This study is aimed at evaluating the effects of dietary protein-to-energy ratios on the growth, immunological response, antioxidative capacity, liver and intestinal histology, and growth-related gene expression of hybrid yellow catfish (Pelteobagrus fulvidraco ♀ × Pelteobagrus vachelli ♂). Eight diets were formulated to form different protein/energy ratios of 84, 88, 90, 93, 95, 96, 99, and 103 mg/kcal (P/E84, P/E88, P/E90, P/E93, P/E95, P/E96, P/E99, and P/E103), respectively. These diets contain different levels of gross energy (GE), ranging from 4.13 to 4.76 kcal g^-1. Seven hundred and twenty healthy fish ( g) were randomly dispersed into 24 rectangular fiberglass tanks with 8 treatments in triplicate groups. The fish fed a P/E ratio of 95 mg/kcal demonstrated the best growth and feed utilization. A significant ( ) increase in percent weight gain (WG%) and specific growth rate (SGR) was seen as the dietary P/E ratio ameliorated from P/E84 to P/E95, followed by a decreased pattern in these parameters. Feed conversion ratio (FCR) and daily feed intake (DFI) were significantly impacted by dietary P/E ratios ( ). Additionally, an optimum P/E ratio improved intestinal morphology. However, low or high P/E ratio diets can cause oxidative stress, impaired liver function, and significantly reduced nonspecific immunity. The expression of target of rapamycin (TOR) and insulin-like growth factor-1 (IGF1) genes in the liver was considerably influenced by dietary protein-to-energy ratios ( ). Based on the statistical analysis of WG% against the dietary P/E ratio, the optimal P/E ratio for the studied species was estimated to be 92.92 mg/kcal.

Histidine (His) is a nutritionally significant amino acid and plays a key regulatory role in growth-related cellular functions. The present study was conducted to assess the effects of dietary His levels on hybrid grouper growth performance, feed utilization, and related gene expression. Seven isoenergetic, isoproteic, and isolipidic diets were made up to contain 6.9, 7.2, 8.6, 9.2, 10.7, 11.1, and 11.7g/kg dietary His levels. The results revealed that protein productive value (PPV), feed efficiency (FE), and weight gain (WG) were increased with an increase in dietary His levels up to 10.7g/kg. Based on the analysis of the second-order polynomial model, the optimal dietary His level for maximum PPV, WG, and FE was 9.24, 9.62, and 9.47g/kg, respectively. The gut morphology was affected by various dietary His levels. Furthermore, dietary His levels can affect the relative mRNA levels of growth hormone receptor (GHR1) insulin-like growth factor1 (IGF1), the target of rapamycin (TOR), tumor necrosis factorα (TNF-α), and S6 kinase1 (S6K1). Dietary 9.2g/kg His revealed a high level of Nrf2 expression in the head kidney. Generally, the optimal dietary His level for the best growth performance of hybrid grouper was found to be 9.62g/kg dry matter or 21.08g/kg dietary protein.