From a nutritional point of view, protein utilization indexes can be considered as good indicators of “protein sparing effect” (Goda, 2008). In the present study, the diet containing a protein content of 30% CP + C low, provided the highest growth indexes and dietary protein utilization efficiency compared with the higher dietary carbohydrate content. These results suggest that S. spinifrons could be using dietary carbohydrate as main energy source at lower contents to spare dietary protein for maximum growth. Similar results were obtained by D’Abramo & New (2000), and also by Goda (2008) for M. rosenbergii.