Use of isotopic enrichment to assess the relationship among dietary protein levels, growth and nitrogen retention in juvenile Totoaba macdonaldi

Stable isotope analysis was used to investigate the relationships among dietary protein levels, growth, protein utilization efficiency, and isotopic enrichment per amino acid for juvenile Totoaba macdonaldi. Four isocaloric diets were formulated to contain different protein levels (400, 430, 460 and 490 g kg⁻¹ DM). After 60 days, growth increased proportionally as dietary protein content increased. The isotopic discrimination factor in bulk samples, revealed increases of 0.83 to 1.24‰ and 3.31 to 3.55‰ for Δδ¹⁵N values of liver and muscle tissues, respectively, of fish fed diets containing 40% to 49% dietary protein. Compound-specific isotope analysis (CSIA) revealed that Δδ¹⁵N for Glu in the liver and Ala in the muscle tissues were enriched by up to 9.37% and 9.0%, respectively, for fish in the 49% dietary treatment, possibly due to transamination processes. Using the traditional parameters such as protein efficiency ratio (PER) failed to show significant differences. Growth of totoaba increased in conjunction with a lower retention of nitrogen according to bulk isotopic values, suggesting that proportionately more protein was being used as an energy source. Relationships among protein productivity values (PPV) corresponded well to those among bulk isotopic values. Considering the low isotopic enrichment in bulk from treatment D40, a diet containing 400 g kg⁻¹ crude protein is recommended for use in future investigations, through the addition of each particular essential amino acids to reach the amount found in a diet contained with 49% protein. And thereafter to determine whether dietary levels of non-essential amino acids might be limiting using the CSIA for amino acids.