FATTY-ACID AND LIPID-CONTENT OF OREOCHROMIS-NILOTICUS L. IN ETHIOPIANLAKES - DIETARY-EFFECTS OF PHYTOPLANKTON

Herbivorous tilapia fish, Oreochromis niloticus L. (Cichlidae), collec ted from five Ethiopian lakes were analyzed for lipid and fatty acid c ontents of the dorsal muscle using spectrophotometry and gas-liquid ch romatography. The results showed remarkable variation between the lake s in both the lipid and total fatty acid content of the fish, 1.7-21 a nd 1.6-9.3% of dry weight (DW), respectively: Most fish from Lakes Hai q and Chamo contained higher levels of fat (greater than or equal to 1 0% DW) compared to fish from Lakes Ziway, Langeno and Awassa which con tained less than or equal to 5% DW fat. A total of 28 Fatty acids were identified. The saturated fatty acids ranged from 5.3-30 mg.g(-1) DW monounsaturated fatty acids from 1.3-30 and polyunsaturated fatty acid s from 6.8-29 mg.g(-1) DW, The most abundant fatty acids were palmitic acid, docosahexaenoic acid (DHA); oleic acid, steal-ic acid, palmitol eic acid, arachidonic acid and eicosapentaenoic acid (EPA), EPA and DH A alone amounted to 2.6-15 mg.g(-1) DW. The omega 3/omega 6 ratios ran ged from 1.3-7.6 and L. Haiq fish showed the highest ratios, 5.1-7.6. indicating that the fat was of high nutritional duality. The reason fo r such variation is probably the varied diet available to the fish in the various lakes. This study, along with previous reports, confirms t hat the phytoplankton flora constituting the diet of O. niloticus vari es greatly between the study lakes. The variation in phytoplankton is also reflected by the fatty acid pattern of net samples taken from the study lakes.