Department of Fisheries and Allied Aquacultures
Alabama Agricultural Experiment Station
Auburn University, Auburn, Alabama 36849
ABSTRACT Commercial production and processing of cultured catfish in the United States was begun in the 1950s in Alabama, and in 1960 the entire industry consisted of only 182 ha of farm ponds and production of 272,727 kg of catfish. In 1992, fish farms, mainly in Mississippi, Alabama, Arkansas, and Louisiana, produced 228,683,500 kg of catfish for processing. Catfish presently accounts for more than half of all U.S. aquaculture yield. During the period 1960-1992, commercial catch of catfish from natural waters declined from 17,456,363 to 4,931,818 kg. This trend was attributed to consumer concerns about safety of fish products from public waters and increasingly competitive marketing and pricing by the farm-raised catfish industry.
Catfish are mainly produced in ponds; some state and federal agencies stock hatchery-reared fish in public waters. Cultured catfish are sometimes lost during flooding from aquaculture facilities to natural waters, where genetic influences and competitive interactions might impact indigenous populations of catfish. However, these influences and interactions have not been evaluated. More significant influences are those manifested when mature catfish are captured from the wild and added to broodstock populations on farms. Through natural selection and genetic drift, gene frequencies are significantly altered when the wild fish are reproduced and grown in farm ponds under intensive management.
On-farm domestication, with no directed selection, has resulted in performance increases of approximately 5% per generation; directed mass selection has increased body weight by 30% in 3 generations. Crossbreeding between certain strains improved catfish yields by 15% in a single generation; hybridization between species of catfishes improved growth and food conversion by 20% and significantly elevated harvestability by seining and angling.
Genetic engineering efforts through electroporation and microinjection of catfish embryos with genetic material from salmonids have resulted in families of progeny with 40% improvement in growth.
Transgenic fish, projected to be less fit and competitive than individuals in naturally occurring catfish populations, are still secured in U.S. government-approved confinement facilities and will not be released to farms or the wild until several geneations of evaluation on inheritance and expression have been completed.
INTRODUCTION Commercial production and processing of cultured catfish in the United States was begun in Alabama (Swingle 1959); in 1960 the entire industry consisted of only 182 ha of farm ponds and production of 272,727 kg of catfish (Ivers 1981). In 1992, fish farms, mainly in Mississippi, Alabama, Arkansas and Louisiana, produced 228,683,500 kg of catfish for processing (USDA 1993). Catfish presently accounts for more than half of all U.S. aquaculture yield. During the period 1960-1992, commercial catch of catfish from natural waters declined from 17,456,363 to 4,931,818 kg (USDC 1993). This trend was attributed to consumer concerns about safety of fish products from public waters and increasingly competitive marketing and pricing by the farm-raised catfish industry.
Catfish are mainly produced in private ponds; some state and federal agencies stock hatchery-reared fish in public waters. Cultured catfish are sometimes lost during flooding from aquaculture facilities to natural waters, where genetic influences and competitive interactions might impact indigenous populations of catfish. However, these influences and interactions have not been evaluated. This paper will review what is known about genetics and performance of domesticated channel catfish, Ictalurus punctatus, and discuss possible effects of genetically altered, domesticated catfish on naturally occurring catfish populations.
ANCESTRY OF DOMESTICATED CHANNEL CATFISH
Catfish have been important commercial and sport fish for many years. The first known spawning of channel catfish in captivity occurred in 1892 (Leary 1908). The Kansas State Fish Hatchery at Pratt began propagating channel catfish as early as 1910. Although channel catfish having ancestry from many river systems are currently propagated (Dunham and Smitherman 1984), the majority of the stock originated near the Denison Dam, Lake Texoma, Oklahoma. These fish were captured in 1949 by the Arkansas Game and Fish Commission in pools formed in the Red River behind Denison Dam after its construction. The fish were spawned in the Arkansas state hatchery system and were the basis of broodstock for some of the earliest catfish farms such as Leon Hill, Edgar Farmer, Anderson-Nelson, and War Eagle Minnow. These fish were also some of the founder stocks in federal hatcheries and research institutions in Alabama, Arkansas, Louisiana, and Mississippi. They were widely distributed in Arkansas and Mississippi via the Hill and Farmer operations. Probably one-half of the Auburn University founder stock and all of the Marion National Fish Hatchery, and Stephens, Inc., founder stock came from Anderson-Nelson or War Eagle Minnow Farm. In turn, Auburn University, Marion National Fish Hatchery, or Stephens, Inc., provided stock for the majority of catfish farms in Alabama. Thus, the ancestry o stocks for the majority of catfish cultured in Alabama, Arkansas, Louisiana, and Mississippi (locations of 95 % of the U.S. pond area devoted to catfish farming) can be traced to a single source of fish: Red River, Denison Dam, Oklahoma.
A number of other stocks have had major impact on the gene pools in Arkansas and Mississippi. Two major fingerling farms in Mississippi, Thompson-Anderson and Transfisheries, have widely distributed fish traced primarily to the Yazoo River and, to a lesser degree, Red River and Kansas. Several farmers have also obtained stock from the Rio Grande River, Texas, or from the Mississippi River, Mississippi. The first catfish farm in Mississippi (V. C. Hammett) used fish captured from the Mississippi River. This influx of "new blood" and the large brood populations used by commercial operations has probably minimized inbreeding in commercial operations.
Another widely distributed stock originated from state and federal fish hatcheries in Kansas, Oklahoma, and Texas. These fish came from many rivers within each state and were exchanged among hatcheries. This stock is common in Kansas, Oklahoma, and Texas and is closely related to Alabama stocks via distribution by Auburn University.
The most widely distributed stock on commercial farms in California is from the Mississippi River, via Osage Fisheries, Missouri. A lesser proportion of stock originated from Kansas.
DEVELOPMENT OF DOMESTICATED CATFISH
Strain EvaluationAcquisition of the best available strains is one of the quickest ways to improve the quality of broodstock. A strain is a breeding population having a similar history and possessing unique characteristics.
Strains of channel catfish originating from different geographic locations within the United States differ in growth rate, and domesticated strains grow faster than wild strains (Smitherman and Pardue 1974, Chappell 1979, Green et al. 1979, Youngblood 1980, Dunham and Smitherman 1981). The domestication process increases growth rate 2 to 6% per generation (Dunham and Smitherman 1983b). Differences exist in growth rate during winter (Dunham and Smitherman 1981) as well as during summer. Variation of length is more pronounced in some strains than others (Brooks 1977). The fast growth of some strains is caused by a combination of increased feed conversion efficiency (Chappell 1979) and increased feed consumption (Al-Ahmad 1983).
Strains also differ in resistance to viral, bacterial (Plumb et al. 1975, Dunham 1981), and parasitic infections (Shrestha 1977). The oldest domestic strain, Kansas, is one of the fastest growing and most disease resistant strains (Dunham and Smitherman 1984). In contrast, the Rio Grande strain is susceptible to several diseases--channel catfish virus disease (Plumb et al. 1975), Ichthyopthirius, and Flexibacter columnaris (Dunham and Smitherman 1984) This susceptibility has been observed at more than one geographic location (Broussard 1979, Dunham and Smitherman 1984). Variation in total hemoglobin among strains may be correlated to disease resistance (Taylor et al. 1984). No relationship was apparent between hematocrit and disease resistance among strains.
Dressing percentage varies among strains. Differences in body conformation among strains was correlated to differences in dress-out percentage among strains (Dunham et al. 1983).
Seine escapeability varies among strains (Dunham and Smitherman 1984). Some evidence exists that the practice of retaining progenitors from fish remaining in a pond after most of the population has been removed by hook and line or seining results in populations difficult to capture by seine.
Time of spawning is a dramatic example of strain variation. A Minnesota strain from the St. Louis River, a feeder stream of Lake Superior, spawned 10 to 12 days earlier than most strains. The Rio Grande strain from Texas spawned approximately 2 weeks later than most strains when both were located at College Station, Texas (Broussard and Stickney 1981). A north-south trend in spawning date was apparent. Smitherman et al. (1984) also found that the strain of female was important in determining spawning date and may impede crossbreeding success. Other reproductive characteristics, such as age of maturation, are distinctive. For example, Rio Grande mature a year earlier, at 2 years of age, than most strains. In contrast, Kansas matures a year later, at 4 years of age, than most strains.
Crossbreeding
Crossbreeding is a breeding program that can produce immediate improvement through heterosis or hybrid vigor. Crossbreeding has improved body weight in channel catfish, but the tested strains had different combining abilities; 55% of the crosses resulted in positive overdominant growth by the P1 (Dunham and Smitherman 1983a). Domestic x domestic crosses were more likely to give positive heterosis (80%) than domestic x wild crosses (33%). Four domestic crosses resulted in positive heterosis, and one resulted in negative heterosis. Domestic x wild crosses were more likely to result in fish with growth rates intermediate to their parent strains, two exhibiting positive heterosis, three intermediate to their parents, and one exhibiting negative heterosis. Nine of eleven crossbreeds grew better than at least one of their parents (Dunham and Smitherman 1983a).
Reciprocal crossbreeds did not grow at the same rate (Dunham and Smitherman 1983a). Males and females of specific strains had different combining abilities with other strains. Crossbreeds from Auburn female channel catfish grew faster than crossbreeds from Auburn male channel catfish. A maternal effect for combining ability was evident. Bondari (1983) also observed this maternal effect for females utilized for crossbreeding that had been bidirectionally selected for body weight.
Crossbreeding can also affect resistance to disease; increased resistance to bacterial (Dunham and Smitherman 1984), viral (Plumb et al. 1975), and parasitic (Shrestha 1977) diseases has been expressed with channel catfish.
Crossbred and pure-strain channel catfish have been compared for spawning date, spawning rate (percent of replicate pairs that spawned), fecundity, egg size and hatchabilty, and survival of offspring (Dunham et al. 1983). Crossbred fish usually spawned earlier than pure-strain channel catfish. As 3-year-olds, crossbred fish had higher spawning rates and fecundity than purebred fish, and their fingerling output per kilogram of female parent was greater. Fingerling output/kg female is a good measure of reproductive efficiency and equals spawning percentage x fecundity x hatchability x fry survival. As 4-year-olds, pure-strain fish improved their performance, and crossbreeds lost most of their relative advantages (Dunham et al. 1983). As was the case for rate of growth, heterosis decreased with age. Broodstock derived from crosses of four strains spawned earlier than those from two-strain F2 crosses, but their surviving offspring were no more numerous. The main value of cross-strain breeding is to produce channel catfish that mature earlier in life and spawn earlier in the season than purebreds.
Hybridization
Hybridization between species is another form of crossbreeding. Different species of catfish have been grown, and they exhibit different culture traits. Channel catfish grow the fastest to harvestable size (Chappell 1979) and are the most disease resistant but are difficult to capture by seining. Blue catfish Ictalurus furcatus have superior dressing percentage (Chappell 1979), are very seinable (Chappell 1979), are more uniform in length (Brooks et al. 1982b), but are prone to disease. White catfish Ictalurus catus tolerate low dissolved oxygen, have the fastest growth during winter (Dunham and Smitherman 1981), but have slower growth to harvest size and poor dressing percentage (Chappell 1979) caused by a large head (Benchaken 1979). Channel catfish and white catfish become sexually dimorphic in size (males larger) at 6 months (Brooks et al. 1982a), but blue catfish exhibit no sexual dimorphism in size until they are older than 3 years (Dunham 1979).
Attempts have been made to take advantage of these specific characteristics and find crosses exhibiting heterotic growth rates through hybridization. Twenty-eight interspecific hybrids have been produced and evaluated for growth rate (Giudice 1966, Dupree and Green 1969, Yant et al. 1976, Dunham and Smitherman 1984). Only one interspecific hybrid, channel catfish female x blue catfish male, has shown increase in body weight of 20 percent above that of channel catfish (Giudice 1966, Yant et al. 1976, Smitherman et al. 1983). The feed conversion efficiency of channel x blue hybrids was also 11 to 14 percent better than channel catfish. The reciprocal, blue x channel, does not exhibit heterotic rates of growth (Dunham and Smitherman 1987). Dupree and Green (1969) indicated that the channel x white hybrid exhibited superior growth in aquarium studies. Chappell (1979) found that compared to channel catfish and the channel x blue, the channel x white hybrid catfish grew slowly to harvestable size in ponds.
Use of the channel x blue hybrid could reduce losses of cultured catfish due to oxygen depletion. This hybrid exhibits heterosis for resistance to critically low oxygen levels (Dunham et al. 1983). When 90 percent of a channel catfish population succumbed due to low dissolved oxygen, only 50 percent of the hybrids died. When 50 percent of a channel catfish population died from low dissolved oxygen, only 10 percent of the hybrids died.
Fishing sucess in catfish fee-fishing ponds could be improved by stocking the channel catfish x blue catfish hybrid. Fee-fishing ponds are an important part of the catfish industry in the United States (McCoy and Crawford 1975). Fee-fishing ponds provide both a source of income for the pond owner and a source of recreation and protein for the public. Any management program that could improve fishing success would be beneficial to both parties. Reciprocal channel-blue hybrids are more catchable by hook and line than their parent species (Tave et al. 1981). The channel x blue was the most catchable reciprocal. The parent species did not differ. The channel x blue is also much easier to catch by seining (Dunham and Smitherman 1987), as well as by hook and line, compared to channel catfish.
Yant et al. (1976) found that dress-out percentage was higher in the channel x blue hybrid than in channel catfish. The average dress-out percentage for the hybrid catfish was 64.5 and for the channel catfish was 61.2. Again, blue x channel did not exhibit heterosis and had lower dress-out percentage than its reciprocal (Chappell 1979). The higher dress-out percentage of the channel x blue channel hybrid may be related to its deep body conformation and small head. In contrast, the blue x channel, channel x white, and white x blue hybrids have tremendous fat deposits in the viscera (Chappell 1979), and this appears to cause poor dressing percentage in these hybrids. Abnormal sexual development is also associated with these fat deposits (LeGrande et al. 1984).
Mass Selection
Dunham and Smitherman (1983b) determined the response to selection and realized heritability for body weight of channel catfish, grown in earthen ponds at 7,500 fish/ha. One generation of mass or individual selection for increased body weight has been successful in all channel catfish populations evaluated (Dunham and Smitherman 1983b). Bondari (1983) also obtained significant response to selection in the Tifton strain in tanks by using a combination of crossbreeding, family selection, and mass selection. The largest 10 percent of each population was selected in these experiments.
Responses to selection of 63, 73, and 54 g (17, 18, and 12% increase in body weight) were obtained (Dunham and Smitherman 1983b) from Rio Grande, Marion, and Kansas strains, respectively, for the fish grown in ponds. Generally, the fish with shorter periods of domestication had greater response. Realized heritability for Marion, 0.50 + 0.13, was higher than that for Rio Grande, 0.24 + 0.06. Kansas, 0.33 + 0.10, did not differ from Rio Grande or Marion. Responses for male and female body weights were the same in Marion, but responses by Kansas males was higher than for Kansas females. There were no significant differences in realized heritabilities for male and female body weights.
Selection for body weight for three generations has further improved growth of channel catfish (Rezk 1993). Three generations of selection resulted in selected lines of Kansas and Marion strains growing 30 and 20 percent faster, respectively, than controls.
Mass selection improved body weight (7 to 10% per generation) in channel catfish more rapidly than the domestication process. Domesticated strains grow faster than wild strains (Smitherman and Green 1973, Burnside et al. 1975, Broussard 1979, Chappell 1979, reen et al. 1979, Dunham and Smitherman 1981). The average increased growth performance of hatchery fish (five strains) over wild fish (six strains) is 3 percent per generation (Smitherman and Green 1973, Smitherman and Pardue 1974, Burnside et al. 1975, Broussard 1979, Chappell 1979, Green et al. 1979). Assuming slow turnover of brood and long generation intervals, 6 percent improvement per generation has occurred. Mass selection for three generations improved growth rate two to three times faster than that which had occurred during 3 to 15 generations of domestication (undirected selection).
Genetic Engineering
Human and salmonid growth hormone genes have been transferred to channel catfish (Dunham et al. 1987, 1992). One to nine copies of the foreign DNA were inserted in either head-to-tail tandem array at single insertion sites or single copies at multiple insertion sites. All F1 transgenic catfish evaluated produced salmonid growth hormone regardless of the construct. The spawning rate and fertility of these P1 transgenics in artificial spawning conditions was comparable to that of normal channel catfish. In 2 of 3 years, 100 percent spawning and 100 percent hatch were obtained. Percent transgenic progeny observed in the five matings were 20, 52, 7, 47, and 0, which was lower than the 75 percent inheritance expected assuming the F1 broodstock had at least one copy of the foreign gene integrated and were not mosaics in the germ-line. At least 7 of 10 F1 were mosaics, and a minimum of 2 of 10 F1 did not possess the salmonid growth hormone gene in their germ-line. F1 transgenics grew at the same rate as their non-transgenic full-siblings, which was not surprising since the F1 were mosaics. F1 transgenic progeny in two families grew 26 percent faster to 40-50 g than their non-transgenic full-siblings when evaluated communally. F1 progeny groups grew at the same rate as normal full-siblings when grown communally to 25 g. In families where F1 progeny grew faster than controls, the range in body weight and coefficient of variation for the transgenic full-siblings was less than that for controls. In families where F1 progeny grew at the same rate as controls, range in body weight and coefficient of variation were similar for transgenic and normal individuals. The percent deformities observed in P1 transgenics, 13.6 percent, was higher than in microinjected P1 non-transgenics, 5.1 percent. Percent deformities in transgenic and control F1 channel catfish was not different, 0.5 and 2.8, respectively.
Results obtained for F1 transgenic common carp (Zhang et al. 1990) and channel catfish containing salmonid trout growth hormone gene were similar. The presence or absence of increased growth can vary among families and may be related to family effects, genetic background, epistasis, or dosage effects of the foreign growth gene expression. Apparently, a combination of both family selection as well as gene transfer is needed to optimize increased growth from the insertion of salmonid growth hormone genes.
COMPARISON OF DOMESICATED AND WILD POPULATIONS OF CATFISH
Domesticated catfish grow faster than wild catfish when cultured in ponds; production differences can be as great as 250 percent. Domestication has resulted in an average growth increase of 3 to 6 percent per generation. Survival of domestic and wild strains is similar in the aquaculture environment.
A large number of polymorphic enzyme loci have been observed for channel catfish (Hallerman et al. 1986, Hallerman 1984, Dunham and Smitherman 1984). Percentage of loci polymorphic, number of alleles/locus, and mean heterozygosity varied considerably among these domesticated populations but was less than that for the only wild population examined. Carmichael et al. (1992) observed similar results in the only other study of channel catfish isozyme variation; however, their sole wild population was non-variable. This illustrates the need to study the genetics of wild populations and their interactions with domestic populations thoroughly.
POSSIBLE EFFECTS OF CULTURED CATFISH ON NATURALLY OCCURRING POPULATIONS OF CATFISH
Effect of Domestication on Genetic VariabilityFish culture in the United States and the world is increasing rapidly. When salmonids are removed from the natural environment and placed in the culture environment, random genetic drift and domestication effects (new and greatly different selective forces act upon fish in the domestic environment compared to the natural environment) alter gene frequencies and reduce genetic variation as measured by isozyme analysis (Allendorf and Phelps 1980, Hallerman et al. 1986, Khana et al. 1975, Koljonen 1989, Ryman and Stahl 1980, Stahl 1983). Then the domesticated populations with reduced genetic variability are propagated in large numbers, sometimes reaching population numbers much greater than that found in natural populations. Purposeful or accidental (such as flooding or escape during harvest) introduction of the domestic fish may then allow mixing of the domestic and natural populations resulting in either greater or less variability depending on the competitiveness, survival, and reproduction of the introduced and natural populations.
Competition Between Domestic and Wild Fish
Initial studies indicate that wild fish generally outcompete domestic strains of fish in the natural environment. Almost all of these oservations were on salmonids (Maclean et al. 1981; Buettner 1962; Flick and Webster 1962; Fraser 1974, 1981; Gordon and Nicola 1970; Flick and Webster 1964, 1976), and coldwater fish and were localized experiments or observations. Larger scale examples with more species, including warmwater species such as channel catfish, are needed to determine the interactions between wild and domestic fish and if wild catfish generally outcompete domestic catfish in the natural environment as seen with salmonids.
Reproductively Isolated Sympatric Populations
Another possible interaction between domestic and wild populations of fish is the establishment of sympatric, but reproductively isolated, populations. Although strains of fish usually do not have reproductive isolating mechanisms preventing them from interbreeding, occasionally behavioral mating blocks prevent or decrease the rate of inter-strain matings. We have found that Marion channel catfish females preferentially mated with their own strain rather than Kansas males (Smitherman et al. 1984), and Ghana strain of Oreochromis niloticus was more likely to mate with its own strain than other strains (Smitherman et al. 1988). The existence of reproductively isolated, sympatric populations of trout (Lerder et al. 1984, Brown et al. 1981, Ryman and Stahl 1981), especially brown trout, Salmo trutta, is well-documented. Some strains of domestic and wild rainbow trout are sympatric but reproductively isolated or near to reproductive isolation. This occurs because of behavioral differences, including temporal or spatial differences in spawning (Smitherman et al. 1988).
The relationships studied for domesticated and wild populations of salmonids have not been evaluated for catfish. Survival, growth, behavior, and reproductive success of genetically altered populations must be studied to assess the fitness of cultured catfish in the natural environment.
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