SELECTION AND CROSSBREEDING RESPONSES FOR CULTURED FISH R. A. DUNHAM, USA

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1 SELECTION AND CROSSBREEDING RESPONSES FOR CULTURED FISH R. A. DUNHAM, USA Department of Fisheries and Allied Aquacultures, Alabama Agricultural Experiment Station, Auburn University, AL SUMMARY Although aquaculture is a relatively young farming enterprise, divergent strains of cultured species of fish have been developed. The selective pressures of domestication have produced strains of fish superior to wild strains in only a few generations. Generally, individual selection has further improved the body weight of these domesticated strains. When exceptions have been observed, selection for decreased body weight has been more successful than selection for increased body weight. Selection has also altered the body conformation and disease resistance of fishes. Correlated responses to selection have generally been beneficial. Intraspecific crossbreeding improves disease resistance; however, improvements in weight gain have been more variable and less promising than those obtained by selection. Domestic X domestic crossbreeds are more likely to exhibit heterotic growth than domestic X wild crossbreeds. Combining abilities vary among strains and between sexes within a strain. Interspecific hybridization seldom results in hybrids exhibiting heterotic growth or having potential for commercial use. Hybrids are easier to capture than parental species. Genotype-enviroment interactions usually affect crossbreeds and hybrids more than strains and lines, except in species that can be raised on natural or on artificial feeds. INTRODUCTION Most research on genetics and breeding of cultured fish has occurred during the last two decades. Strain evaluation has received much attention because of the availability of strains from separate geographic locations that possess different breeding histories and characteristics. Heritabillty estimates for several traits in many species have been determined, but few responses to selection have been measured. The effects of intraspecific crossbreeding has received some attention. Several species of fish have been cultured, and interspecific hybridization has also been an active area of research in aquaculture. A comparison of the relative success of strain evaluations, mass selection, intraspecific crossbreeding and interspecific hybridization is reviewed. STRAIN EVALUATION Aquaculture is a new animal husbandry. Since little breeding of fish has occurred, genetic changes of cultured fish as they are domesticated can be observed. The ancestry of farmed fish and the genomes originally introduced 391

2 from the native environment could affect the success of selection and crossbreeding programs. Consequently, knowledge of breeding histories may allow more efficient evaluation of breeding programs. The majority of cultured channel catfish, Ictalurus punctatus, originate from the Red River, Oklahoma (Dunham and Smitherman, 1984), and rainbow trout, Salmo gairdneri, from the McCloud River, California (Kincaid, 1981). Although the ancestry of most strains of channel catfish and rainbow trout can be traced to single collections, several strains of lesser impact from other river systems have since been introduced to cultured populations. Inbreeding is probably not a factor in selection and crossbreeding responses in channel catfish, rainbow trout, and other cultured fishes. Large strain differences exist for several traits of cultured fishes. Channel catfish strains differ in growth, disease resistance, body conformation, dressing percentage, seinability, vulnerability to angling, age of maturity, time of spawning, fecundity and egg size (Dunham and Smitherman, 1984; Smitherman and Dunham, 1985). Like catfish, salmonid strains of Atlantic salmon, Salmo salar, brook trout, Salvelinus fontlnalis, brown trout, Salmo trutta, and rainbow trout differ for growth, disease resistance, resistance to acid water, body conformation, vulnerability to angling and age of maturity (Kincaid, 1981). Tilapia nilotica strains vary for growth rate of reproduction, seinability and cold tolerance (Khater, 1985). Cold tolerance is related to geographic origin in terms of distance from the equator. Divergent strains of common carp, Cyprlnus carpio, also vary for growth, disease resistance, reproduction and harvestability (Hines et al., 1974, Moav et al., 1975; Wohlfarth et al., 1975ab). Domestication is one of the most important factors causing divergence among strains. The domestic environment provides selective pressure on fish introduced from the wild even without intentional selection by hatchery managers. In all comparisons between domestic and wild strains of channel catfish and rainbow trout, domestic fish grew faster than progeny from wild fish (Embody and Hayford, 1925; Greene, 1951; Flick and Webster, 1976; Kincaid, 1981; Smitherman and Dunham, 1985). Domestic trout also have deeper bodies (Greene, 1951; Flick and Webster, 1976) and greater survival (Embody and Hayford, 1925; Greene, 1951) than wild trout in the hatchery environment. Domestication without artificial selection increases growth rate in catfish 3-6% per generation (Smitherman and Dunham, 1985). The faster growth of the wild Nile strain of T. nilotica (Khater, 1985) is the only exception to the growth relationship between domestic and wild strains. This species is also unusual because it lacks additive genetic variation for increased body weight (Tave and Smitherman, 1980; Hulata et al., 1986; Teichert-Coddington and Smitherman, 1986). Behavioral differences exist between domestic and wild fish. Progeny of wild fish are more wary. Domestic trout were more vulnerable to angling than wild trout (Greene, 1951; Kincaid 1981). The wild Nile strain of T. nilotica exhibits more nervousness, jumps more and is more difficult to seine than domesticated T. nilotica (Khater, 1985). 392

3 SELECTION Over 200 heritability estimates have been obtained for several traits of cultured fishes (Tave, 1986), but few responses to selection have been measured. In general, selection programs have been successful for cultured fishes. Body weight of channel catfish has been improved 12-20% by 1 to 2 generations of mass selection (Dunham and Smitherman, 1983a; Bondari, 1983; Smitherman and Dunham, 1985). Selection has been successful in all eight lines of channel catfish that were evaluated, and the fastest growing strains yielded the fastest growing select lines (Smitherman and Dunham, 1985). The best select line grew twice as fast as average strains of channel catfish (Patino, 1986). Selection for body weight in salmonids has been successful. Embody and Hayford (1925) increased growth rate in brook trout through selection. Six generations of selection increased body weight by 30% in rainbow trout (Kincaid, 1983). One generation of selection increased body weight by 7% in Atlantic salmon (Gjedrem, 1979). Five generations of bidirectional selection did not improve body weight of common carp in Israel, but body weight was decreased in the line selected for small body size (Moav and Wohlfarth, 1976). However, different strains of common carp may possess varying amounts of additive genetic variation. Smisek (1979) estimated heritabilities for body weight of in a Czechoslovakian strain of common carp. Mass selection has improved body weight in an inbred line of mossambica (Ch'ang, 1971), and increased body weight 15% in aurea (Bondari et al., 1983). However, selection for increased body weight in T. nilotica has not been successful (Hulata et al., 1986; Teichert-Coddington and Smitherman, 1986). Teichert-Coddington and Smitherman (1986) were able to decrease body weight of T. nilotica, just as Moav and Wohlfarth (1976) were able to decrease body weight in common carp that did not respond to selection for increased body weight. Selection for body conformation in common carp has also been advantageous. Heritability for body depth was high (Ankorion, 1966) and deep-bodied lines have been developed. Resistance to dropsy has been improved for common carp through selection (Kirpichnikov et al., 1979). Mass selection has also improved the resistance of brook trout and brown trout to Aeromonas salmonicida (Ehlinger, 1964; 1977). Age of maturity has been genetically altered in salmonids. Fall, winter and spring spawning lines of rainbow trout have been developed. Atlantic salmon responded to individual selection for age of maturity (Gjerde, 1984); the h^ based on the offspring-dam regression was Although responses to selection have not been measured for several traits of cultured fishes, heritability estimates indicate many characteristics could be improved via selection (Tave, 1986). In general, heritabilities for fat percentage; tolerance of acid, formalin and heavy metals are greater than 0.3; for dressing percentage and early survival are less than 0.15, and for reproductive traits are variable. Heritabilities also vary with age and environment (Tave, 1986). 393

4 Correlated responses to selection have generally been positive. Increased fecundity, fry survival and disease resistance were correlated responses to selection for increased body weight in channel catfish (Dunham and Smitherman, 1983a; Smitherman and Dunham, 1985). Lines of catfish selected for large body weight have correlated increases in food consumption and feed conversion efficiency (Dunham and Smitherman, 1983a; Patino, 1986). Increased percentage hatch and fry survival were correlated responses to selection for increased body weight in rainbow trout (Kincaid et al., 1977). However, brook trout resistant to furunculosis were more susceptible to gill disease (Ehlinger, 1977). Fast growing common carp have increased seine escapability (Moav and Wohlfarth, 1970), but this relationship does not exist for catfish (Smitherman and Dunham, 1985). INTRASPECIFIC CROSSBREEDING Intraspecific crossbreeding yields more variable results than selection for improving body weight. Usually, both catfish (Dunham and Smitherman, 1983b) and rainbow trout (Gall 1969; 1975; Klupp et al., 1978; Ayles and Baker, 1983; Linder et al., 1983) crossbreeds express dominance and grow as fast as the best parent strain. Heterotic growth in excess of both parent strains occurred in 55% and 22% of channel catfish and rainbow trout crossbreeds, respectively. Common carp crossbreeds also expressed heterosis in a low percentage of the crosses examined (Moav et al., 1964; Moav and Wohlfarth, 1974; Nagy et al., 1984). All six possible T. nilotica crossbreeds from three strains expressed dominance and grew at the rate of their best parent strain (Khater, 1985). Domestication is also an important factor in crossbreeding of catfish. Domestic X domestic channel catfish crosses were more likley to exhibit heterotic rates of growth than domestic X wild crosses (Dunham and Smitherman, 1983b). Domestic crosses of rainbow trout were also more likely to express heterosis than domestic X wild crosses (Gall, 1969; Gall and Gross, 1978; Kincaid, 1981, Ayles and Baker, 1983). Combining abilities vary among channel catfish strains, and reciprocal crossbreeds do not grow at the same rate (Dunham and Smitherman, 1983b). Crossbreeds of Auburn strain females have expressed heterosis while crossbreeds of Auburn males have expressed no heterosis or negative heterosis. Ayles and Baker (1983) also demonstrated that reciprocal crossbred rainbow trout have different rates of growth. Crossbred common carp and channel catfish also have increased resistance to several diseases (Hines et al., 1974; Smitherman and Dunham, 1985). Improved survival was exhibited by 54% of rainbow trout crossbreeds (Ayles and Baker, 1983). Crossbreeding also increased resistance to low ph in brook trout (Kincaid, 1981). Some reproductive characters of fish are improved through crossbreeding. Early sexual maturity was expressed by crossbred catfish (Smitherman and Dunham, 1985). Gall (1969;1975) observed heterosis for egg size in crossbred rainbow trout. 394

5 Mating incompatabilities can limit the spawning frequency in some interstrain matings in species in which gametes are difficult to strip. Smitherman and Dunham (1985) observed mating incompatabilities for some crosses of channel catfish; the female strain was dictating spawning date. Intra-strain matings of T. nilotica also resulted in greater spawning frequency and fry production than inter-strain matings (Khater, 1985). INTERSPECIFIC HYBRIDIZATION Interspecific hybridization has been attempted extensively for cultured fish, but it has produced few positive results. The channel catfish female X blue catfish, I. furcatus, male hybrid is the only one of 28 catfish hybrids evaluated with commercial potential (Smitherman and Dunham, 1985). This hybrid has increased growth, growth uniformity, disease resistance, tolerance of low oxygen, dressing percentage and harvestability. However, mating blocks between the two species has prevented their commercial utilization. Like reciprocal catfish crossbreeds, the growth and characteristics of reciprocal catfish hybrids differ. Paternal predominance, the hybrid possessing traits more like the male parent than its reciprocal, was observed in blue-channel hybrids (Dunham et al., 1982). Interspecific carp hybrids have not expressed heterosis. The grass carp, Ctenopharyngodon idella, X bighead carp, Aristichthys nobilis, exhibits poor growth and weed control (Young et al., 1983). Reciprocal bighead x silver carp, Hypothalmichthys molitrlx, hybrids failed to express positive heterotic growth or plankton control (Green and Smitherman, 1984). Several salmonid hybrids have been produced. As expected, salmonid hybrids were more viable when made within genera than between genera (Chevassus, 1979). Salmonid hybrids do not express heterosis for growth rate. Some diploid salmonid hybrids are potentially valuable because of disease resistance inherited from the parent species that is usually not cultured, but these hybrids have low viability. The induction of triploidy can increase the hatchability of these potentially important hybrids (Parsons et al., 1986). Hybridization among tilapia may result in all male progeny (Mires, 1977). Some species are homogametic for maleness, and other species are homogametic for femaleness (Chen, 1969). When homogametic males are hybridized with homogametic females, monosex male progeny are produced. Monosex male populations are desirable because of the early sexual maturation and subsequent unwanted reproduction in mixed-sex tilapia culture ponds, and the slow growth rate of females. Although few good evaluations of the growth rate of tilapia hybrids exist, apparently there was no heterosis for growth rate when monosex hybrids were compared to the males of the parent species (Lovshin and DaSilva, 1975; Hanson et al., 1983). The mean of the male hybrids may be higher than that for the combined sexes of the parent species. Reproductive isolating mechanisms limit the success of interspecific matings, but the use of tilapia hybrids is common in Israel. Backcrosses between species has potential for transferring genes. and Smitherman (1984) developed more cold-tolerant red tilapia through introgressive hybridization. Behrends 3 9 5

6 Generally, interspecific hybrids are extremely catchable. Channel-blue catfish hybrids (Smitherman and Dunham, 1985), Chinese carp hybrids (Green and Smitherman, 1984) and tilapia hybrids (Wohlfarth and Hulata, 1981) were as much or more seinable than their most catchable parent species. Channel-blue catfish hybrids (Smitherman and Dunham, 1985) and centrarchid hybrids (Childers, 1967) expressed heterosis for vulnerability to angling. Centrarchid hybrids exhibited such aggressiveness that up to 90% of a hybrid population was caught by angling in a three-day period in an 8-ha lake (Childers, 1967). GENOTYPE-ENVIRONMENT INTERACTIONS Genotype-environment interactions are more important for crossbreeds and hybrids of fishes than strains or select lines. Strains and select lines of catfish performed similarly in aquaria, cages and ponds (Smitherman and Dunham, 1985). Channel catfish selected for increased body weight at one stocking density in ponds also grew faster than their control populations at other stocking densities (Brummett, 1986). Similar results have been obtained for common carp. Relative growth rates of several strains of common carp remained constant in three widely different environments (Suzuki et al., 1976). Gunnes and Gjedrem (1978; 1981) found little evidence for genotype-environment interactions among strains of both Atlantic salmon and rainbow trout. However, distinct interactions among strains of rainbow trout were observed for different stocking densities (Gall, 1969), different ponds (Ayles, 1975) and different culture units, cages and ponds (Klupp et al., 1983). However, genetic rank did not change in different ponds (Ayles, 1975), and the same strain grew the fastest in both cages in ponds (Klupp et al., 1978). Crossbred and hybrid catfish exhibited genotype-environment interactions when grown in aquaria, cages and ponds (Smitherman and Dunham, 1985). Interactions for the channel x blue catfish hybrid were related to its nervous behavior and tolerance of low oxygen environments. Steffens (1974) also found genotype-environment interactions for crossbred common carp and Klupp et al. (1978) found interactions for crossbred rainbow trout grown in cages and ponds. Genotype-environment interactions are more likely to occur for cultured fish that can be grown with natural foods and artificial feeds. This type of interaction has been observed in both strains of common carp (Moav et al., 1975; Wohlfarth et al., 1983) and strains of T. nilotlca (Khater, 1985) when the fish were grown in ponds fertilized with manure or in ponds receiving pelleted fish feed. REFERENCES ANKORION, Y Investigations on the heredity of some morphological traits in the common carp, Cyprinus carpio L. M.S. thesis, the Hebrew University, Jerusalem. (in Hebrew). AYLES, G. B Influence of the genotype and the environment on growth and survival of rainbow trout (Salmo gairdneri) in central Canadian aquaculture lakes. Aquaculture Jj,

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