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The aim of this research was to assess the effects of different feeding regimes on growth performance, plasma immune indices and related genes' expression of hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) juveniles fed two dietary energy levels. A 3 × 2 factorial experiment was designed including three feeding regimes (two meals a day, one meal a day, one meal every two days; designated as 2 M, 1 M, 2DM) and two dietary available energy levels (3.4 and 3.6 kcal/g; designated as E3.4, E3.6), with three replicates per treatment. Juvenile hybrid grouper (average initial body weight: 19.39 ± 0.04 g/fish) were cultured for 6 weeks in 18 floating cages (125 cm × 80 cm × 40 cm) placed in six concrete ponds sharing the same water source. Results revealed that the 2 M and 1 M treatments had significantly higher weight gain rate (WGR), protein efficiency ratio (PER), expression of hepatic IGF-1, TOR, and S6K1 than the 2DM treatment (P < 0.05), whereas feed conversion ratio (FCR) displayed an opposite trend (P < 0.05). No significant differences in the above parameters were found between the 2 M and 1 M groups. As the feeding frequency decreased, the total feed intake (FI) per fish was significantly reduced (P < 0.05), while feed intake per meal (FIM), stomachic index (SI) and expression of hypothalamus agouti related neuropeptide (AgRP) were significantly increased (P < 0.05). The 2 M treatment supported higher plasma lysozyme contents than the 1 M and 2DM treatments (P < 0.05). The E3.6 treatment had higher intraperitoneal fat ratio and intestinosomatic index, but lower plasma lysozyme and immunoglobulin M contents than the E3.4 treatment (P < 0.05). Based on our results, a feeding regime of two meals/day of a diet containing 3.4 kcal DE/g of dry matter is recommended for the culture of juvenile hybrid grouper with an initial size of 19.39 g. • The feeding regime of two meals a day is suitable for hybrid grouper. • The feeding regime of one meal every two days impairs fish growth and health. • The feeding regimes affect expression of genes involved in the TOR/S6K1 pathway. • The high dietary energy level (358 kcal per 100 g) impairs immune indices of hybrid grouper. [ABSTRACT FROM AUTHOR]

Ascorbic acid (Vitamin C - VC) is one of the essential micronutrients required for normal growth and physiology of fish. In this research, the optimum dietary VC requirement of juvenile hybrid grouper (Epinephelus fuscoguttatus ♀ × Epinephelus lanceolatus ♂) was evaluated in a comparative feeding trial and subsequent oxidative stress test. Six isoenergetic (340 kcal/kg), isoproteic (49%) and isolipidic (7%) experimental diets were supplemented with graded amounts of L-ascorbyl-2-polyphosphate (analyzed VC concentrations of 76, 86, 104, 126, 137 and 152 mg/kg dry matter) and hand-fed to fish (initial weight of 15 ± 0.1 g/fish (means ± S.D.)) in three replicate cages twice a day (08:00 h and 16:30 h) to apparent satiation for 7 weeks. Percent weight gain (WG%), together with the VC concentrations in liver and head kidney, were elevated with increasing dietary VC contents, reaching a plateau in the VC137 treatment. The optimal dietary VC requirement of hybrid grouper was estimated to be 148 mg/kg dry matter based on broken-line regression analysis of WG%. Serum immunoglobulin M (IgM) concentrations and the relative mRNA expression of hepatic insulin-like growth factor-1 (IGF-1) displayed a similar variation tendency as WG%. At the transcriptional level, the activity of Nrf2-Keap1 pathway was up-regulated by VC supplementations, with the VC104 treatment showing the highest value. In summary, the optimal dietary VC requirement for the highest value of WG% of juvenile hybrid grouper was 148 mg/kg dry matter, and appropriate dietary VC supplementation improved oxidation resistance and serum IgM concentrations of the fish. • VC affects growth and expression of hepatic IGF-1 of hybrid grouper. • At the transcriptional level, the activity of Nrf2-Keap1 pathway was up-regulated by VC supplementations. • Optimal VC requirement of hybrid grouper fed low fishmeal diets was 148 mg/kg. [ABSTRACT FROM AUTHOR]