Effects of dietary n-3 LC-PUFA/n-6 C18 PUFA ratio on growth, feed utilization, fatty acid composition and lipid metabolism related gene expression in black seabream, Acanthopagrus schlegelii

An 8-week feeding trial with black seabream, Acanthopagrus schlegehii, was conducted to evaluate effects of dietary n-3 LC-PUFA/n-6 C-18 PUFA (LC-PUFA, long-chain polyunsaturated fatty acid; C-18 PUFA, PUFA with 18 atoms of carbon) ratio on growth, feed utilization, fatty acid composition and lipid related gene expression. Five isoproteic and isolipidic diets were formulated to contain different n-3 LC-PUFA/n-6 C-18 PUFA ratios: 0.1, 0.4, 0.5, 0.8, 1.0. Each diet was assigned randomly to feed triplicate net cage of 90 fish (30 fish per net cage) of initial weight 3.0 +/- 0.0 g. As a result, final weight (FW), weight gain (WG), specific growth rate (SGR), feed efficiency (FE) and protein efficiency ratio (PER) were all significantly affected by dietary n-3 LC-PUFA/n-6 C-18 PUFA ratio (P < 0.05). Fish fed the diet with a n-3 LC-PUFA/n-6 C-18 PUFA ratio of 0.8 showed the highest FW, WG, SGR, FE, PER. There was no significant differences in survival rate (SR), hepatosomatic index (HIS), viscerosomatic index (VIS), intraperitoneal fat ratio (IPF) among all treatments (P > 0.05). Likewise,The moisture contents, crude protein, crude lipid and ash contents in the whole body and muscle were all not affected by experimental diets (P > 0.05). Tissue fatty acid profiles reflected the diets. The highest concentration of triacylglycerol (TG) was observed in fish fed the lowest ratio (0.1) (P < 0.05). However,the highest activities of alanine aminotransferase (ALT) and aspartate transaminase (AST) as well as the highest content of cholesterol (CHOL) were all found in fish fed the diet containing n-3 LC-PUFA/n-6 C-18 PUFA ratio of 1.0 (P < 0.05). Hepatic G6pd, Lpl and Hsl activities were significantly affected by dietary n-3 LC-PUFA/n-6 C-18 PUFA ratio (P < 0.05). Similarly, the expression levels of fas, acca, g6pd, cpt1a and atgl were influenced significantly among treatments, the highest fas, acca, g6pd, cpt1a and atgl expression levels were obtained in fish fed the lowest dietary n-3 LC-PUFA/n-6 C-18 PUFA ratio of 0.1 (P < 0.05). In conclusion, according to quadratic regression analysis, the optimum dietary n-3 LC-PUFA/n-6 C-18 PUFA ratio was 0.7 for juvenile black seabream to obtain the maximum growth. Moreover, this study also indicated that dietary n-3 LC-PUFA/n-6 C-18 PUFA ratio affected fatty acid profiles and significantly influenced lipid metabolism.