The effects of replacing fish meal with defatted black soldier fly meal on the growth, apparent digestibility, and protein metabolism of subadult Chinese mitten crab(Eriocheir sinensis)

The unstable protein content of black soldier fly larvae (BSFL) limits its use in aquaculture. This study investigated the effects of substrate protein levels on larval composition and evaluated the effects of replacing fish meal (FM) with defatted black soldier fly larvae meal (BSFLM)—produced on a 45 % protein substrate—on the growth, amino acid profile, protein digestibility, and protein synthesis of subadult E.sinensis . BSFL were reared on diets with 10 % – 55 % protein for 20 days. Larval protein content increased with dietary protein and plateaued at 45 %, reaching 59.18 % in defatted BSFLM. Defatted BSFLM then replaced 33 %, 66 %, and 100 % of FM in crab diets (initial weight: 45.30 ± 0.08 g). The 33 % replacement group (BSFLM33) significantly improved survival without affecting growth, whereas total replacement (BSFLM100) reduced both survival and growth. No significant differences were found in crab body composition. Hepatopancreatic protease activity decreased in all replacement groups, and amylase activity declined significantly in the BSFLM100 group. The apparent digestibility of BSFLM protein was 77.99 %. In terms of amino acid composition, the content of sulfur-containing amino acids in the hepatopancreas was significantly lower than that in the control group. Downregulation of genes related to protein synthesis was also observed. In conclusion, rearing BSFL on a 45 % protein diet produces high-protein defatted BSFLM, which can effectively replace up to 33 % of FM in feeds for subadult E. sinensis. • The body protein of BSFL is positively correlated with substrate protein. • Replacing 33 % of fish meal with defatted BSFLM can improve the survival rate of crab. • The apparent digestibility of defatted BSFLM crude protein in crab is 77.99 %. • Defatted BSFLM reduced protein level in hepatopancreas and muscle by inhibiting the Pi3k/AKT/mTOR pathways.