Effects of Partial Replacing Fish Meal with Dietary Degossypolled Cottonseed Protein on Growth Performance, Intestinal Barrier, Immune Function and Antioxidant Response in Largemouth Bass (Micropterus salmoides)

The increasing need for aquafeed protein sources calls for sustainable substitutes for fish meal (FM). Degossypolled cottonseed protein (DCP) represents a promising plant-derived protein, but its impact on the intestinal health of carnivorous fish is not well understood. This study assessed how partial replacing FM with DCP affects growth performance, intestinal barrier integrity, immune function, and antioxidant status in juvenile largemouth bass. Five isonitrogenous and isoenergetic diets were prepared, in which DCP replaced 0%, 20%, 40%, 60%, or 80% of the FM. Following an 8-week feeding period, the group receiving 20% DCP replacement showed no significant differences in growth or feed efficiency relative to the control, while 80% replacement caused marked growth reduction and increased visceral fat accumulation. High replacement levels (≥60%) downregulated antioxidant-related genes, damaged intestinal villi, compromised barrier integrity (mucins and tight junctions), and activated inflammatory responses via the NLRP3 inflammasome pathway, along with suppression of immune parameters. In conclusion, replacing 20% of FM with DCP was safe for largemouth bass, while excessive substitution (≥60%) induces intestinal oxidative stress, barrier dysfunction, inflammation, and immunosuppression. These findings support DCP as a viable partial substitute for FM in commercial diets for this species.