Effects of dietary protein level on growth performance, plasma parameters, nutritional metabolism, and mTOR pathway of Chinese perch (Siniperca chuatsi)

This study aimed to investigate the impact of dietary protein levels (DPLs) on the growth performance, plasma parameters, nutritional metabolism, and the mTOR pathway in Chinese perch. Three replicates of Chinese perch (34.39 +/- 0.16 g) were provided with five diets containing varying protein levels (39.99 %, 43.09 %, 46.04 %, 49.14 %, and 52.12 %) over an 8-week period. The results revealed that, as DPLs increased, the final weight (FW), daily weight gain (DWG)), specific growth rate (SGR) and protein efficiency ratio (PER) initially rose and then declined, reaching their peak at 49.14 % DPL group. The feed conversion ratio (FCR) decreased first and then increased. In addition, compared to 46.04 % DPL group, the 39.99 % DPL group significantly suppressed whole body crude protein, liver crude protein, and liver crude lipid contents. The 52.12 % DPL group significantly increased liver crude lipid content and aspartate aminotransferase (AST) activity, and increased plasma total cholesterol (TC), total triglyceride (TG) and total lipase (TL) concentrations. However, in Chinese perch fed with 46.04 % DPL, the expression of genes associated with liver amino acid catabolism (ast, gdh and ampd), lipid hydrolysis, and oxidation (hsl, lpl and cs), as well as glycolysis (pk), was significantly increased. And the expression levels of mTOR pathway-related genes (s6k and mtor) and the phosphorylation level of p-S6 were significantly elevated in the liver of fish fed with 46.04 % DPL. In summary, a low DPL (39.99 %) significantly impedes the growth performance of Chinese perch, resulting in an elevated feed conversion ratio. And a high DPL (52.12 %) can induce lipid deposition in the liver of Chinese perch, increase metabolic stress, and lead to liver damage. Additionally, piecewise regression analysis and second-degree polynomial analysis revealed that optimal protein level in Chinese perch feed is suggested to range between 46.10 % and 48.71 %. This study provides partial theoretical basis for the design of commercial practical feed formulas for Chinese perch.