Use of soybean meal and phytase for partial replacement of fish meal in the diet of red sea bream, Pagrus major

This study was designed to determine the optimum dosage of phytase in soybean meal diets for red sea bream to produce good growth performance, digestibility and retention efficiencies of nutrients and energy, and to reduce phosphorus discharge of effluent rearing waters. Six isoenergetic diets were formulated: fish meal 65% (F), fish meal 40% + soybean meal 30% (S), and S supplemented with phytase at 1000, 2000, 3000 and 4000 phytase activity units (FTU) kg− 1 (SP1000, SP2000, SP3000 and SP4000, respectively). Twenty fish (initial mean weight ca. 24 g) were stocked into each of twelve 300-l tanks and two tanks were assigned randomly to each diet. Fish were fed their respective diets for 6 weeks, twice daily until satiation to investigate the growth performance. Fish were then transferred into separate fecal collection columns to investigate digestibility. This experiment was run for another 4 weeks with a similar feeding protocol and 0.5% chromic oxide (Cr2O3) was included as an inert marker.The partial replacement of fish meal by soybean meal in diet S resulted in reduced weight gain, specific growth rate (SGR), digestibility and retention efficiencies of nutrients and energy; however, all parameters were increased significantly in fish fed diet SP2000 (P < 0.05). Phosphorus discharge was also significantly reduced in fish fed diet SP2000 compared to the fish fed diets F and S. The growth performance in fish fed diet SP2000 was as good as those fed diet F. Phytase dosage higher than 2000 FTU kg− 1, however, did not further improve growth performance. This suggests that the optimum dosage of phytase in the diet including 40% fish meal and 30% soybean meal was approximately 2000 FTU kg− 1 for red sea bream.