Effects of dietary protein levels on the growth, survival, amylase and trypsin activities in large yellow croaker, Pseudosciaena Crocea R., larvae

A feeding trial was conducted to investigate the effects of dietary graded protein levels on the growth, survival, amylase and trypsin activities of large yellow croaker (Pseudosciaena crocea R.) larvae from 12 to 42 days after hatching (DAH). Five approximately isoenergetic microbound diets (16.65MJ/kg diet) were formulated to contain different protein (47.1%, 52.0%, 57.1%, 62.2% and 67.5%) levels. Frozen copepods, containing 54.5% crude protein (CP), 6.0% crude lipid, 27.2% ash and 6.7% glycogen, were used as a control. Each diet was randomly fed to triplicate groups of larvae with an initial mean body weight of 1.76 ± 0.09mg (mean ± SD) in 180L white plastic tanks, and each tank was stocked initially with 3500 larvae. Both the survival and the specific growth rate (SGR) of large yellow croaker larvae significantly increased with increasing dietary protein level up to 57.1%, and decreased thereafter. Frozen copepods resulted in intermediate survival and low SGR compared with the other diets. Whole-body moisture and protein of larvae were not significantly affected by the dietary protein level. In contrast, whole-body lipid of larvae fed diet with 47.1% CP was significantly higher (P<0.05) than those from fish fed the diets containing more than 57.1% CP. Additionally, fish fed the frozen copepods had the lowest whole-body protein and lipid. The amylase-specific activity increased with increasing dietary carbohydrate level during the period of this experiment. However, trypsin activity was not significantly affected by the dietary protein content before 42DAH, indicating a later onset of trypsin than amylase in the regulation of enzymatic synthesis induced by a dietary substrate.