THE EFFECTS OF DIETARY FISH OIL REPLACEMENT BY PLANT OILS ON THE GROWTH, WHOLE BODY AND TISSUE FATTY ACID AND NUTRIENT COMPOSITION AND DIGESTIVE ENZYME ACTIVITIES IN NILE TILAPIA (Oreochromis niloticus L.)  

This study aimed at demonstrating the effects of dietary fish oil replacement by plant oils on the growth, whole body and tissue fatty acid and nutrient composition and digestive enzyme activities in Nile tilapia (Oreochromis niloticus L.). Five iso-nitrogenous and iso-lipidic diets (34 % CP and 10 % CL on a dry matter basis) were formulated to replace dietary Fish Oil (FO) by sunflower seed oil (SF), Canola Oil (CO), Linseed Oil (LO) and equal combination (BLD;1:1 w:w) of these vegetable oils. A total of 375 fish (32.5±0.5 g) were randomly assigned to 15 190 l rectangular fiberglass tanks and fed 3 % BW/d two times in equal amounts both for morning and afternoon feeding.                                                                                                                                                                 Fish tripled its initial weight and no significant difference was observed among growth rates of fish in different dietary treatments. Fish fed the diets containing vegetable oils had lower Hepato Somatic Index compared to that of fish fed FO diet and the difference was statistically significant (p<0.05) between fish fed SF and FO diets. Fatty acid compositions analysis of whole body and tissues of muscle, liver and gill showed that the intermediates of n-3 LC HUFA biosynthesis pathway from α-linolenic acid were more favourably expressed in fish fed SF diet compared to that of fish fed other dietary treatments. It was also evident that EPA was readily oxidized regardless of the dietary treatments whereas DHA was proportionally retained in significantly higher (p<0.01) rates in fish fed diets containing vegetable oils compared to that of fish fed FO diet. Dietary treatments did not affect the trypsin activities in fish significantly but slight increases were recorded with fish oil inclusion in diets. α-amylase activity of fish fed CO diet was found to be significantly higher (p<0.05) than that of fish fed other dietary treatments. Furthermore, lipase activities in fish fed SF and CO diets were also measured significantly higher (p<0.01) than that of fish fed other diets indicating lipolytic activity in Nile tilapia was stimulated by medium chain fatty acids specifically linoleic and oleic acids.                                                                                                                                This study demonstrated that up to 80 % of total fish oil in Nile tilapia diets could be replaced by either solely or in equal combinations of sun flower, canola and linseed oils without compromising growth rates, whole body and tissue fatty acid compositions and nutrient absorbent efficiencies. Further research targeting total fish oil replacement by these vegetable oils with special consideration to fatty acid metabolism, organ histology and immune response would be highly useful in this species.