Supplementing graded levels of Fish oil in the feeds of Labeo rohita fingerlings: Effects on tissue n-3 LC-PUFA deposition, Δ6 fad gene expression, blood metabolites and fish performance

A 60-day feeding trial was conducted to study the effects of levels of fish oil (FO), 0 (FO-0), 30 (FO-30), 60(FO-60), 90 (FO-90),120 (FO-120) and 150 (FO-150) g kg(-1) feed on growth, nutrient utilisation, whole-body chemical composition, tissue fatty acids profile, blood metabolites and Delta 6 fad gene expression in Labeo rohita fingerlings. Two hundred and seventy fingerlings of average body weight 13.03 +/- 0.03 g were randomly stocked in 18 500 L concrete tanks, each with 15 fingerlings. Triplicate tanks, all fitted with flow-through water systems, were allocated under each dietary treatment. The fish were hand-fed twice daily till satiation. The feed intake decreased with increasing dietary FO levels. The body weight gain, specific growth rate (SGR) and protein efficiency ratio (PER) increased significantly with increasing dietary FO supplementation level of 60 g kg(-1) feed. Above this level, the values plateaued. The feed conversion ratio (FCR) showed the reverse trend of growth. The fish fed with FO-120 had the highest whole-body lipid deposition. Increasing dietary FO levels up to 90 g kg(-1) feed enhanced muscle n-3 LC-PUFA content significantly (P < 0.05), and further increasing the FO level did not further influence the fatty acid. Supplementation of FO in the feeds significantly (P < 0.05) suppressed the Delta 6 fad gene expression in muscle, liver, and intestine tissues. In muscle and liver tissues, the suppression of the Delta 6 fad gene expression was the highest when the dietary FO level was above 90 g FO kg(-1) feed, whereas in the intestine tissue above 60 g FO kg(-1). Increasing dietary FO levels increased serum cholesterol and high-density lipoprotein (HDL) contents and decreased low-density lipoprotein (LDL) and very low-density lipoprotein (VLDL) concentrations. The second-order polynomial regression graphs suggest a 104 g lipid kg(-1) feed is optimum for the best growth and nutrient utilisation, whereas 113 g FO kg(-1)feed is required to maximise n-3 LC-PUFA deposition in the muscle of L. rohita fingerlings.