Journal or Publishing Institution: CABI Publishing
Author(s): Dunham, R.A.
Article Type: Book
Record ID: 575
Abstract: Improvements in the performance of fish species used in aquaculture are being accomplished using a variety of approaches, including both traditional and molecular genetic methodologies. Historical gains in productivity have been achieved by domestication, selection, interspecific and interstrain crossbreeding, polyploidy, and synthesis of monosex populations. More recently, transgenesis has been explored as a technique to enhance growth rate and other performance characteristics.
Domestication of species, with directed selection, can yield improvement in production characteristics. Domesticated strains of farmed fish usually grow faster than wild strains, and this effect can be achieved fairly rapidly: for example in channel catfish, Ictalurus punctatus, domestication can improve the growth rate by approximately 2-6% per generation. In contrast, directed selection (mass selection) for body weight in fish has result in an up to 55% increase in body weight after four to ten generations of selection. In channel catfish, correlated responses to selection include higher dressing percentage, but a decreased ability to tolerate low concentrations of dissolved oxygen.
Intraspecific crossbreeding can increase growth in channel catfish, common carp and salmonids, but crossbreeding does not always result in heterosis. Interspecific hybridization seldom results in overdominant performance in fish. However, one catfish hybrid, channel catfish female x blue catfish (I. furcatus) male, exhibits improved performance for several traits including growth, disease resistance, survival, tolerance of low dissolved oxygen, angling vulnerability, seinability, dressing and fillet %.
Ploidy manipulation and sex-control technologies have also played an important role in enhancing production performance. Induction of triploidy does not improve performance in cat hybrids, but in salmonids triploidy can enhance flesh quality by preventing sexual maturation, although growth rate is somewhat reduced relative to diploids. Monosex male populations can increase growth rate in some strains of channel catfish, and monosex female populations of salmon have a reduced incidence of precocious maturation and thus overall improved flesh quality.
In comparison with traditional selective breeding, transgenesis in channel catfish can increase the growth rate by 30-40% by the introduction of salmonid growth hormone (GH) genes. For several species of salmonids, insertion of GH transgenes can result in dramatic weight increases of up to 11-fold after 1 year of growth. A variety of effects on commercially important characteristics other than growth are also observed in GH transgenic fish. Feed conversion efficiency is enhanced in transgenic catfish, common carp (Cyprinus carpio) and salmonids, an effect also observed in catfish improved for growth by traditional breeding approaches. Transgenic catfish and carp have increased protein levels and decreased fat, however, alterations in ratios among amino acids and among fatty acids in the flesh are slight or non-existent. Transgenic catfish demonstrate improved flavour and sensory scores, and GH transgenic common carp display improvements in dressing %.
Due to the potential for farmed transgenic fish to escape into natural ecosystems and breed with wild conspecifics, research has also been conducted into examining the fitness (morphological, physiological and behavioural characteristics) of transgenic animals relative to wild-type. As has been observed in other transgenic systems, body shape and physiological performance is altered in transgenic fish. In common carp, transgenic animals have larger heads, and deeper and thicker bodies. In GH transgenic salmonids, morphological disruptions analagous to acromegaly can be observed in the cranium of individuals with extraordinary growth rates. While reproductive traits are not affected in transgenic channel catfish and common carp, GH transgenic common carp display enhanced disease resistance and tolerance of oxygen levels which could affect their ability to survive in natural systems. Foraging ability of transgenic and control catfish is similar, and under conditions of competition and natural food source, as would be the case in nature, growth is not different between transgenic and control catfish. Predator avoidance was also slightly impaired for GH transgenic catfish compared with control individuals. Swimming ability is reduced in transgenic salmon, which has implications for foraging ability and predator avoidance, as well as ability to complete arduous river migrations for spawning. Although transgenic fish may be released to nature by accident, it appears that ecological effects of transgenic fish developed and evaluated to date will be unlikely because of these examples of reduced fitness. However, each new variety of transgenic fish should be evaluated for potential environmental risk prior to utilization in aquaculture.
Great potential exists to improve production characteristics by transgenic and other molecular genetic approaches. However, future genetic improvements will continued to be achieved from traditional approaches, and, by utilizing a combination of both approaches simultaneously, maximum genetic gain should be accomplished.
Keywords: aquaculture, intraspecific crossbreeding, transgenic fish
Citation: Dunham, R.A., 1999. Comparison of traditional breeding and transgenesis in farmed fish with implications for growth enhancement and fitness. In J.D. Murray, G.B. Anderson, A.M. Oberbauer, M.M. McGloughlin (eds.), Transgenic Animals in Agriculture. New York: CABI Publishing, pp.209-229.
