Impact of dietary sodium fulvate inclusion on growth, intestinal health, antioxidant and immune responses in largemouth bass (Micropterus salmoides)

In response to current restrictions on antibiotic use in aquaculture, developing eco-friendly feed additives as alternatives is essential to sustain industry growth. This study evaluated the effects of dietary sodium fulvate (SF) on growth, intestinal health, antioxidant capacity, and immune responses in largemouth bass ( Micropterus salmoides ). Six experimental diets containing 0%, 0.05%, 0.1%, 0.2%, 0.4%, and 0.8% SF were formulated and fed to fish maintained in three replicate cages over 9 weeks. Dietary SF inclusion did not significantly affect growth performance (weight gain rate, specific growth rate) or feed utilization (feed conversion rate, protein efficiency ratio) ( P > 0.05). However, second-order polynomial regression indicated that dietary inclusion of 0.17% SF maximized intestinal trypsin activity ( P < 0.05), whereas supplementation with 0.30%–0.58% SF reduced plasma diamine oxidase, endothelin-1, and lipopolysaccharide concentrations. Furthermore, supplementation with 0.44%–0.56% SF significantly enhanced the Shannon diversity index and increased the relative abundance of Fusobacteria, Ralstonia, and Cetobacterium . Dietary inclusion of 0.06%–0.46% SF enhanced intestinal superoxide dismutase and catalase activities; increased serum complement 3, complement 4, immunoglobulin M levels, and the relative expression levels of TNF-α, IL-10, and TGF-β1; and reduced intestinal malondialdehyde level. Collectively, these findings indicate that dietary supplementation with 0.06%–0.58% SF promoted fish health by improving intestinal digestive capacity, reinforcing mucosal barrier integrity, and enhancing antioxidant and immune responses, underscoring its potential as a functional feed additive for antibiotic-free aquaculture.