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Common carp varieties resulting from selective breeding and genetic improvement
Common carp varieties resulting from selective breeding and genetic improvement
Courtesy of Fish Culture Research Institute, Szarvas, Hungary

In contrast with livestock and plant crops where improvements in production have been based on modern breeding approaches, only a few examples of such breeding programmes exist for fish (e.g. Atlantic salmon in Norway, Nile tilapia in Asia, channel catfish in the USA). Thus, principles of genetics can be applied to the farming of marine species to increase cost effectiveness through improved breeds. The application of such technologies can be divided into two broad groups: those for short-term and those for long-term improvement.

Different technologies

Hybridization, chromosome set manipulation, and sex reversal can be considered short-term improvements made over 1 - 2 generations; the improvements generally are non-cumulative, i.e. one time events. Selective breeding represents a long-term improvement programme where small gains accumulate over generations; gene transfer may be considered long-term where the gains may be substantial, but may not accumulate each generation. Many of these technologies can and should be combined and used together. Recently, there has been a call for the application of genetic technologies to reduce the risk of adverse environmental effects should a farmed species escape into the natural environment. At the same time, some genetic technologies are being criticized on moral and ethical grounds.

Several standard techniques for genetic improvement have not yielded good results with marine shrimp. Hybridization has been difficult because of pre-zygotic and post-zygotic reproductive isolating mechanisms; when it has been accomplished it has generally not produced heterosis in the F1 generation for either growth rate or disease resistance. Nor has hybridization been an effective means of combining desirable traits from different species. Chromosome manipulation (polyploidy) has not been practical, and selective breeding programmes have been hindered by difficult reproduction of key species, such as P. monodon, by difficulty in marking individuals, by low heritabilities for growth-related traits (that is, environmental components greatly affect growth) and by the generally low priority afforded genetic research by private industry. The majority of the shrimp culture industry relies on wild caught post-larvae because of their ease in collecting and their superior performance under culture conditions.

Genetic improvement strategies
Genetic manipulation Improvement
Selective breeding for:
Long-term strategies
growth rate As high as 50% increase after 10 gen. in coho salmon; gilthead sea bream mass selection gave 20% increase/generation; mass selection for live wt and SL in Chilean oysters found 10 - 13% gain in one generation
body confirmation High heritabilities in common carp, catfish and trout
physiological tolerance (stress) Rainbow trout selected for high response showed increased levels of plasma cortisol levels
disease resistance Increased resistance to dropsy in common carp but disease resistance difficult to select for
pollutant resistance Tilapia progeny from lines selected for resistance to heavy metals Hg, Cd, and Zn survived 3 - 5 times better than progeny from unexposed lines
maturity and time of spawning 60d advance in spawning date in rainbow trout
gene transfer Coho salmon with a growth hormone gene and promoter from sockeye salmon grew 11 times (0 - 37 range) as fast as non-transgenics Atlantic salmon grew 400% faster than normal during first year
Short-term strategies
Intra-specific crossbreeding Heterotic growth seen in 55 and 22% of channel catfish and rainbow trout crosses, respectively Chum salmon and largemouth bass showed no heterosis Heterosis for wild x hatchery S. aurata; crossbreeds of channel catfish common carp showed 30 - 60% improvement over parents
Sex reversal and breeding All male tilapia show improvements in yield of almost 60% depending on farming system and little unwanted reproduction and stunting All female rainbow trout grew faster and had better flesh quality
Chromosome manipulation Pagrus major triploids had similar growth rate to diploids at 10 months of age, but were smaller and presumed to be sterile at 18 months. Dicentrarchus labrax triploids showed inconsistent growth in relation to diploids and had lower GSI. Improved growth and conversion efficiency in triploid rainbow trout, channel catfish, at plaice flounder hybrids; triploid Nile tilapia grew 66-90% better than diploids and showed decreased sex-dimorphism for body weight, but other studies found no advantage. Genotype by Environment interactions also influence performance. Triploid Pacific oysters show 13 - 51% growth improvement over diploids at 8 - 10 months of age and better marketability due to reduced gonads; triploid Sydney rock oysters showed 41% increase in body weight at 2.5 yrs. Polyploidization makes certain interspecific crosses viable

Transgenic shrimp have been reported by, but there has been no successful development of a transgenic shrimp for culture. The use of transgenic animals in aquaculture is controversial and not well accepted by industry at the present for any species.

As breeding of aquatic species becomes easier and more aquatic species become domesticated, genetically differentiated strains will undoubtedly increase and aquaculture development will be faced with the problem of how best to manage and promote the new diversity, while conserving the natural genetic diversity of aquatic species. Socio-economic, as well as technical and biological, factors will play a vital role in this regard.

 
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