Periodical changes of plasma free amino acid levels and feed digesta in yellowtail after feeding non-fishmeal diets with or without supplemental crystalline amino acids

ABSTRACT: Postprandial changes of free amino acid (FAA) concentrations in plasma and feed digesta contents were examined in yellowtail, Seriola quinqueradiata (220–280 g bodyweight) fed non-fishmeal diets either with or without supplemental crystalline amino acids (lysine, methionine, threonine, and tryptophan) in order to evaluate the availability of supplemental amino acids. Non-fishmeal diets containing 30% soy protein concentrate as the major protein source were prepared in three diet forms: soft dry pellet, extruded pellet, and single moist pellet. The level of plasma FAA and feed digesta content were determined at 0, 3, 6, 9, 12, 18, and 24 h after feeding. Plasma levels of four supplemental amino acids in fish fed the non-fishmeal diets with EAA were higher than those of fish fed diets without EAA, suggesting that yellowtail can efficiently absorb supplemental crystalline amino acids irrespective of diet form. However, a remarkable difference was observed in the periodical patterns of these four amino acids between FAA derived from supplements and those from feed protein. Moreover, FAA patterns in fish fed the non-fishmeal diets with EAA were different from those of the control fishmeal diet. Of note, methionine concentration was markedly high during the whole experimental period, resulting in an amino acid imbalance that may have caused lower feed performances in fish fed the non-fishmeal diets with EAA compared to the control fishmeal diet.