Assessment of amino acid requirements for Totoaba macdonaldi at different levels of protein using stable isotopes and a non-digestible protein source as a filler

Stable isotope analyses, in bulk and compound-specific per amino acid (AA), were used to evaluate the isotopic enrichment (Δδ13C, and Δδ15N) in bulk and per amino acid, using different available protein levels at fixed dietary levels of lipids and carbohydrates. Combinations of a blend of dietary protein sources, consisting of digestible and non-digestible amounts were used to produce Totoaba macdonaldi diets containing different proportions of metabolically available protein. The non-digestible part of the protein source blend was prepared by crosslinking the protein using a treatment of exposure to formaldehyde and heat. As larger amounts of available protein were included in the diet, the growth of totoaba correspondingly increased. However, at higher levels, a proportionately greater amount of protein was also metabolized as the source of energy. Moreover, at all protein inclusion levels, no differences were observed in PER, regardless of whether calculation was based on total protein, available protein, or digestible protein. Higher growth at higher dietary protein intake was accompanied by an increase in the isotopic enrichment factor for nitrogen in muscle mass increasing from 2.7 to 3.3‰ indicating an increase in the use of protein as an energy source. A similar pattern of increase in the deposition was observed for the amino acids (AAs) such as Ala, Asp, and Glu (1.0–2.7‰) as well as Leu (4.2‰), Met (1.1‰), and Ile (4.2‰). These results support that protein is preferentially used as an energy source by totoaba especially at higher levels of available protein, as evidenced by the inefficient use of EAAs for growth in each treatment. Thus, the use of stable isotopes demonstrates the usefulness of a methodology that can precisely discriminate whether specific amino acids are being used as a source of energy or for deposition.