WORLD AQUACULTURE 11 were improved by 9 percent and 14 percent, respectively. Further, feed conversion ratio (FCR) was improved by 20 percent after five generations of selection. In terrestrial livestock, genetic gain in growth rate obtained in selection programs is typically 5 percent per generation or 1-2 percent per year, which is 5-6 times less than in aquatic species (Gjedrem and Baranski 2009). It is also possible to obtain high genetic gain in fish by selecting for disease resistance. This has been documented for resistance against IPN virus in Atlantic salmon (Storset et al. 2007, Housten et al. 2008, Moen et al. 2009), pancreatic necrosis in rainbow trout (Okamoto et al. 1993), Areomonas hydrophila in rohu carp (Sahoo et al. 2011), and bacterial cold water disease in rainbow trout (Leeds et al. 2010). Applying challenge tests under standardized environmental conditions opens possibilities for developing resistant strains for specific diseases. It is only possible to obtain high genetic gains when breeding programs are well planned and carried out. The genetic gain (ΔG) per generation depends on three parameters: ΔG = i • h2 • σ p where i is selection intensity, h2 is heritability and σ p is phenotypic standard deviation. The primary reasons for the high genetic gain obtained for aquatic species are: 1. Fecundity is very high when females contribute thousands or even millions of eggs to each spawn. Selection intensity can, therefore, be very high. 2. Phenotypic and genetic variation is large for growth rate and most other traits of economic importance. 3. Heritability for economically important traits is of medium magnitude. Because the generation interval is rather low (compared to terrestrial livestock), the yearly genetic gain can be very high. Reasons for Few Breeding Programs in Aquaculture There may be many reasons for the infrequent use of breeding programs for aquatic species improvement. In general, aquaculture farmers have not adopted the genetic improvements obtained. Some of the reasons are: 1. We deal with small animals, each individual with a low economic value. 2. There are several examples of individual selection being performed over multiple generations using few broodstock without control of the relationship between mating animals. This has resulted in inbreeding, leading to high mortality and low performance. This leads to a loss of confidence in selective breeding among farmers. 3. Fry and fingerlings are easily available from wild stocks at a low price for a number of species. 4. Lack of knowledge in quantitative genetics and selective breeding among farmers and researchers. 5. Lack of knowledge of the high benefit/cost ratio of investing in breeding programs. 6. Lack of farmers’ co-operatives and private companies willing to develop breeding programs. 7. High investment required to begin family-based breeding programs. 8. Farmers are not willing to pay more for improved stocks. 9. National and international organizations have not advocated and stimulated development of breeding programs for aquatic species, with the World Fish Center (formerly ICLARM) a notable exception. Initiating Breeding Programs for Aquatic Species It is essential to start a breeding program with a broad genetic base. For species with ongoing breeding programs, such as those for the GIFT strain of Nile tilapia and Atlantic salmon, it is possible to get a ‘flying start’ by importing stock that has already been genetically improved. Most governments will require detailed health certificates for translocated animals and that quarantine and other conditions are met. This procedure can be advocated because genotype by environment interaction is low and insignificant for tilapia (Eknath et al. 1993), Atlantic salmon (Gunnes and Gjedrem 1978), rohu carp Labeo rohita (Reddy et al. 2002) and white shrimp Litopenaeus vannamei (Fjalestad et al. 1997). The alternative is to start from scratch by collecting animals of different origin to form a synthetic base population. There are two recommended breeding strategies: individual selection and family-based breeding programs. Individual selection may be efficient in improving growth rate. If mass spawning is used without knowledge of the relationship between mating animals, inbreeding will increase rapidly and (CONTINUED ON PAGE 10) TABLE 1. World aquaculture production of various species groups in 2009 (FAO 2011). Species group Production (million t) Value (US$ 1000) Value (US$ per kg) Freshwater fishes 30.64 44.19 1.44 Diadromous fishes 3.53 14.00 3.96 Marine fishes 1.95 7.10 3.64 Crustaceans 5.30 24.13 4.55 Molluscs 13.52 13.13 0.97 Miscellaneous aquatic animals 0.73 2.75 3.77 Aquatic plants 17.34 4.82 0.28 Total aquaculture production 73.02 110.12 1.51
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