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The study aimed to investigate the effects of oregano oil (OO) supplement in the diet on the growth performance, disease resistance, immunity, and intestinal health of hybrid grouper. Six isonitrogenous and isolipidic diets were fed to the grouper with an initial weight of 19.10 ± 0.12 g for 8 weeks, of which the dietary OO levels were 0 (Control), 0.1 (OO-0.1), 0.3 (OO-0.3), 0.6 (OO-0.6), 0.9 (OO-0.9), or 1.2 (OO-1.2) g kg-1. The results showed that there were no significant differences in growth performance among the six groups, while the OO supplement could improve the disease resistance ability of grouper. Our findings have demonstrated that the OO supplement, especially 0.3 g kg-1, could improve the intestinal morphology via increasing the villus number and muscle thickness and elevate digestive enzyme activities such as those of amylase (AMS) and lipase. The 0.3-g kg-1 OO supplement also can activate the antioxidant defense system, reduce nuclear factor erythroid 2-related factor 2-like 2 (Nrf2)/Kelch-like-ECH-associated protein 1 (Keap1) expression, and then enhance the immune ability via acid phosphatase (ACP), lysozyme (LYS), complement C3 (C3), and tight junction-associated signaling pathways in hybrid grouper. Meanwhile, the relative abundance of Firmicutes and Bacteroidetes in the OO-0.3 group was significantly higher than that of Control group (P< 0.05), while the relative abundance of Proteobacteria showed a basically opposite trend (P< 0.05). These findings suggested that dietary OO supplementation could significantly improve intestinal health and induce the antioxidant defense system and immunity, which means that OO could be used as a functional feed additive to protect the intestine, and the appropriate addition level of OO should be 0.3 g kg-1.

This study investigated the effect of lipid levels on growth, body composition, lipid metabolism enzymes activity and related genes expression in hybrid grouper (♀Epinephelus fuscointestinestatus × ♂E. lanceolatu). Three diets with different lipid levels of 6% (low-lipid feed, LF), 11% (medium-lipid feed, MF) and 16% (high-lipid feed, HF) were designed and each diet was fed to triplicate groups of 30 fish each (6.73 ± 0.00g) for eight weeks. The results revealed the following. 1) The maximum weight gain rate (WGR), specific growth rate (SGR), protein efficiency ratio (PER) and the lowest feed conversion ratio (FCR) values were found in the HF group. There was no significant difference in the survival rate (SR) among the groups (P > 0.05). 2) Condition factor (CF), hepatosomatic index (HSI) and visceralsomatic index (VSI) increased with increasing dietary lipid. 3) Crude lipid content in the whole-body and muscle increased significantly as the dietary lipid level increased (P < 0.05). 4) Principal component analysis of fatty acids showed that sum n-3 polyunsaturated fatty acids (∑n-3PUFA)/sum n-6 polyunsaturated fatty acids (∑n-6PUFA) was significantly higher in the HF group than in the other two groups (P < 0.05). 5) Biochemical serum indexes showed higher triglycerides (TG), total cholesterol (TC), alkaline phosphatase (AKP) and low-density lipoprotein-cholesterol (LDL-C) levels but lower high-density lipoprotein-cholesterol (HDL-C) level in the HF group. 6) Liver lipid metabolism enzymes activity and biochemical indicators showed that fish fed the MF diet had higher lipoprotein lipase (LPL), hepatic lipase (HL) and triglyceride lipase (ATGL) activity but lower fatty acid synthase (FAS) activity and very-low density lipoprotein-cholesterol (VLDL-C) level. 7) The relative expression levels of lipoprotein lipase (lpl), adipocyte-type fatty acid-binding protein (a-fabp), liver-type fatty acid-binding protein (l-fabp), heart-type fatty acid-binding protein (h-fabp) and hormone-sensitive lipase (hsl) genes in the fish liver were higher in the MF group with increasing dietary lipid (P < 0.05), while the fatty acid synthase (fas) gene was expressed at lower relative level (P < 0.05). 8) Histological oil-red sections of the liver showed that fish in the HF group had larger and more hepatic lipid droplets than those fed the other diets. It was concluded that fish fed the HF diet had better growth performance but this diet was not conducive to the progress of lipid metabolism, resulting in lipid accumulation and affecting the quality of the fish. This study provides useful information for the formulation of environmentally friendly and cost-effective diets for hybrid grouper.

There is a trend towards using high-lipid diets in aquaculture, which can be optimized by using certain additives. This study investigated the effect of vitamin E (VE) supplementation of a high-lipid diet on the growth, serum and liver biochemical indexes, hepatic histology, and lipid metabolism of hybrid groupers (♀ Epinephelus fuscoguttatus × ♂Epinephelus lanceolatus). Six groups of iso-protein (50.23%) and iso-lipidic high-lipid (15.36%) experimental diets were prepared by adding VE at concentrations of 49.6 (basic diet, controls), 100, 110, 163, 206, and 245 mg/kg. Each treatment consisted of three replicates and 30 fish (10.20 ± 0.02 g) in an 8-week feeding experiment. The results show that 1) compared with controls, growth performance was not affected by VE addition. However, the weight gain rate (WGR) and specific growth rate (SGR) were significantly lower in the high-VE group (245 mg/kg) than in the 163 and 206 mg/kg groups. 2) Compared with controls, VE significantly increased the contents of alpha-tocopherol in the liver and muscle. 3) Adding VE can reduce the fatty livers caused by high-lipid diets and significantly reduce total serum cholesterol, reduce the activity of lipid synthesis enzymes, and significantly increase the activity of lipolytic enzymes in the liver. 4) Compared with controls, appropriate amounts of VE significantly increased the expressions of the liver lipid-metabolism-related genes PPAR-α, PPAR-β, PPAR-γ, H-FABP , A-FABP, and L-FABP. Based on observations of lipid metabolism enzyme activity, histological sections, and lipid-metabolism-related gene expression, we conclude that the addition of 163–206 mg/kg VE to a high-lipid diet can promote lipid metabolism in groupers.