A 9-week feeding trial was conducted to evaluate the effects of dietary phytase on phosphorus (P) utilization of juvenile red drum (Sciaenops ocellatus) . Two control diets were designed to contain 40% crude protein primarily contributed by menhaden fishmeal and plant-protein feedstuffs. One diet without extra monocalcium P supplementation was the negative control diet and contained 0.56% available P (0.77% of total P ). The positive control diet was supplemented with monocalcium P to achieve the previously established minimum dietary P requirement (0.7% of dry diet) . Another four experimental diets were formulated by adding phytase (Quantum Blue Phytase, AB Vista) at 1000, 2000, 4000, or 8000 FTU/kg to the negative control diet. A total of 480 fish (initial weight 4.99 ± 0.09 g/fish) were assigned as groups of 20 fish to 24, 110-L glass aquaria connected as a closed-recirculating system maintained at 7 ppt salinity. E ach dietary treatment was randomly assigned to quadruplicate aquaria. Fish were fed twice daily at representative percentage of body weight that approached apparent satiation and was adjusted weekly after group weighing . In addition, at the end of the trial, P and nitrogen (N) excretion of fish after feeding was quantified at different time intervals as well as disappearance of N and P from different sections of the gastrointestinal tract .
After the 9-week feeding period, there were no significant (P < 0.05) differences i n survival, hepatosomatic index, or intraperitoneal fat ratio among fish fed with various experimental diets. However, c ompared to fish fed the negative control diet, those fed all other diets had significantly increased weight gain and feed efficiency (Table. 1) . In addition, fish fed the diet supplemented with 8000 FTU/kg of phytase had significantly higher weight gain and muscle yield than fish fed the positive control diet. More results will be supplied after remaining samples are analyzed.