Effects of Dietary Protein Level on Growth and Utilization of Protein and Energy by Juvenile Mangrove Red Snapper (Lutjanus argentimaculatus )

A feeding trial was conducted in a recirculating water system to investigate the effects of dietary protein levels on growth, feed utilization, hepatosomatic index and liver lipid deposition of juvenile red snapper, Lutjanus argentimaculatus(average initial wet weight 8.0 ± 0.39 g and total length 3.14±0.3 cm). In the experiment, six fishmeal-based diets were formulated to contain various protein levels (20% to 45% in 5% increments), with dietary energy ranging from 2 210.7 kJ 100 g to 2 250.2 kJ 100 g dry matter. The protein to energy ratios of diets ranged from 8.58 mgprotein kJ-1 to 20.03 mgprotein kJ-1.Diets were fed for 90d to triplicate groups of fish stocked in 0.128 m3 seawater tanks, 25 individuals each. The daily ration of 2% wet body weight was offered to the fish thrice a day. The fish at the end of the study had more than ten-fold (77.0 g) increase in weight compared to the initial (8.0 g). Fish fed diets of 40% and 45% protein produced significantly (P ＜ 0.05) higher weight gain of 77.2 g and 76.5 g, and specific growth rate (SGR) of 2.65% and 2.62% than those of 67.0 g and 68.3 g,and 2.49% and 2.51% of the other diets. The broken-line regression of SGR against dietary protein level yielded an optimum dietary protein requirement of 42.6% ( Y=- 1.6295 + 0.1114 X2, P ＜ 0.05). Survival remained 100% among groups. Feed conversion ratio decreased from 0.45 for fish fed 20% dietary protein to 0.35 for fish fed 45% dietary protein. Nitrogen intake increased with an increase in dietary protein, which in turn resulted in an increase in nitrogen gain of fish whole body.Fish fed 40% and 45% protein diets showed higher (P ＜ 0.05) nitrogen gain (0.27 g and 0.26 g) than those (0.23 g and 0.25 g)fed all other diets. Gross energy intake (GEI)in fish fed 45% protein was lower (600.67 kJ)than that (607.97 kJ) of 40%protein diet, though the differences were not statistically significant (P ＞0.05); GEI ranging from 677.31 kJ to 663.20 kJ at remaining four diets (20% to 35% protein) did not appear to differ significantly (P ＞0.05). The highest energy gain of518.33 kJ was obtained with fish fed 40% protein, resulting in the highest energy retention efficiency of 85.26%. The hepatosomatic index of fish fed diets of 20%, 25%, 30% and 35% protein were significantly (P ＜ 0.05) higher (2.09% to2.57%) than those (1.44% and 1.41%) of fish fed diets containing 40% and 45% protein. Liver lipid contents decreased from 8.72% to 7.0% in fish fed dietary protein of 20% to 45% in 5% increments. Results suggest that the diet containing40% to 42.6% protein with a P/E ratio of 17.6 mg protein kJ-1 is required for good growth of L. argentimaculatus weighing between 8.0 g and 85.2 g under the culture conditions of the present study.