Optimizing Feeding Regimens with Differential Protein Levels to Enhance Growth and Health in Juvenile Largemouth Bass (Micropterus salmoides)

Largemouth bass (Micropterus salmoides) represents an important freshwater aquaculture species. Dietary protein strategies significantly influence growth performance, immune markers, and intestinal microbiota homeostasis. This study evaluated the physiological effects of five phased protein regimens on juvenile largemouth bass (9.77 ± 0.03 g) over 60 days. A constant 46% protein diet (CON) served as the control, compared with four dynamic regimens (LLH, LMH, LHH, HML) comprising low (L, 43%), medium (M, 46%), or high (H, 50%) protein levels across three 20-day phases. Seven hundred fish were randomly allocated into five groups (four replicates each). Daily feed intake was recorded, body weight was measured at the start and end, and survival rates were monitored. Post-trial assessments included body composition, serum biomarkers, intestinal morphology, and intestinal microbiota. Results showed that the LHH regimen significantly increased immunoglobulin M, complement C3 and C4, lysozyme activity, catalase, glutathione peroxidase, and total antioxidant capacity compared to the CON group. The LHH group also exhibited a reduced relative abundance of Stenotrophomonas_A. Functional predictions indicated downregulation of limonene/pinene degradation and the renin–angiotensin system. The LHH feeding regimen synergistically enhances immune-antioxidant function and optimizes intestinal microbiota in juvenile largemouth bass.