Mixed plant protein sources balanced with crystalline amino acids as a substitute for fish meal: Effects on growth, health and gut microbiota of juvenile largemouth bass (Micropterus salmoides)

To evaluate the effects of replacing fishmeal (FM) with mixed plant proteins (corn gluten meal + soybean meal) supplemented with crystalline amino acids (CAAs) on the growth, health, and gut microbiota of carnivorous Micropterus salmoides ( M. salmoides ), an 8-week trial was conducted using fish with an initial body weight of (6.0 ± 0.5) g. Five isolipidic, isonitrogenous, and amino acid-balanced diets were formulated: a control (FM100%, 560.00 g/kg FM) and four test diets (FM75%, FM50%, FM25%, FM0%) with FM replaced by the mixed plant proteins. Growth performance (weight gain, WG; specific growth rate, SGR; feed conversion ratio, FCR; condition factor, CF; hepatosomatic index, HSI; survival rate, SR) and whole-body composition (moisture, crude protein, crude lipid, ash) did not differ significantly between FM75% and FM100% ( P > 0.05). However, FM substitution > 25% impaired growth and reduced ash content significantly ( P < 0.05). Serum indices (alanine transaminase, ALT; aspartate transaminase, AST; superoxide dismutase, SOD; catalase, CAT; lysozyme, LYZ; complement 3, C3; complement 4, C4; total immunoglobulin, TIg) and intestinal histomorphology (villus height, thickness, circumference) were similar between FM75% and FM100%, but both decreased notably when FM replacement exceeded 25% ( P < 0.05). Plant protein substitution significantly altered gut microbiota abundance, composition, and particularly diversity. Dominant genera included MNG7-norank , Mycoplasma , Lactococcus , Streptococcus, and Lactobacillus ; notably, Lactococcus and Lactobacillus abundances declined significantly with FM replacement > 25% ( P < 0.05). In conclusion, replacing 25% FM with CAAs-supplemented mixed plant proteins had no adverse effects on M. salmoides ’ growth, serum biochemistry, antioxidant, immune function, intestinal histomorphology, or gut microbiota, whereas substitution ratios > 25% were detrimental. • 25% FM replacement with mixed plant proteins + CAAs had no significant adverse effects on largemouth bass. • Levels of substitution above 25% are harmful to health of largemouth bass. • Replacing 25% fishmeal with mixed plant proteins plus CAAs is a viable dietary strategy for largemouth bass.