Effects of Crayfish Shell Meal Processing Methods on Growth Performance, Antioxidant Capacity, Intestinal Function and Gut Microbiota of Largemouth Bass (Micropterus salmoides)

This study evaluated the effects of crayfish shell meal (CSM) processed by drying, enzymatic hydrolysis, or fermentation on juvenile largemouth bass (Micropterus salmoides). A total of 320 fish (initial body weight, approximately 5.69 g) were distributed into 16 tanks, with 20 fish per tank and four replicates per treatment. Four isonitrogenous and isoenergetic diets were prepared: a fish meal-based control diet (FM) and three diets containing 4.5% dried CSM (DCSM), enzymatically hydrolyzed CSM (ECSM), or fermented CSM (FCSM), in which fish meal was reduced from 45.0% to 39.6%, and were fed for 8 weeks. Compared with the FM group, DCSM significantly reduced final body weight, weight gain rate, specific growth rate, and protein efficiency ratio, and increased feed conversion ratio (p < 0.05). ECSM generally showed intermediate responses, whereas FCSM did not differ significantly from FM in these growth and feed utilization indices (p > 0.05). DCSM also significantly increased serum aspartate aminotransferase and blood urea nitrogen levels and induced a pro-inflammatory intestinal response, as indicated by decreased il10 and increased il1b and tnfa expression (p < 0.05). Compared with DCSM, FCSM significantly improved these serum and inflammatory indices, increased intestinal trypsin, Na+/K+-ATPase and γ-glutamyl transferase activities (p < 0.05), and showed more favorable antioxidant and gut microbiota responses. Overall, fermentation was more effective than enzymatic hydrolysis in improving the feeding value of CSM for largemouth bass. These results suggest that FCSM is a promising alternative ingredient for aquafeeds and may contribute to the high-value utilization of crayfish processing by-products.