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.
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.
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.