Effects of different dietary ratios of linolenic to linoleic acids or docosahexaenoic to eicosapentaenoic acids on the growth and immune indices in grouper, Epinephelus coioides

In order to investigate whether juvenile grouper (Epinephelus coioides) has biosynthetic ability of long-chain polyunsaturated fatty acids (LC-PUFA) and the proper ratio of docosahexaenoic (DHA) to eicosapentaenoic (EPA) acids in formula diets, a feeding trial was conducted with seven isonitrogenous (55.2%) and isolipidic (10.8%) diets (D0-D6) in sea floating net cages. D0 (control diet) used fish oil (FO) as lipid resource, D1-D3 contained blends of perilla and corn oils at linolenic/linoleic acids (ALA/LA) ratios of 1.0, 2.0 and 3.0, while D4-D6 contained blends of corn oil and purified EPA- or DHA-enriched oil at DHA/EPA ratios of 1.0, 2.0 and 3.0. After 9 weeks of the feeding trial, the weight gain (WG) and specific growth rate (SGR) of D1-D3 groups were significantly lower than those of D4-D5 groups (P < 0.05), but showed no significant differences compared with that of D0 or D6 group (P > 0.05). D4 group showed the highest WG and SGR as well as the lowest estimated feed conversion rate (FCR), followed by the D5 group. The levels of EPA, DHA and docosapentaenoic acid (DPA) in liver of D1-D3 groups were significantly lower than those of D0 or D4-D6 groups (P < 0.05), while those of EPA, DHA and total n-3 LC-PUFA in the muscle of D1-D3 groups were markedly lower than that of D4 group (P < 0.05). Both the albumin and IgM concentrations in the serum of D1-D3 groups were significantly lower than those of D4 and D5 groups, respectively (P < 0.05). Whereas the serum lysozyme activity of D3 group and alkaline phosphatase (AKP) activity of D1 group were respectively significantly lower than that of D0 and D4 groups (P < 0.05). These results indicated that juvenile E. coioides might have a low biosynthetic capacity for production of LC-PUFA from LA and ALA, and good growth performance and health as well as high muscle LC-PUFA contents were obtained in fish fed D4 or D5 diets where the DHA/EPA ratio was about 1:1 or 2:1 and total n-3 LC-PUFA level was approximate to 1.83% in current study.