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 (Micropterus salmoides) 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 (p < 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 (nf-κb, il-8) was downregulated in VC4 and VC5, while anti-inflammatory il-10 was upregulated in VC4 after stress. Apoptosis-related genes (bcl-2, caspase, bax) and TUNEL assays indicated the strongest anti-apoptotic effects in VC3 and VC4 under heat stress (p < 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.