EFFECTS OF FEEDING SUPPLEMENTARY DIETS CONTAINING VARYING LEVELS OF LIPID ON GROWTH, FOOD CONVERSION, AND BODY-COMPOSITION OF NILE TILAPIA, OREOCHROMIS-NILOTICUS (L)

Young, male tilapia, Oreochromis niloticus (L.) were stocked in 3500-litre outdoor concrete tanks at densities of 1.7 fish/m2 and fed diets containing 5, 9 and 12% lipids, or 34, 29 and 26 mg crude protein/kJ DE at a rate of 3% of their wet biomass per day. The diets were labelled the low-lipid (LL), medium-lipid (ML), and high-lipid (HL) diet. Increasing the dietary lipid level produced no significant effects on growth rate (SGR from 1.55 to 1.58%/day), food conversion ratio (FCR from 0.60 to 0.65 g gain/g feed), and protein gain (e.g. Trial 1: % PD from 20.8 to 23.0%). In contrast, percent energy retained varied between the trials (P < 0.05), and between fish fed the different diets (e.g. Trial 1:35.8-40.4%), being highest in fish fed the LL and ML diets in Trial 1, and in fish fed the ML diet in Trial 2 (P < 0.05). It was recognized that the effects of the diets may have been masked by an unquantified input of food energy from the natural production of micro-algae within the water of the tanks. Changes in body composition were not significant with respect to moisture and protein content of the carcass (moisture 73-75%; protein 13.8-15.2% in Trial 1 and 16.2-16.6% in Trial 2 with a significant difference between the trials [P < 0.05]). Increasing the dietary lipid level effected a significant increase in the level of carcass and visceral lipid (carcass lipid from 1.2% in control fish to 4.5-7.9% in fish fed the supplementary diets; visceral lipid from 1.3% to 10.7-18.5% in control and supplementary-fed fish, respectively). It was concluded that the tilapia were able to store significant quantities of lipids in the carcass and the viscera, but were not able to utilize this energy source to improve growth or food utilization efficiency, at least in diets containing adequate levels of protein, or under conditions where it was possible for natural productivity in the water to add unknown, but possibly considerable quantities of dietary protein and energy. Diets for tilapia culture may therefore have to be compounded on the basis of protein rather than energy considerations.