Optimizing Low Fishmeal Diets with Vitamin C Supplementation: A Comprehensive Study on Growth, Immunity, and Heat Stress Resistance in Largemouth Bass (Micropterus salmoides) Juveniles

Six dietary groups were supplemented with graded vitamin C (VC) levels: VC1 (control, 0.39 g/kg), VC2 (0.51 g/kg), VC3 (0.66 g/kg), VC4 (0.81 g/kg), VC5 (0.97 g/kg), and VC6 (1.11 g/kg). Largemouth bass (<i>Micropterus salmoides</i>) with an initial weight of 2.21 ± 0.00 g were fed these diets for 8 weeks to evaluate the effects of different VC levels on growth performance, immune response, and heat stress resistance. Heat stress was induced at a constant temperature of 33.00 ± 0.16 °C for one week. The VC3 and VC4 groups showed significantly improved growth performance (FBW, WGR, SGR) compared to VC1 (<i>p</i> < 0.05). VC4 exhibited lower ALT and AST levels before and after heat stress. Antioxidant capacity (T-AOC, GSH-Px, CAT) was significantly enhanced in VC3-VC5, with VC5 showing the highest after stress activity (except CAT). Expression of pro-inflammatory genes (<i>nf-κb</i>, <i>il-8</i>) was downregulated in VC4 and VC5, while anti-inflammatory <i>il-10</i> was upregulated in VC4 after stress. Apoptosis-related genes (<i>bcl-2</i>, <i>caspase</i>, <i>bax</i>) and TUNEL assays indicated the strongest anti-apoptotic effects in VC3 and VC4 under heat stress (<i>p</i> < 0.05). These findings suggest that VC supplementation in low-fishmeal diets enhances growth, immune response, apoptosis resistance, and acute heat stress tolerance in fish.