella, CYPRINIDAE) and big head carp (Aristichthys nobilis, CYPRINIDAE). Grass carp are commonly produced for aquatic weed control, but there is concern regarding their use in biological control that they may become established in natural waterbodies and adversely impact desirable vegetation. These triploid hybrids have reduced fertility, but a small portion of the progeny are diploid and could be fertile (Allen and Wattendorf, 1987). However, crosses of some sturgeon species (ACIPENSERIDAE) with different chromosome numbers, as well as most tilapia crosses, produce fertile offspring (Steffens et al., 1990). The cross between the black crappie and white crappie is often recommended for stocking small impoundments because of reduced fertility of the F 2 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 was found in Georgia and South Carolina (USA) hatcheries and rivers (Avise and Van den Avyle, 1984). The gilthead sea bream x red seabream hybrid is also sterile and this may be an important quality in marine aquaculture to reduce impacts on local stocks in the event of escapes from aquaculture (Hulata, 1995). The tiger trout, a brown trout (Salmo trutta, SALMONIDAE) and brook trout (Salvelinus fontinalis, SALMONIDAE) cross, is sterile with poor early survival, but good growth rate and therefore is useful for stocking areas where reproduction is unwanted (Scheerer and Thorgaard, 1983).

7 331 Overall product improvement One function of hybridization is to combine traits from different species to increase the overall production or marketability of a cultured species. The main catfish cultured in Thailand is a cross between African (Clarias gariepinus, CLARIIDAE) and Thai (C. macrocephalus, CLARIIDAE) catfish; this cross 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 bester an intergeneric cross between the beluga (Huso huso, ACIPENSERIDAE) and the sterlet sturgeon (Acipenser ruthenus, ACIPENSERIDAE) combines the growth-rate and salinity tolerance of the beluga with the freshwater tolerance of the sterlet. The bester grows nearly as rapidly as the beluga, and may be raised in fresh or saline waters (Steffens et al., 1990). The rohu x catla 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). Catla x fringed-lipped peninsula carp (Labeo fimbriatus, CYPRINIDAE) hybrids were reported to have small heads of the fringedlipped peninsula carp and deep body and nearly equal growth rate to the catla; dressing percentage also was improved in this hybrid (Basavaraju et al., 1995). The sunshine bass hybrid (white bass x striped bass) has a suite of advatageous traits including good osmoregulation, high thermal tolerance, resistance to stress and disease, high survival in culture and modified waterbodies, and ability to utilize soy beans as a protein source (see refs. in Smith, 1988 and Colombo et al., 1998). In many parts of the world, e.g. Cuba, Venezuela, Thailand, Europe and the United States of America, red tilapia are more desirable than darker skinned tilapia (personal observation). Most red tilapia are descended from the Nile x blue tilapia cross (Verdegem et al., 1997), but red tilapia also result from the cross of Wami tilapia (O. urolepis hornorum,cichl- IDAE) with x Mossambique tilapia (Ernst et al., 1991). Welcomme (1999) reported that red tilapia from Nile tilapia x Mossambique tilapia, and Nile tilapia x Wami tilapia are being farmed in central Thailand and have been moved to Lao PDR (Peoples Democratic Republic) for aquaculture. The latter cross is also salt tolerant and used for coastal aquaculture in parts of South East Asia, e.g. Malaysia, Thailand and the Philippines, and elsewhere, e.g. Mauritias (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 and improve its stability (Koren et al., 1994; Hussain, 1994). Culturists hybridizing channel catfish (Ictalurus punctatus, ICTALURIDAE) and blue catfish (I. furcatus, ICTALURIDAE), hoped to combine the 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 (see references in Dunham and Argue, 1998). Recreational fisheries for esocid fishes are supported by hatchery stocking in many areas of the USA. The tiger muskellunge is a sterile hybrid between the muskellunge (Esox masquinongy, ESOCIDAE) and the pike (E. luscious, ESOCIDAE) that is adaptable to intensive culture systems being used to raise progeny and has some of the desirable sport fish characteristics of the pure muskellunge (Brecka et al., 1995). However the protein requirements of the hybrid are higher than for both parental species (Brecka et al., 1995). Increased environmental and disease tolerances An organism becomes sick, or begins to perform poorly, because of the interactions among the organism, the environment, and pathogens; environmental stress can increase fishes susceptibility to disease (Bunch and Bejerano, 1997). Because increasing general environmental tolerance also will help keep cultured populations healthy, these topics are treated simultaneously. Hybridization may be used to improve disease resistance by breeding a resistant species with a less resistant one. Dorson et al. (1991) reported on hybridization attempts using coho salmon (Oncorhynchus kisutch, SALMONIDAE), which are considered to be resistant to a variety of salmonid viruses, in which disease resistance in the hybrids was improved, but overall viability was very poor. Viability was increased when hybridization was followed with triploidization, and Dorson et al. (1991) stated that the rainbow trout (O. mykiss, SALMONIDAE) x char

8 332 (Salvelinus spp., SALMONIDAE) triploid hybrids had increased resistance to several pathogenic salmonid viruses (Dorson et al., 1991). Hybrids may have increased environmental tolerances when one parental species has a wide range of tolerance (e.g. euryhaline species), a specific tolerance (cold tolerant species), or because of increased heterozygosity sometimes being associated with a broad niche (Nelson and Hedgecock, 1980; Noy et al., 1987). Mossambique tilapia and Wami tilapia can reproduce in saline waters, however Nile tilapia has improved culture performance in many aquaculture settings. Hybridization between the Mossambique and Nile tilapias yields a red tilapia with salinity tolerance (Lim et al., 1993). Hybrids between Mossambique and Wami tilapia, called the Florida red strain, have high growth rates and can reproduce in salinities of 19ppt (Ernst et al., 1991). Crosses between Nile tilapia and blue tilapia also resulted in progeny with good salinity tolerance (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, SALMONIDAE)) and brook trout (S. fontinalis, SALMONIDAE), can tolerate reduced ph levels of of acid lakes in 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 (Ihssen et al., 1982). Hybridization and polyploidization Chromosome-set manipulation can be combined with hybridization to increase the viability of hybrid fishes and provide increased developmental stability of the hybrids 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 on the hybrids (here viability refers to the ability of the hybrid to survive and grow, but does not imply that the hybrid is fertile or capable of reproducing) (Grey et al., 1993). Triploidization of Atlantic salmon (Salmo salar, SALMONIDAE) x brown trout (S. trutta, SALMONIDAE) hybrids increased survival and growth rate to a level comparable to Atlantic salmon (Galbreath and Thorgaard, 1995). Triploid Pacific salmon hybrids between chum salmon (Oncorhynchus keta, SALMONIDAE) and chinook salmon (O. tshawytscha, SALMONIDAE) have earlier seawater acclimation times (Seeb et al., 1993). 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 hematopoetic necrosis virus (IHN), but the latter hybrids grew more slowly (Dorson et al., 1991). New or experimental hybrids As the domestication of fish species increases, the possibilities to increase production through appropriate hybridization will also increase. New hybrids will need to be tested as to their performance, viability, and fertility (their ability to reproduce). There is substantial interest in hybridizing marine fishes because of their high market value and improvements in efficiency of larval culture. The Kuwait Institute of Scientific Research has produced a hybrid bream by crossing the Sheim (Acanthopagus latus, SPAR- IDAE) and the sobiaty (Sparidentex hasta, SPAR- IDAE). Its culture performance currently is being investigated; the hybrid has good growth and body quality and appears to be fertile (Khaled Al-Abdul- Elah, Kuwait Institute of Scientific Research, pers. comm.). The camouflage grouper (Epinephelus polyphekadion, SERRANIDAE) is more resistant to environmental stress and disease than the marbled grouper (E. fuscoguttatus, SERRANIDAE). Experimental trials demonstrated faster growth and improved conversion efficiency in the hybrid grouper, but other parameters were not evaluated (James et al., 1999). To overcome problems with early development and feeding in grouper (Epinephelus spp.) culture, hybrids between dusky grouper (E. marginatus, SPARIDAE) and the white grouper (E. aeneus, SPARIDAE)were tested, but the hybrids, along with the controls, all died by 10d post-hatch (Glamuzina et al., 1999). Following on the success of the bester, trials with the hybrid between the Beluga and Russian (A. güldenstäti, ACIPENSERIDAE) sturgeons indicated a wide salinity tolerance and good growth (Gorshkova et al., 1996). These hybrid sturgeon now are being considered for culture in Russia and Iran (pers. comm. Shilat, Iranian Fisheries Company).

9 333 Hybrids between the black drum (Pogonias cromis, SCIANIDAE) and the red drum (Scianenops ocellatus, SCIANIDAE) resulted in better growth, similar flavor characteristics, but higher susceptibility to parasites than exhibited by parental lines (Henderson- Arzapalo and Maciorowski, 1994). Such a hybrid may be advantageous in marine stock enhancement programmes or in meeting the consumer demand for red drum in light of the declining numbers of red drum. The fertility of the cross is unknown and should be determined to ensure that the hybrid does not represent another threat to the red drum genetic diversity. Aquaculturists in the Republic of Korea hybridized two species of loach, popular farmed fish for food and Buddist ceremonies (Kim et al., 1995). The hope was to combine the fast growth and large size of the mud loach (Misgurnus mizolepis, COBITIDAE) with the desirable body color of the cyprinid loach (M. anguillicaudatus, COBITIDAE). Fertilization, hatching, and survival of the hybrids were equal to the parents. Karyology of the parental species is very similar and triploidization of the hybrids produced lower survival and hatching rates, possibly due to the method of triploidization used (cold shock) (Kim et al., 1995). Continued studies are planned on production characteristics of the hybrids and their fertility. The Thais now are crossing the giant catfish (Pangasiodon gigas, PANGASIIDAE) with the giant pangasius (Pangasius sanitwonsei, PANGASIIDAE) (Welcomme, R.L. unpublished report to the Mekong River Commission, Phnom Pehn, Kingdom of Cambodia) for cage culture. Both of these catfish are extremely large, reaching 3 m and 300 kg, and the giant catfish is an endangered species whose trade is restriced under the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). These hybridization efforts should also be evaluated so as not to endanger further the giant catfish through excessive take of brood fish from the wild or through genetic introgression of the two pure species. Due to the wide geographic distribution of yellow bass (Morone mississippiensis, MORONIDAE), hybridization tests with striped bass and comparisons with the hybrid 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 DeMay, 1996). Accidental hybridisation Although many aquaculturists purposefully produce inter-specfic hybrids, some hybrids are produced inadvertently through mixed spawning of different species in a hatchery, misidentification of species by hatchery personnel, or by contamination of the aquaculture facility with wild fish. To produce Indian Major carp seed, different species often are induced to spawn in a common spawning pond thus providing the opportunity for unintentional hybridization (Padhi and Mandal, 1997). Silver carp and bighead carp sometimes are hybridized inadvertantly because of their similar appearance and because of shortage of the correct species at spawning time due to differences in maturation times between male and female carp. 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 (Shilat, Iranian Fisheries Company, pers. comm.). Accidental hybridization with wild fish is especially prevalent in tilapia ponds that are connected to natural water bodies that contain natural or feral tilapia populations. Such uncontrolled and unintentioned hybridization could undermine the performance of cultured stocks and restrict future use of the contaminated stocks as broodstock. For example, wild three-spotted tilapia (Oreochromis andersoni,cichl- IDAE) invaded Nile tilapia ponds in Mozambique and produced hybrid tilapia that were less marketable than pure Nile tilapia (pers.obs.). The breakdown of monosex production of tilapia often has been attributed to misidentification and subsequent use of the wrong species in breeding plans. Inadvertent hybridization at a chinook salmon hatchery was suggested as the probable explanation for the appearance of chinook x coho salmon hybrids in a California stream (Bartley et al., 1990). The level of unintentional or accidental hybridization has important considerations for the conservation of aquatic biodiversity and will influence risk assessment on the use of hybrid fishes in aquaculture and fisheries (see next section).

10 334 Discussion Reporting hybrid production The Food and Agriculture Organization of the United Nations (FAO) is the intergovernmental body serving as repository for global fishery and aquaculture data. To publish data on aquaculture and fishery production, FAO sends out questionnaires to each member country. The Fisheries Information, Data and Statistics Unit compiles this information into a database (FAO, 1997). Member countries are requested to report production to species, and because the FAO database cannot easily incorporate hybrid nomenclature, many hybrids are simply reported and recorded to genus or as species nei (not elsewhere included). The nei category also includes fish not identified to species. The only hybrid fish for which production is reported to FAO is the hybrid striped bass. There have been numerous studies on hybridization of fishes (see references in Colombo et al., 1998 and Lutz, 1997) and certainly not all of the hybrids reported are contributing to commercial aquaculture production. However, we believe the contribution that hybrid fishes make to global aquaculture production is underestimated. For example, tilapia production in Indonesia is reported to FAO (FAO Fisheries Information, Data and Statistics Unit, pers. comm.) as Nile tilapia and Mossambique tilapia, but we have observed production of tilapia hybrids. Although most countries report tilapia nei as a category, it is not known if these are hybrid tilapia. In Venezuela the only tilapia with market value is the red tilapia, which is one of several hybrids described earlier. However, Venezuelan statistics for tilapia production only report 2,200 mt of tilapia nei. The Characid hybrids in Venezuela are thought to contribute about 29% to the country s overall aquaculture production (FAO unpublished report) and similar hybrids are being created in Brazil, but no mention is made of these hybrids in the country s reports to FAO. Approximately 80% of Thai catfish production is from hybrids and there is a growing concern that these hybrids may be impacting native gene pools (Thai National Institute of Fisheries, pers. comm.). The tilapia hybrids in Israel are the main tilapia produced (Hulata, 1995), but the 6,691 mt reported to FAO were not identified as hybrid tilapia. Production of 4,257 mt of hybrid striped bass was reported in 1998 from the USA (FAO unpublished data), but production from no other hybrid fishes was reported, in spite of the fact that red tilapia and other tilapia hybrids are being produced and sold in Florida (see In the countries of the former Soviet Union that farm the bester, such as Russia and Ukraine, no mention is made of this hybrid in their reports to FAO. Accurate identification of hybrids is important not only for sustainable aquaculture development, guiding aquaculture domestication efforts, assessing aquaculture production, and identifying useful crosses, but also to allow for a better understanding of biodiversity issues. It would be unfortunate to experience widespread loss of pure species in aquaculture as happened with tilapia as a result of widespread introduction 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. If disease resistance can be transferred via hybridization, can disease susceptibility also be transferred? These are some of the concerns associated with the use of hybrids in aquaculture that should be addressed. We believe that reporting on the use of hybrid fishes aquaculture and fisheries to FAO and to other information users needs improvement; we offer the following suggestions to understand better the role of hybrids in aquacutlure and fisheries: Reporting data on hybrid production should be standardized and consistent Parental origins and stock (strain) identity should be recorded for each hybrid. When crossses are made, the female species should be listed first; random crosses in regards to sex of each parent should be so identified. Countries (and aquaculturists) need to establish that a hybrid is going to be part of a longterm production system. Short term production of hybrids is often recorded in the nei category. For a hybrid to be included as a separate entry in country reports and subsequently in the FAO database, reporting should be consistent over a period of years. 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 or 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

11 335 aquaculture and fisheries. The sunshine bass and bester are two examples of accepted names that signify specific crosses of specific species. Some aquaculturists working on the hybridization of sparids in the Mediterranean have adopted an informal nomenclature where the cross between the genera Dentex and Pagrus was nicknamed Dentagrus, while the reciprocal cross was named, Pantex (Colombo et al., 1998). Hybrids as genetically modified organisms Hybrid fish represent a genetic modification wherein genes from different species are combined together in a single animal. However, the international community and many national governments have restricted the use of the term genetically modified organism (GMO) to those organisms created by recombinant DNA technology, i.e. hybrids would not be GMOs (UNEP, 1994; Bartley and Hallerman, 1995; Hallerman and Kapuscinski, 1995). There is legislation that covers hybridization in some areas, for example the State of California (USA) has laws prohibiting the hybridization of fish without a proper license (pers. obs.). Government oversight, regulation, and applicable laws such as containment, labelling, and trade requirements, focus heavily on organisms with recombinant DNA, but hardly at all on hybrids (Bartley and Hallerman, 1995; Hallerman and Kapuscinski, 1995). As Colombo et al. (p. 95, 1998) point out, The fact that foreign genes were inserted into a species by genetic intermingling through hybridization rather than by genetic addition through egg microinjection or gamete electroporatoin does not seem to justify, in such trials with fertile fish, the total lack of those containment measures that are usually imposed on experiments with transgenic fish. The United States Department of Agriculture (USDA) considers hybrids to be applicable organisms to their Performance Standards for conducting research on GMOs if there is insufficient information on the phenotypic change due to the hybridization, i.e. how the hybrid interacts with its biotic and abiotic environments (ABRAC, 1995). The USDA s position mirrors the philosophy oulined by Pullin et al. (1999), who proposed that the product of the genetic modification and the amount of uncertainty as to the phenotypic changes imparted by the genetic modification should be major deciding factors on level of regulation for an organism used in aquaculture, rather than focusing on a particular process per se. The position of the USDA to include some hybrids as being considered GMOs is a practical compromise that forces regulators and users of hybrids and other GMOs to consider phenotypic traits and how an organism will interact in the environment and emphasises the need to increase our understanding of the fishes being farmed. The level of unintentional or accidental hybridiztion will have a bearing on evaluating the risk of farming different species. It is generally assumed that in aquaculture facilities matings will be controlled and the genetic structure of the offspring will be known. This information is then used to evaluate the risk of using certain species. If this information is not known or is mistakenly known, accurate risk assessment will be impossible because the genetic structure, phenotype, viability and fertility of the offspriing will be unknown or incorrectly surmised. We do not have an indication at present of the extent of unintentional of accidental hybridization in global aquaculture other than to know that it exists or has existed in diverse situations such as tilapia culture in Africa, Chinese carp culture in Iran, and Pacific salmon enhancement in the USA. In conclusion, it should be emphasized that it is not our intention to promote hybridization as the preferred method of genetic improvement, but rather as one method of improvement that has potential for some immediate gains. Desirable traits usually can be passed to the hybrid in one generation, but it should be appreciated that hybridization can be a hit and miss proposition. Proper evaluation of hybridization depends on knowing very well what is being crossed and what is being produced from that cross. Knowledge of the genetic structure of the brood stock is critical. The Nile x blue tilapia crosses meant to produce monosex progeny will produce mixed-sex offspring if the parental stocks have been contaminated with genes from other species. Hulata et al. (1993) reported differences in culture performance and sex ratio in crosses of Nile x blue tilapia that were dependent on the specific strain being used. Attention must be paid to the sex in crosses; the sunshine bass (female white bass x male striped bass) is superior to the palmetto bass (the reciprocal cross) and the bester is superior to its reciprocal cross. Lutz (1997) stresses the value of replicate tests because of non-genetic factors that may influence the results, such as weather conditions, culture conditions, season, and stresses associated with collecting, handling and rearing broodstock and progeny.

12 336 Furthermore, evaluation of the progeny and especially the fertility of hybrid progeny is essential in areas where reduced reproduction is preferred, as in biological control by grass carp x bighead carp hybrids, in tilapia grow-out facilities or in many of the hybrid marine fishes that are being considered for coastal aquaculture. Although hybridization may not require the same type of data management that a long term selective breeding programme would require, proper broodstock management, including the documentation and maintenance of the correct parental lines, avoidance of inbreeding, avoidance of inadvertant hybridization or backcrossing, the appropriate choice of sex for the matings, and subsequent monitoring of the results will be necessary to utilize fully this method of genetic improvement. Acknowledgements We gratefully acknowledge the suggestions of two anonymous referees. The unpublished reports and publications of FAO and other specialized agencies of the United Nations cited here are available on request. References ABRAC (1995) Performance Standards for Safely Conducting Research with Genetically Modified Fish and Shellfish. Part 1. Introduction and Supporting Text for Flowcharts. Agricultural Biotechnology Research Advisory Committee. US Department of Agriculture. Document No , Washington, D.C. 63 pp. plus annexes. Allen S.K.J. and Wattendorf R.J. (1987) Triploid grass carp: status and management implications. Fisheries 12, Avise J.C. and Van den Avyle M.J. (1984) Genetic analysis of reproduction of hybrid white bass x striped bass in the Savannah River. Transactions of the American Fisheries Society 113, Bartley D.M and Hallerman E.M. (1995) Global perspective on the utilization of genetically modified organisms in aquaculture and fisheries. Aquaculture 137, 1 7. Bartley D.M., Gall G.A.E. and Bentley B. (1990) Biochemical genetic detection of natural and artificial hybridization of chinook and coho salmon in northern California. Transactions American Fisheries Society 119, Basavaraju Y., Deveraj K.V. and Ayyar S.P. (1995) Comparative growth of reciprocal carp hybrids between Catla catla and Labeo fimbriatus. Aquaculture 129, Bhikajee M. (1997) Mariculture of the red tilapia in enclosed bays and in cages the Mauritian experience. In: Fitzsimmons, K. (ed.), Tilapia Aquaculture, Northeast Regional Agricultural Engineering Service 106, Volume 2, New York, pp Brecka B.J., Kohler C.C. and Wahl D.H. (1995) Effects of dietary protein concentration on growth, survival, and body composition of muskellunge, Esox masquinongy, and tiger muskellinge, Esox masquinongy x E. luscius, fingerlings. J. World Aquacult. Soc. 26, Bunch E.V. and Bejerano I. (1997) The effect of environmental factors of the susceptibility of hybrid tilapia Oreochromis niloticus x Oreochromis aureus to Streptococcosis. Israeli J. Aquaculture 49, Colombo L., Barbaro A. Francescon A., Libertini A., Bortolussi M., Argenton F., Dalla Valle L., Vianell S. and Belvedere P. (1998) Towards an integration between chromosome set maipulation, intergeneric hybridization and gene transfer in marine fish culture. In: Bartley D. and Basurco B. (eds.), Genetics and Breeding of Mediterranean Aquaculture Species. Cahiers Options Méditerranéennes Vol. 34. CIHEAM Zaragoza, Spain, pp Danzmann R.G., Ferguson M.M. and Allendorf F.W. (1985) Does enzyme heterozygosity influence developmental rate in rainbow trout? Heredity 56, Dorson M., Chevassus B. and Torhy C. (1991) Comparative susceptibility of three species of char and rainbow trout x char triploid hybrids to several pathogenic salmonid viruses. Dis. Aquat. Org. 11, Dunham R.A. (1987) American catfish breeding programmes. In: Tiews K. (ed.), Selection, Hybridization and Genetic Engineering in Aquaculture of Fish and Shellfish, Vol. 2. FAO European Inland Fisheries Advisory Commission and International Council for the Exploration of the Sea, Rome, Italy and Copenhagen, Denmark, pp Dunham R.A. and Argue B.J. (1998) Seinability of channel catfish, blue catfish, and their F 1,F 2,F 3 and backcross hybrids in earthen ponds. Prog. Fish. Cult. 60, Dunham R.A., Brummet R.E., Ella M.O. and Smitherman R.O. (1990) Genotype-environment interactions for growth of blue, channel, and hybrid catfish in ponds and cages at varying densities. Aquaculture 85, Ernst D.H., Watanabe W.O., Ellington L.J., Wicklund R.I. and Olla B.L. (1991) Commercial-scale production of Florida red tilapia seed in low- and brackish-salinity tanks. J. World Aquaculture Soc. 22, FAO (1997) Review of the State of the World Aquaculture. FAO Fisheries Circular 885, Rev. 1. Food and Agriculture Organization of the United Nations, Rome, Italy. Galbreath P.F. and Thorgaard G.H. (1995) Sexual maturation and fertility of diploid and triploid Atlantic salmon x brown trout hybrids. Aquaculture 137, Glamuzina B., Kozul V., Tutman P. and Skaramuc B. (1999) Hyrbridization of Mediterranean groupers: Epinephelus marginatus x E. aeneus and early development. Aquaculture Research 30, Gorshkova G., Gorshova S., Gordin H. and Knibb W. (1996) Karyological studies in hybrids of Beluga, Huso huso (L.) and the Russian Acipenser guldenstati Brant. Isr. J. Aquacult. Bamidgeh 48, Grey A.K., Evans M.A. and Thorgaard G.H. (1993) Viability and development of diploid and triploid salmon hybrids. Aquaculture 112, Hallerman E.M. and Kapuscinsky A.R. (1995) Incorporating risk assessment and risk management into public policies on genetically modified finfish and shellfish. Aquaculture 137, Head W.D., Zerbi A. and Watanabe W.O. (1994) Preliminary observatoins on the maketability of saltwater-cultured Florida red tilapia in Puerto Rico. J. World Aquacult. Soc. 25, Henderson-Arzapalo A. and Maciorowski A.F. (1994) A comparison of black drum, red drum, and their hybrid in salwater pond culture. J. World Aquacult. Soc. 25,