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PROJECT TOPIC: AN ASSESSMENT OF THE EXTENT OF GENETIC INTROGRESSION IN EXOTIC CULTURE STOCKS OF TILAPIA

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CHAPTER ONE INTRODUCTION 1.1   BACKGROUND TO THE STUDY Aquaculture of tilapias provides a classic example of a success story of a species group outside its natural range of distribution. The group currently contributes about 3.8 percent to the cultured fish and shellfish production of about 40 million tonnes globally (FAO FishStat, 2002). The current aquaculture production of tilapias in 2002 is about 1.5 million tones accounting for nearly 80 percent of the total world production. It is important to note, however, that tilapia culture in Africa including Nigeria is also increasing. Prior to the mid-1990s, the yield of tilapia from capture fisheries was greater than that from aquaculture. Currently, the later accounts for approximately 2.5 times the production from capture fisheries. Tilapia aquaculture production increased from 28 000 tonnes to 1.504 million tonnes globally from 1970 to 2002. This increment can be said to have occurred has result of better breeds of tilapia occasioned by hybridization through genetic introgression (Amarasinghe, 2002). Culture practices of tilapias in the world are very diverse, perhaps the most diverse among all aquaculture species in the world. It is a group of fish that could be cultured at many desired intensities, thus, appealing to all socio-economic strata, enabling the culture practices to be adjusted to suit their economic capabilities. Tilapia (Oreochromis niloticus)is commonly cultured in backyard and/or home garden ponds to supplement the income of poor households as well as provide a fresh source of animal proteins to the family. In such situations, the cultured stock is often fed with kitchen waste and supplemented by relatively readily available, often low cost agricultural by-products such as rice bran. However, the direct nutritional value of the latter to the stock is not known and in all probability rather low; the inputs act more as a fertilizer. Oreochromis niloticus is cultured in relatively poor quality waters, including sewage fed ponds and primary and secondary treated waste effluents. So far, there have not been any reports of detrimental effects of consumption of fish reared in sewagefed farms on human health even as the practice has been in operation since the 1930s (Nandeesha, 2002). Genetic introgression of most cultured aquatic species lags far behind that of farmed plants and animals. Availability of genetically improved seeds is considered as the single most important factor in the green revolution that became responsible for averting a famine in the developing world during the second half of the last century (Gjedrem, 2002). Gjedrem (2002) in reviewing the degree of response of aquaculture species to selection concluded that the mean genetic gain per generation among ten species was 13.3 percent generation with a range of 9.0 to 17.5 percent in clams and channel catfish, respectively.


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