Effects of defatted soybean protein levels on growth performance and nitrogen and phosphorus excretion in Asian seabass (Lates calcarifer)

The effects of defatted soybean meal protein levels in Asian seabass diets on growth, feed utilization, protein digestibility and nitrogen and phosphorus excretion were investigated. Six isonitrogenous diets were formulated to contain defatted soybean meal to replace fish meal at 0, 10%, 20%, 30%, 40% and 50% of fish meal protein, respectively. Trash fish was also used in the experiment. The fish were cultured in aerated flow-through 81 l aquaria filled with 54 l of seawater. Each diet was fed twice daily to satiation to three replicate groups of juvenile seabass with an average initial weight of 0.95 +/- 0.04-0.99 +/- 0.02 g/fish for 12 weeks. Growth of fish fed diets 1 and 2 (0 and 10%) was significantly higher than that of fish fed diets 3-6 (20-50%) and trash fish. Fish fed diet 6 had the lowest weight gain and specific growth rate but, was not significantly different from those of fish fed diet 5 and trash fish. Feed conversion efficiency (FCE), protein efficiency ratio (PER) and productive protein value (PPV) of fish fed diets 1, 2 and 3 were significantly higher than those of the other diets, particularly the trash fish fed groups. The ammonia excretion of fish fed experimental diets peaked at 10 and 12 h after the first meal and was lowest for fish that were fed diet 1. On the other hand, ammonia excretion of fish fed trash fish was the highest. Fish fed diet 1 had the highest phosphorus excretion, whereas fish fed trash fish had the lowest. Loss values calculated from supply and gain confirmed data obtained through excretion measurements. Furthermore, replacing fish meal with soybean meal in the diets tended to increase nitrogen loss. Feed costs for production of 1 kg of fish was also determined. The cost of diet 2 with soybean meal replacing 10% of fish meal protein was 34.20 baht (US$0.81)/kg of fish whereas that of diet 6 with soybean meal replacing 50% of fish meal protein was the highest at 40 baht (US$0.95)/kg of fish. The results of the present study indicated that soybean meal can replace fish meal at the level of 10% of fish meal protein in diets for Asian seabass with good growth and feed utilization. In addition, replacing fish meal with soybean meal at the stated levels resulted in a reduction of nitrogenous waste in comparison with trash fish.