1.1 BACKGROUND TO THE STUDY
Global demand for mud crabs has risen over the past decade, led by expanding wealthier markets such as those in Asia, America and parts of Africa. This demand has largely been met by exploitation of wild stocks, causing many to go into decline. Current trends in these fisheries suggest this exploitation is unsustainable. This situation continues to be exacerbated by rising demand for seafood. Mud crabs (Scylla species) are widely distributed across the West Africa sub-region, mainly in coastal and estuarine areas, making them ideal for fishing. This does also make them highly suitable for aquaculture, providing some barriers to production can be overcome.
Past researches has developed laboratory-scale technologies for hatching crabs from larvae, a first step in aquaculture development. Large-scale hatchery production is now under way where a leading centre for crab aquaculture has been established especially in advanced countries of the world.Until diets suitable for crab grow-out can be formulated, based on meeting their nutritional needs, further advances will be limited. Most aquaculture of crabs uses ‘trash-fish’ collected from marine inshore areas or mussel meat from intertidal areas. This can damage these environments and not all feed is likely to be consumed, fouling hatchery ponds. Growing exploitation of trash-fish is also leading to declining numbers, threatening the viability of aquaculture. A cost-effective replacement diet is needed to ensure the benefits gained to date are not lost. Species identification of mud crab has been controversial, and for many years only one species in the genus Scylla was recognized (Fuseya, 1998). Based on genetic and morphological discoveries, however, Keenan (1999) identified four species form the genus Scylla and revised their taxonomic nomenclature as S. serrata, S. olivacea, S. paramamosain and S. tranquebarica. Among the four mud crab species, S. serrata has the widest distribution range, and is commonly found from southern Africa to Tahiti, including the northern half of Australia, north to Okinawa, and south to the Bay of Islands in New Zealand (Keenan, 1999). S. serrata is exploited throughout its range, and in Nigeria it is estimated to accounts for 95% of the total mud crab harvest (DPI&F, 2007).
One of the main challenges for mud crab hatcheries is the lack of appropriate larval diets. Seed production of most aquatic animals, including Scylla spp., relies on live foods such as rotifers and Artemia nauplii (Hamasaki, 2003), as these live foods have several beneficial characteristics; they are slow swimmers, available in different sizes and are hardy enough to be suitable for mass culturing (Verischele, 1989). From a nutritional perspective, however, rotifers and Artemia are far from ideal, as they show nutritional inconsistency depending on source, age and culture technique (Tucker, 1992). They also lack certain highly unsaturated fatty acids (HUFA) essential for growth and survival of marine larvae (Southgate, 2003), and they are known to be a vector for introduction of pathogens into the larvae culture (Person-Le Ruyet, 1990). To overcome the nutritional deficiency it is common practice to ‘enrich’ live foods to enhance their levels of important HUFA, particularly decosahexaenoic acid (22:6n-3, DHA) and eicosapentaenoic acid (20:5n-3, EPA), prior to introduction to the larval rearing tanks (Southgate and Lou, 1995). This practice results in increased growth and survival of larvae for many species and has been viewed as a solution to the nutritional deficiency problem associated with the live prey. However, recent experiments conducted on the effectiveness of this enrichment method has shown that despite elevated HUFA levels in Artemia following enrichment, these fatty acids may not be readily available to crustacean larvae. For example, a study conducted on the larvae of rock lobster, Panulirus Cygnus showed that DHA in tissue of larvae fed enriched Artemia was low compared to the level in Artemia itself, indicating that crustacean larvae have a limited ability to absorb DHA directly from Artemia (Liddy et al., 2004). This may be linked to Artemia ’s ability to metabolize dietary DHA for energy (Danielsen et al., 1995) or alternatively, DHA is being convert to EPA in Artemia shortly after enrichment (Han et al., 2001).
Live foods production in aquaculture hatcheries is further disadvantaged by the need for specialized personnel, dedicated equipment and facilities, and the need for micro-algae culture as a food source for the live food culture. On this basis, live prey production may account for 50-75% of the total running costs of aquaculture hatcheries (Dainteath and Quin, 1991). Finally, as Artemia cysts are collected from the wild environment they are subject to an inconsistent supply, unpredictable prices and varying quality. These factors influence the sustainability of the use of Artemia as a food source and may present a serious bottleneck for the global aquaculture industry in the coming years (Lavens and Sorgeloos, 2000).
1.2 STATEMENT OF THE PROBLEM
In response to the problems associated with the use of live foods in the aquaculture of mud crabs, research into the development of alternative diets has become increasingly important. Several different types of formulated diet particles with potential for use with mud crab larvae have been developed. These are better known as microparticulate diets and have commonly been categorized as either micro encapsulated diets (MED) or microbound diets (MBD) (Kanazawa, 1986). Although less common, commercial larval foods are also available as microcoated diets (MCD), flakes, granulated foods and liquid foods (lipid walled capsules). A shared advantage of these formulated diet particles is that unlike live foods, the size and composition of micro particulate diet particle can be adjusted to suit the exact requirements of the various species and the different larval stages, the cost cannot be said to that low. However, this study is assessing the potential for low cost formulated diets for mud crab aquaculture in Nigeria.
1.3 OBJECTIVES OF THE STUDY
The following are the objectives of this study:
1. To examine the potential for low cost formulated diets for mud crab aquaculture in Nigeria.
2. To examine the nutritional constituents of a low cost formulated diet for mud crab.
3. To identify the effect of low cost formulated diets on the productivity of mud crab aquaculture.
1.4 RESEARCH QUESTIONS
1. What are the potential for low cost formulated diets for mud crab aquaculture in Nigeria?
2. What are the nutritional constituents of a low cost formulated diet for mud crab?
3. What is the effect of low cost formulated diets on the productivity of mud crab aquaculture?
1.5 SIGNIFICANCE OF THE STUDY
The following are the significance of this study:
1. This study will provide a clear understanding to fish farmers on approaches of assessing the potential for low cost formulated diets for mud crab aquaculture in Nigeria. It will also educate on the nutritional benefits of low cost formulated diets for mud crab.
2. 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.
1.6 SCOPE/LIMITATIONS OF THE STUDY
This study will cover the the potential for low cost formulated diets for mud crab aquaculture in Nigeria.
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.