Effects of protein levels on growth,feed utilization,body composition,amino acid composition and physiology indices of juvenile chu's croaker,Nibea coibor

A 49‐day feeding trial was conducted to determine the optimal dietary protein level for juvenile chu's croaker Nibea coibor in sea cages. Five isocaloric diets were formulated to contain graded levels of protein: 360 (D36), 400 (D40), 440 (D44), 480 (D48) and 520 (D52) g kg^?1 diet. Each diet was randomly assigned to triplicate groups of 25 fish (mean 76.0 ± 0.8 g) in a total of 15 floating cages. Specific growth rate (SGR), protein retention efficiency (PRE), feed efficiency and feed intake were significantly higher at D44 and D48. Consumed protein (CP) obtained the maximum at D48, and protein efficiency ratio was significantly higher in fish fed with D44 and D48 compared with fish fed with D36 and D52. Hepatosomatic index and condition factor were not affected. Body protein and moisture content increased with increasing dietary protein, while lipid and ash content were not affected. Individual and total essential amino acids maximized at D44. Digestive enzymes and physiology indices in intestine, liver and plasma were differentially affected. Based on polynomial regression analysis of SGR, PRE and CP, we suggested that the optimum dietary protein level for juvenile chu's croaker was 444–478 g kg^?1 diet.