Effects of dietary protein levels with various stocking density on growth performance, whole body composition, plasma parameters, nitrogen emission and gene expression related to TOR signaling of juvenile blunt snout bream (Megalobrama ambylcephala)

A 70-day experimental trial was conducted to examine the interaction effects of dietary protein levels supplemented with essential amino acid (EAA) and stocking density on growth performance and physiological responses of blunt snout bream juvenile (Megalobrama ambylcephala). 3 x 3 factorial design is used to formulate three isoenergetic (18 KJ/g(-1)) diets having three levels of dietary protein (30%, 33%, and 36%) and stocking density (20, 40 and 60 fish/m(3)). Triplicate groups of blunt snout bream juvenile (initial weight 4.01 +/- 0.01 g) were distributed into the floating cage (1 m(3)) and feed on the experimental diets according to the design three times daily fill apparent satiation. Results showed that growth performances of the fish were not affected by the level of dietary protein and their interaction but decreased significantly with increasing stocking density. The moisture content of whole fish body was significantly increased, while lipid content was decreased with increasing level of dietary protein. However, moisture content was decreased significantly while lipid and protein contents were increased with increasing stocking density. Total ammonium nitrogen emission was increased significantly with increasing level of dietary protein and stocking density. Plasma glucose was affected positively by the level of dietary protein whereas total cholesterol (TC) was affected negatively. In the meantime, plasma protein, triglyceride, and cholesterol were affected positively by stocking density. Expression levels of the target of rapamycin, insulin-like growth factor-1, eukaryotic initiation factor 4E binding protein 2, and ribosomal protein S6 kinase 1 gene were not affected by the level of dietary protein but affected significantly by stocking density. Generally, using minimum levels of dietary protein supplemented with EAA and optimum stocking density has economic and environmental benefits in blunt snout bream production.