AN ASSESSMENT OF THE EXTENT OF GENETIC INTROGRESSION IN EXOTIC CULTURE STOCKS OF TILAPIA
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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. The gain in tilapia was estimated to be about 13.5 percent.
While developments are taking place on all-male tilapia production (hormone treated or genetically), it was increasingly recognized that tilapia genetic resources in its native habitats need to be conserved, wild stocks protected and an international research programme on tilapia genetics be established (Pullin, 1988). Pullin and Capili (1988) also took into consideration the possible bottleneck effects of tilapia introduction to Africa and addressed the need for genetic improvement of cultured tilapias, particularly O. niloticus. The increasing interests in tilapia culture and the almost unanimous acceptance that cultured tilapia stocks needed genetic assessment and improvement led to the birth of the regional research and development programme "Genetic Improvement of Farmed Tilapia - GIFT", under the leadership of the WorldFish Centre - WFC, based in Penang, Malaysia (then referred to as the International Centre for Living Aquatic Resources Management - ICLARM, based in Manila, Philippines).
The "GIFT Fish" was the result of this carefully conducted genetic introgression, selection and improvement programme based on broodfish collected from four African countries (Egypt, Ghana, Kenya and Senegal) and four commercial O. niloticus strains (from Israel, Singapore, Taiwan Province of China and Thailand) used in the Philippines (Eknath et al., 1993; Dey and Gupta, 2000). In the initial phase of the research, it was evident that the gain in growth and survival through crossbreeding was less than expected. This was followed by a pure-breeding strategy among the best performing purebred and crossbred groups that led to the build-up of a genetically-mixed base population. This population formed the basis for the final selection programme through a combined family and within-family selection strategy (Eknath, 1995). Subsequent selection resulted in the emergence of the GIFT strain, which is purported to have an 85 percent cumulative genetic gain compared to the base population (Eknath et al., 1993). The development of a better strain by itself does not complete the task particularly in regions where tilapia culture is widespread, often rural and very diverse, unless the findings are extended to practitioners to enable them to reap the benefits.
1.2 STATEMENT OF THE PROBLEM
Unfortunately, there is no information available on the impact of the genetic introgression on tilapia culture in general. However, what is common knowledge is that GIFT tilapia has been introduced into a number of African and Asian countries including Nigeria. Oreochromis niloticus is widely distributed in Nigeria already and there are no reports, even anecdotal ones, that it has been responsible for the decline of indigenous species. As such it may be argued that the "GIFT Fish" may not cause any negative impacts on the environment when introduced and/or established. In contrast, it could also be suggested that "GIFT Fish", because of its genetic superiority, it could be more invasive and would increase its range of distribution and thereby bring about detrimental environmental impacts which were not evident with O. niloticus. The reverse also could occur because of its rather specialized traits (e.g. fast growth) that may have reduced fitness in the wild. However, this study is assessing the extent of genetic introgression in exotic culture of stocks of Tilapia.
1.3 OBJECTIVES OF THE STUDY
The following are the objectives of this study:
- To examine the extent of genetic introgression in exotic culture of stocks of Tilapia.
- To examine the process of genetic introgression in exotic culture of stocks of Tilapia.
- To examine the prospects of genetic introgression in exotic culture of stocks of Tilapia
1.4 RESEARCH QUESTIONS
- What is the extent of genetic introgression in exotic culture of stocks of Tilapia?
- What is the process of genetic introgression in exotic culture of stocks of Tilapia?
- What are the prospects of genetic introgression in exotic culture of stocks of Tilapia?
1.6 SIGNIFICANCE OF THE STUDY
The following are the significance of this study:
- The outcome of this study will educate aquaculture experts on the improvements achieved through the hybridization and genetic introgression of exotic culture stocks of Tilapia.
- This research will be a contribution to the body of literature in the area of the effect of personality trait on student’s academic performance, thereby constituting the empirical literature for future research in the subject area
1.7 SCOPE/LIMITATIONS OF THE STUDY
This study will cover the process involved in the hybridization and genetic introgression in exotic culture stocks of Tilapia.
LIMITATION OF STUDY
Financial constraint- Insufficient fund tends to impede the efficiency of the researcher in sourcing for the relevant materials, literature or information and in the process of data collection (internet, questionnaire and interview).
Time constraint- The researcher will simultaneously engage in this study with other academic work. This consequently will cut down on the time devoted for the research work.
Amarasinghe, U.S. 2002. The fishery and population dynamics of Oreochromis mossambicus and Oreochromis niloticus (Osteichthyes, Cichlidae) in a shallow irrigation reservoir in Sri Lanka. Asian Fisheries Science 715: 7 - 20.
Dey, M.M., & Gupta, M.V. 2000. Socioeconomics of disseminating genetically improved Nile tilapia in Asia: introduction. Aquaculture Economics and Management 4: 5 - 11.
Eknath, A.E. 1995. Managing aquatic genetic resources. Management example 4: the Nile tilapia. In J. Thorpe, ed. Conservation of fish and shellfish resources: managing diversity, pp. 176 - 194. London, Academic Press.
Eknath, A.E., Tayamen, M.M., Palada-de Vera, M.S., Danting, J.C., Reyes, R.A., Dionisio, E.E., Capili, J.B., Bolivar, H.L., Abella, T. A., Circa, A.V., Bensten, H.B., Gjerde, B., Gjedrem, T. & Pullin, R. S. V. 1993. Genetic improvement of farmed tilapias: the growth performance of eight strains of Oreochromis niloticus tested in different farm environments. Aquaculture 111: 171 - 188.
FAO. 1999. Irrigation in Asia in figures. Water reports 18, Rome, Italy, FAO, 228 pp.
Gjedrem, T. 2002. Selective breeding. Essential for further productivity, sustainability in aquaculture. The Advocate 5 (1): 46 - 47.
Nandeesha, M.C. 2002. Sewage fed aquaculture systems of Kolkata, a century - old innovation of farmers. Aquaculture Asia VII (2): 28 - 32.
Pullin, R.S.V. (ed.). 1988. Tilapia genetic resources for aquaculture. ICLARM Conference Proceedings 16, Manila, Philippines. 108 pp.
Pullin, R.S.V. & Capili, J.B. 1988. Genetic improvement of tilapia: problems and prospects. In R.S.V. Pullin, T. Bhukaswan, K. Tonguthai & J.L. Maclean, eds. The Second International Symposium on Tilapia in Aquaculture, pp. 259 - 266. Bangkok, Thailand, Department of Fisheries and Manila, Philippines, International Centre for Living Aquatic Resources Managemen