Effects of Partial Fishmeal Replacement With Methanotrophic Bacterial Protein ( Methylococcus capsulatus ) in Siberian Sturgeon ( Acipenser baerii ) on Grow

This study evaluated the effects of dietary methanotrophic bacterial protein from Methylococcus capsulatus (MCP) as a partial replacement for fishmeal in juvenile Siberian sturgeon ( Acipenser baerii ). Four isonitrogenous, isolipidic, and isoenergetic diets were formulated containing 0%, 5.25%, 10.50%, or 15.75% MCP, corresponding to 0%, 25%, 50%, and 75% replacement of fishmeal, respectively. A total of 540 fish (23.15 ± 0.35 g) were randomly assigned to experimental diets in triplicate groups of 45 fish per tank over 70 days. Dietary MCP did not significantly affect growth performance, organo‐somatic indices, or serum biochemical and hematological parameters. However, MCP inclusion significantly affected the apparent digestibility of dry matter, protein, ash, phosphorus, and several amino acids, as well as immune‐related responses and hepatic lipid metabolism. Complement component 3 and lysozyme activity decreased with increasing MCP inclusion, whereas the expression of selected immune‐related genes in the head kidney and hepatic cholesterol and cholesteryl ester concentrations were altered. The intestinal microbial community remained dominated by Fusobacteriota, Firmicutes, and Proteobacteria, with no significant differences in overall community structure among dietary treatments. Overall, MCP at 5.75% supported biological responses comparable to the control diet, whereas higher inclusion levels were associated with reduced nutrient utilization and alterations in immune‐related and hepatic metabolic responses. These findings indicate that MCP can serve as a partial fishmeal substitute in Siberian sturgeon diets, although its inclusion level should be carefully considered.