EVALUATION OF THE EXTENT OF GENETIC INTROGRESSION IN EXOTIC CULTURE STOCKS OF TILAPIA

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INTRODUCTION

The cultivation of tilapias in aquaculture is a prime illustration of the success that can be achieved by a species group when it is relocated outside of its normal zone of distribution. The current contribution of the group is around 3.8 percent of the total global production of cultured fish and shellfish, which is over 40 million tonnes (FAO FishStat, 2002). The present production of tilapias from aquaculture in 2002 was approximately 1.5 million tons, accounting for close to 80 percent of the total production from all sources worldwide. It is essential to point out, however, that the culture of tilapia in Africa, particularly Nigeria, is also expanding at a rapid rate.
Before the middle of the 1990s, the amount of tilapia that could be harvested from catch fisheries was far higher than the amount that could be harvested via aquaculture. At the moment, the latter accounts for nearly two and a half times the productivity that comes from catch fisheries. From 1970 to 2002, the total amount of tilapia that was produced by aquaculture on a global scale climbed from 28,000 to 1,504 million tonnes. This increase can be attributed to the creation of superior tilapia breeds as a result of hybridization and genetic introgression, which led to the phenomenon of hybridization (Amarasinghe, 2002).
The methods used to raise tilapia around the world are quite varied, to the point where it’s possible they’re the most variable of any aquaculture species on the planet. It is a group of fish that can be cultured at many different intensities, making it appealing to people of all socioeconomic strata and allowing the culture methods to be altered to suit the economic capacities of the people doing the culture. Tilapia, also known as Oreochromis niloticus, is a kind of fish that is frequently bred in backyard or home garden ponds for the purpose of providing a supplementary source of revenue for low-income families as well as a new source of animal proteins for the household. In these kinds of circumstances, the cultured stock is frequently nourished with leftovers from the kitchen and supplemented with agricultural by-products that are easily available and typically inexpensive, such as rice bran. On the other hand, the direct nutritional value of the latter to the stock is unknown and in all likelihood relatively modest; the inputs act more as a fertilizer. This information is not available. Oreochromis niloticus is bred in waters of rather low quality, such as ponds that are supplied by sewage and primary and secondary processed waste effluents. Even though the practice has been in use since the 1930s, there have been no reports of any adverse effects on human health caused by the ingestion of fish that was raised in farms that were fed with sewage up to this point (Nandeesha, 2002).

The majority of aquacultured aquatic species have not undergone nearly as much genetic introgression as have other types of agricultural animals and plants. It is generally agreed that the accessibility of genetically enhanced seeds was the single most essential component in the green revolution, which was ultimately responsible for preventing widespread starvation in the world’s less developed regions throughout the latter half of the 20th century (Gjedrem, 2002). Gjedrem (2002) came to the following conclusion after conducting a study on the degree to which aquaculture species respond to the process of selection: the average genetic gain per generation for all ten species was 13.3 percent, with a range from 9.0 to 17.5 percent in clams and channel catfish, respectively. It was estimated that there was a gain of approximately 13.5 percent in tilapia.

While progress is being made on the production of all-male tilapia (whether through the use of hormones or genetic engineering), it is becoming more widely acknowledged that tilapia genetic resources in their natural habitats need to be preserved, wild stocks need to be protected, and an international research program on tilapia genetics needs to be established (Pullin, 1988). Pullin and Capili (1988) addressed the need for genetic improvement of cultured tilapias, particularly O. niloticus, and took into consideration the potential bottleneck effects of the introduction of tilapia to Africa. The growing interest in tilapia farming combined with the nearly universal consensus that cultured tilapia stocks required genetic evaluation and improvement resulted in the establishment of the regional research and development program known as “Genetic Improvement of Farmed Tilapia – GIFT.” This program is led by the WorldFish Centre – WFC, which is headquartered in Penang, Malaysia (then referred to as the International Centre for Living Aquatic Resources Management – ICLARM, based in Manila, Philippines).

This genetic introgression, selection, and improvement program was 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) that were used in the Philippines. The “GIFT Fish” was the result of this program (Eknath et al., 1993; Dey and Gupta, 2000). During the preliminary stages of the investigation, it became clear that the anticipated increase in growth and survival rates would not be achieved through crossbreeding. After this, a strategy of pure breeding was implemented among the highest performing purebred and crossbred groups, which resulted in the construction of a genetically diverse base population. This population served as the foundation for the final selection program, which utilized an approach that blended family selection with selection from within families (Eknath, 1995). Following this, selection led to the development of the GIFT strain, which is said to have a cumulative genetic gain of 85 percent when compared to the original population from which it originated (Eknath et al., 1993). If the findings are not disseminated to practitioners in order to give them the opportunity to reap the benefits of the improved strain, then the task will not be finished. This is especially true in areas where tilapia culture is widespread, frequently located in rural areas, and has a very diverse range of practices.