EFFECTS OF DIETARY PROTEIN LEVELS ON GROWTH, FEED UTILIZATION AND THE ENZYMES ACTIVITY ON NITROGEN METABOLISM OF GRASS CARP (CTENOPHARYNGODON IDELLUS)

The present study was to investigate the effect of dietary protein levels on growth, feed utilization and nitrogen metabolism of grass carp (Ctenopharyngodon idellus). Five isoenergetic diets (13.71 kJ/g) were formulated with 15%, 20%, 25%, 30% and 35% protein level. Fish (209 10 g) were randomly distributed into five groups each with three replicates and were fed twice daily (08:00 and 16:00) to apparent satiation for 8 weeks. The results showed that weight gain and feed utilization were significantly affected by dietary protein levels (P0.05). Fish fed the diet with 25% protein had the highest weight gain, specific growth ratio and feed efficiency ratio than other groups. Protein retention efficiency and protein efficiency ratio inversely correlated with dietary protein levels (P0.05). There were no significantly differences on survival, activities of serum alanine aminotransferase and serum aspartate aminotransferase (P0.05). The dietary optimal protein for grass carp from SGR and FER estimated using the second order polynomial regression was ranged from 26.5% to 27.2%. Serum total protein, blood urea nitrogen, the activities of aspartate aminotransferase, glutamate dehydrogenase in liver and adenosine 5'-monophosphate deaminase in muscle were significantly increased with the increasing of dietary protein levels (P0.05). This denoted increased catabolism of protein to supply energy with the increasing of dietary protein levels.