Diet composition impacts the fatty acid contents of Nile tilapia, Oreochromis niloticus L., in Ethiopian highland lakes

Abstract

Tilapia are generally known to be warm water fish widely distributed across the tropics. In particular, the matemal mouth brooding tilapia of the genera Saroterodon and Oreochromis have successfully adapted to a wide range o f water bodies because of their plastic feeding habits and wide tolerance to varying environmental conditions (TREWAVAS 1982). The Nile tilapia, Oreochromis niloticus occurs in almost all rivers and lakes ofEthiopia. It is the most important commercial fish, constituting over 60% of the commercial catch o f the capture fishery in the country (REYNTJENS & WUDINEH 1998). Investigations on the fatty acid composition and lipid quality of fishes have been well documented mainly from marine and freshwater systems of the temperate regions (PUUSTINEN et al. 1985, AHLGREN et al. 1994, 1996). Most studies can be broadly examined either from an ecological point ofview or in relation to hurnan health. From the medical point o f view, two o f the ro3 polyunsaturated fatty acids (PUFA), eicosapentaenoic acid (EPA, 20:5ro3) and docosahexaenoic acid (DHA, 22:6ro3) common in fish and fish oil have several biochemical effects on hurnan metabolism. Fish oil ri eh in EPA and DHA can lower blood lipid and reduce the risk of getting coronary heart disease (MEDIORUS et al. 2007). From an ecological view point, it is important to know the origin ofthese important PUFA in fish and other animals at higher trophic levels. Information on the transfer of fatty acids from the lower trophic levels beginning from the phytoplankton can be obtained by quantifying the levels of important fatty acids across the food chain. These fatty acids can serve as biomarkers to elucidate the transfer and conversion of individual fatty acids (NAPOLITANO 1999). Marine fish have been reported to have higher PUFA content than their freshwater counterparts (NAPOLITANO 1999); however, AHLGREN et al. (1994) and references therein think that the difference between marine and freshwater fish is related to other factors, such as feeding and type of fish, rather than the difference in salinity. In our earlier work we also found that tropical fish can be equally important to temperate fish in terms ofPUFA content (ZENEBE et al. 1998a). D ata on the fatty acid content o f tropical fish are scarce and are mostly limited to fish from South America (CLEMENT & LOVELL 1994, ANDRADE et al. 1995). In our earlier study we reported the fatty acid composition of O. niloticus collected from the rift valley lakes located at lower altitudes (ZENEBE et al. 1998a, 1998b ). In this study I report the fatty acid composition of the same fish taken mainly from the highland lakes located outside the Rift system ofEthiopia. This information will provide broad comparisons and more comprehensive data on the fatty acid contents of O. niloticus collected from the wild in Ethiopian lakes