N-3 essential fatty acids in Nile tilapia, Oreochromis niloticus: Quantification of optimum requirement of dietary linolenic acid in juvenile fish

To quantify the optimum requirement of dietary linolenic acid (18:3n-3, LNA) for Nile tilapia ( Oreochromis niloticus ), purified ethyl linolenate was added to the basal diet to provide the dietary LNA level of 0.00, 0.10, 0.32, 0.63, 0.98, 1.56 and 2.04% of dry weight, respectively. All diets contained similar amounts of 18:2n-6 (0.61% of dry weight) and 18:1n-9 from purified ethyl linoleate and oleate , respectively. Basal diet without LNA was considered as a control. After 2-week acclimation with a lipid-free diet, 4 replicate groups of 120 juvenile fish with initial body weight of 2.10   g were fed with experimental diets in a recirculated fresh water rearing system for 10   weeks. Feeding rate was 10% of body weight in the first 2   weeks, 8% in the following 4   weeks, and then gradually reduced to 5% in the next 4   weeks. Fish were fed three times daily at 8:30, 12:00 and 17:00. The results showed that fish fed the diet with 0.63% of LNA had significantly higher weight gain (WG) and specific growth rate (SGR) than fish fed the control diet ( P   <   0.05). Feed efficiency (FE) and protein deposition (PD) in fish fed diets with 0.63, 0.98, 1.56 and 2.04% of LNA were significantly higher than those of the control group ( P   <   0.05). Fish muscle fatty acid composition reflected dietary fatty acids. Increasing dietary LNA levels resulted in increased contents of LNA and 22:6n-3, but decreased contents of 20:4n-6 and 20:5n-3. When WG and FE were subjected to broken-line regression analysis, the breakpoint was achieved at dietary LNA levels of 0.45 and 0.64% of dry weight, respectively. Based on those results, the recommended optimum requirement of dietary LNA for juvenile tilapia is 0.45–0.64% of dry weight.