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

Highlights

• We find that linolenic acid (LNA) is required for juvenile Nile tilapia to obtain optimum growth.

• Based on the growth results, the recommended dietary LNA requirement of juvenile Nile tilapia is 0.45–0.64% of dry weight.

Abstract

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.

Introduction

Fish, like all other vertebrates, require essential fatty acids (EFA) for normal growth, development and reproduction (Sargent et al., 1999, Tocher, 2010). In freshwater fish, the requirement of EFA is usually met by n-6 and n-3 series polyunsaturated fatty acids (PUFA) typified by linoleic acid (18:2n-6, LA) and linolenic acid (18:3n-3, LNA). LA and LNA are then converted to arachidonic acid (20:4n-6, ARA), eicosapentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA) to meet the requirement of essential highly polyunsaturated fatty acids (HUFA) (Sargent et al., 1999).

Tilapia is one of the most widely and successfully cultured freshwater fish worldwide. Previous studies showed that n-6 series (LA or ARA) were essential fatty acids for normal growth and reproduction in tilapia (Lim et al., 2011, National Research Council (NRC), 2011, Santiago and Reyes, 1993). The optimum requirement of dietary n-6 PUFA was estimated to be about 1.0% for redbelly tilapia (Kanazawa et al., 1980), 0.5% for Nile tilapia (Takeuchi et al., 1983) and 1.14% for hybrid tilapia, Oreochromis niloticus × Oreochromis Aureus (Li et al., 2013). Li et al. (2013) suggested that relatively high level of LA (i.e., 1.14%) could alone meet the EFA requirement for optimum growth of O. niloticus × O. Aureus, but the requirement could be reduced when LNA was present simultaneously. No significant differences were observed in growth performance and feed utilization of Nile tilapia fed diets with different levels (0.4 to 2.5%) of n-6 PUFA in combination with above 2.0% of n-3 PUFA (Luo et al., 2012). Besides n-6 series, n-3 series (LNA, EPA and DHA) were also reported to be essential for blue tilapia (Stickney and Wurts, 1986), Nile tilapia (El-Sayed et al., 2005, Santiago and Reyes, 1993, Yildirim-Aksoy et al., 2007) and O. niloticus × O. Aureus (Chou and Shiau, 1999, Chou et al., 2001, Li et al., 2013). Although tilapia, like other warmwater fish, is more inclined to require higher amounts of n-6 PUFA compared to n-3 PUFA for maximal growth (NRC, 2011), the presence of high dietary levels of either n-3 or n-6 PUFA may spare the requirement of the other (Lim et al., 2011, Yildirim-Aksoy et al., 2007). However, growth depression was occurred when tilapia fed diets with above 1% of LNA (Stickney and McGeachin, 1983) or oils (i.e., 5% Pollock liver oil, 10% or 12% Cod liver oil) with high levels of n-3 PUFA but very low levels of n-6 PUFA (Al-Souti et al., 2012, Kanazawa et al., 1980, Ng et al., 2001). Considering the above inconsistent results on the requirement of dietary n-3 PUFA in tilapia and the optimum requirement of dietary LNA for tilapia was not determined (NRC, 2011), further studies on the necessity of LNA for maximum growth and the requirement of dietary LNA in tilapia were needed.

The purpose of this study was to estimate the requirement of dietary LNA for Nile tilapia (O. niloticus) by feeding semipurified diets containing graded levels of LNA, which was one of a series focused on the effects of n-3 essential fatty acids on the growth and lipid metabolism in Nile tilapia.