Dietary Fermented Mulberry Leaf Triggers Dose-Dependent Changes in Growth, Antioxidant Immune Status and Intestinal Microecology of Amur Sturgeon (Acipenser schrenckii)

Mulberry leaf is a promising sustainable plant-based feed resource; however, high dietary inclusion of raw mulberry leaf impairs growth in carnivorous fish. Fermentation degrades anti-nutritional components and enhances nutrient availability, yet the dose-dependent effects and underlying mechanisms of fermented mulberry leaf (FML) in Acipenser schrenckii remain unclear. This study assessed graded dietary FML inclusion (0%, 2%, 4%, and 6%) as a partial replacement for fishmeal in Amur sturgeon, measuring growth performance, intestinal morphology, serum biochemistry, target gene expression, gut microbiota, and metabolome. Moderate FML maintained acceptable growth, increased villus height, activated the Keap1–Nrf2 antioxidant pathway, and raised serum IgM and C3 levels without provoking severe intestinal inflammation. Intestinal microecology underwent prominent dose-dependent remodeling: low-dose FML enriched Lysinibacillus and Clostridium_T, while moderate-to-high FML increased the abundance of beneficial Bacillales taxa that modulated amino acid and energy-associated metabolic pathways. Quadratic regression estimated an optimal FML dosage of 3.66–4.62%. The 4% FML group achieved balanced improvements in growth and intestinal health within this optimal range. Collectively, FML represents a promising sustainable feed ingredient for sturgeon aquaculture, providing new references for applying mulberry by-products in carnivorous fish diets.