Dietary protein requirement of juvenile giant croaker, Nibea japonica Temminck & Schlegel

One 6-week feeding trial was conducted to determine the dietary protein requirement of juvenile giant croaker, Nibea japonica. Semipurified diets were formulated with white fish meal to contain graded levels of protein ranging from 30% to 55% of dry diet. Each diet was fed to triplicate groups of juvenile giant croaker initially weighing 1.86 +/- 0.07 g fish(-1) (mean +/- SD) in a flow-through sea-water system. Giant croaker were raised in aquaria with a water temperature of 16.4-20.0 degreesC for 6 weeks. Weight gain and feed efficiency were best in fish fed diets containing 45% protein or more in the experiment. The protein efficiency ratio value was maximized in fish fed diets containing 45% crude protein, and declined in fish fed diets containing 50% crude protein and higher, Survival was independent of all diets excluding 30% crude protein, At elevated dietary protein level, whole-body moisture decreased and lipid increased (P < 0.05): however, all diets containing different protein levels produced similar condition factors, intraperitoneal fat, whole-body protein and ash. Hepatosomatic index and intestinosomatic index were maximized in fish fed diets containing 50% and 40% crude protein, respectively. Diets containing higher protein levels (45%-55%) produced significantly higher muscle ratio values compared with the diets containing lower protein levels (30%-40%), whereas fish fed all diets containing different protein levels had similar muscle ash, lipid, moisture and protein values. The optimal dietary protein requirement for juvenile giant croaker was deterruined to be 45% of dry diet in natural seawater based on high weight gain, feed efficiency, protein efficiency ratio values and muscle ratio.