Acidifier supplementation in low-fish meal diets improved growth performance and health indices in Asian seabass (Lates calcarifer) juveniles

A feeding trial was done for 60 days to examine the influence of supplementing low-fish meal (FM) diet with a mixture of acidifiers on the performance of Asian seabass (Lates calcarifer) juveniles (initial weight: 54.2 ± 0.5 g, mean ± standard deviation). Dietary FM was replaced (35% and 70%) with a mixture of alternative protein sources, including soybean meal, corn gluten, wheat gluten, and poultry meal then supplemented with two levels (0.5% and 1.0%) of an acidifier mixture (butyric acid, sodium diformate and fulvic acid, 1:1:1 ratio). In this regard, seven isonitrogenous (45%) and isolipidic (15%) diets were formulated including: FM70 (FM-based diet, control), FMR35 (35% FM replacement), FMR35+0.5% (35% FM replacement + 0.5% acidifier mixture), FMR35+1.0% (35% FM replacement + 1.0% acidifier mixture), FMR70 (70% FM replacement), FMR70+0.5% (70% FM replacement + 0.5% acidifier mixture), and FMR35+1.0% (70% FM replacement + 1.0% acidifier mixture). Fish were stocked into twenty-one 2000-L rectangular concrete tanks (53 fish/tank) that were filled with running seawater in a flow-through system (26.5 ± 1.5 °C and 46.0 ± 0.2 ppt). Fish were fed with the diets twice every day up to visual satiation. Before the beginning (day 0), middle (day 30), and after finishing the feeding trial (day 60), fish were individually weighed, and samples were collected from their blood and gut for evaluating hematological, antioxidant and digestive enzymes, respectively. After finishing the feeding trial, fish fed FMR70 had lower weight gain (89.1% lower than FM70) than other groups (P = 0.001) that associated with the lowest feed intake in this group. The trypsin, protease, and α-amylase activities were decreased by increasing FM replacement level, but it enhanced alkaline phosphatase activity (P < 0.05). Serum catalase activity increased by the interactive effects of dietary FM replacement and acidifier level (P = 0.029). The blood hemoglobin, mean cell volume, mean cell hemoglobin, lysozyme activity, serum protein, albumin, globulin, cholesterol, and triglycerides increased by including a 0.5% acidifier mixture in the low FM content diets (P < 0.05). In addition, serum calcium content increased with increasing acidifier mixture level in the diet (P = 0.006). In summary, the inclusion of 0.5% acidifier mixture in a low FM-diet not only improved growth performance and feed utilization but also promoted humoral immunocompetance and hematological indices in L. calcarifer juveniles.