Essential amino acid supplementation to fish meal-based diets with low protein to energy ratios improves the protein utilization in juvenile rainbow trout Oncorhynchus mykiss

Two 10-week feeding experiments were conducted to evaluate the supplemental effect of essential amino acids (EAA) to fish meal-based diets with low digestible crude protein (DCP) to digestible energy (DE) ratio (DCP/DE) on the growth and protein utilization in juvenile rainbow trout. In each experiment, the control diet having DCP/DE of 24 g/MJ was compared to three lower DCP/DE diets of ratios of 17 and 16 g/MJ in Experiment 1 and 2, respectively. The ingredients containing protein in the control diet were proportionally reduced in the low DCP/DE diets. To the low DCP/DE diets, EAA were: 1) unsupplemented; 2) supplemented to meet the requirements; and 3) supplemented corresponding to 1.2 times the requirements. Each diet was fed to apparent satiation twice a day to juveniles (initial body weight of 21 and 17 g, respectively) at 17 degrees C. In both experiments, the growth of fish was not affected by the dietary treatments, however, the feed efficiency was significantly improved in fish fed the low DCP/DE diet supplemented with EAA at 1.2 times the requirements. Whole body fat content increased whereas the crude protein content decreased in fish fed the low DCP/DE diets relative to those for the control diet, however, the crude protein content of fish fed the low DCP/DE diet supplemented with EAA at 1.2 times the requirements was significantly higher than fish fed the low DCP/DE diet without the EAA supplementation. Accordingly, percentage nitrogen retention was 50-52% in the group fed diets with the higher levels of supplemental EAA compared to 35-36% in the control diet group. By the supplementation of EAA at 1.2 times the requirements, the concentrations of most of free EAA in plasma, liver, and dorsal white muscle of the low DCP/DE diet group increased to identical levels of fish fed the control diet. Taurine concentration in the plasma and tissues showed an opposite postprandial change to that of ordinal amino acids: the concentration decreased after feeding and the lowest levels were noted at around 15 h after feeding. The results of these experiments indicate that juvenile rainbow trout grow well on 16 g DCP/MJ DE diets based on fish meal, and EAA supplementation to these diets improves the protein utilization and thus reduces nitrogen loading into the environment.