REPLACING FISHMEAL WITH METHANOTROPH BACTERIA MEAL ON GROWTH PERFORMANCE, PROTEIN METABOLISM, AND HEPATOPANCREATIC HEALTH IN JUVENILE <italic>PROCAMBARUS CLARKII</italic>

To evaluate the feasibility of replacing fish meal with Methanotroph bacteria meal (MBM) in feed for the red swamp crayfish (<italic>Procambarus clarkii</italic>), this study designed five isonitrogenous and isolipidic diets. MBM replaced 0 (MBM0), 25% (MBM25), 50% (MBM50), 75% (MBM75), and 100% (MBM100) of fish meal protein, corresponding to MBM additions of 0, 2.12%, 4.24%, 6.36%, and 8.49%, respectively. A total of 375 juvenile crayfish with initial body weight of (3.44±0.02) g were randomly divided into five groups with three replicates each and reared for 8 weeks. Results showed that as the substitution ratio increased, the final body weight, weight gain rate, specific growth rate, and feed efficiency in the MBM75 and MBM100 groups were significantly lower than those in the MBM0 group (<italic>P</italic>&lt;0.05). Physiologically and biochemically, compared with the MBM0 group, the high-substitution groups (MBM75, MBM100) exhibited significantly reduced pancreatic trypsin activity (<italic>P</italic>&lt;0.05) and significantly increased alanine aminotransferase activity (<italic>P</italic>&lt;0.05). At the molecular level, the MBM100 group showed significantly downregulated expression of hepatopancreatic protein synthesis-related genes (<italic>mtor</italic>, <italic>s6k1</italic>, <italic>akt</italic>, <italic>pi3k</italic>, and <italic>p62</italic>), while proteolysis-related genes (<italic>gcn2</italic>, <italic>atg3</italic>, <italic>atg9</italic>, <italic>ub</italic>, <italic>psmd1</italic>, <italic>psmd6</italic>) were significantly upregulated (<italic>P</italic>&lt;0.05). Intestinal amino acid transporter genes (<italic>eaat3</italic>, <italic>pat1</italic>, <italic>mglur</italic>) showed significantly suppressed expression in the MBM75 and MBM100 groups. Histological examination revealed normal hepatopancreatic structure in the MBM25 group, mild damage in the MBM50 group. The MBM75 and MBM100 groups exhibited hepatopancreatic ductal lumen deformities, blurred basement membrane boundaries, and severe vacuolation. Inflammation-related genes (<italic>tnf-α</italic>, <italic>traf6</italic>, <italic>relish</italic>, and <italic>irak4</italic>) showed significantly upregulated expression (<italic>P</italic>&lt;0.05). In summary, high proportions of MBM replacing fish meal inhibit growth and protein metabolism in juvenile crayfish while inducing hepatopancreatic inflammatory damage. Therefore, the replacement ratio of MBM for fish meal in juvenile crayfish feed should not exceed 50%, corresponding to a maximum feed inclusion rate of 4.24%.