Abstract:This study was conducted to evaluate the effects of replacing fish meal with composite protein on growth performance, diet digestibility, and gut health in Japanese seabass (Lateolabrax maculatus). Five isonitrogenous and isolipidic diets were formulated by replacing 0, 20%, 40%, 60%, or 80% of fish meal in the basic diet with composite protein. Quadruplicate groups of 30 fish [initial body weight (5.80±0.07) g] were fed the test diets to apparent satiation twice daily (08:00 and 16:30), and the experiment continued for 70 days. During the feeding trial, water temperature, salinity, and dissolved oxygen values were maintained at (29.5±1.5)℃, 29±1, and ≥6.5 mg/L, respectively. Samples were collected for analysis after the feeding trial. The main indexes including the growth performance, diet digestibility, intestinal digestive enzymes activities, posterior intestine morphology structure, and expression of inflammation-related genes of the proximal intestine, mid-intestine, and posterior intestine were analyzed to investigate the appropriate replacement proportion of fish meal by composite protein. The results showed that substituting 20% of fish meal with composite protein did not significantly affect growth performance compared to that of the control group (P>0.05), while further increment of fish meal replacement levels resulted in significant reduction of weight gain, survival rate, feed efficiency, and protein efficiency ratio (P<0.05). Moreover, as the replacement level of fish meal with composite protein exceeded 20%, intestinal protease activity and apparent digestibility coefficients of feed dry matter, crude protein, and crude lipid also significantly decreased with increasing level of fish meal replacement by composite protein (P<0.05). Meanwhile, increasing replacement of fish meal with composite protein damaged posterior intestine morphology structure and up-regulated expression of gut inflammatory genes such as TNF-α and IL-1β, while an opposite trend was observed for the expression of anti-inflammatory genes, IL-4 and IL-10. In conclusion, it was recommended that the replacement proportion of fish meal with composite protein should not exceed 20%.
