Self-selection and feed consumption of diets with a complete amino acid composition and a composition deficient in either methionine or lysine by rainbow trout, Oncorhynchus mykiss (Walbaum)

Two feeding experiments were conducted to evaluate the ability of rainbow trout, Oucorhynchus mykiss (Walbaum), to discriminate the sufficiency of a single essential amino acid in diets using self-feeders. In experiment 1, 12 groups of 23-24 fish (initial weight; 25.3 +/- 0.5 g) were acclimated to a casein-based low protein diet (crude protein, CP = 10%) for 2 weeks. They were then offered for 32 days a combination of two wheat gluten-based diets fortified by crystalline amino acids (CP = 40%), with a complete amino acid composition (complete) and a composition deficient in either methionine (Met-def) or lysine (Lys-def) (six groups per treatment). In the complete and Met-def diet selection groups, no specific preference for either of the diets was observed. However, in the complete and Lys-def diets selection groups, a preference for the complete diet increased gradually (P < 0.05, for the last 8-day period). In experiment 2, 12 groups of 21-22 fish (initial weight: 27.6 +/- 0.5 g) were acclimated to a casein-based high protein diet (CP = 40%) for 2 weeks, and then the complete diet and one of the amino acid-deficient diets were offered alternately for 8-day intervals over 32 days (six groups per treatment). In the complete-Met-def diet groups, overall daily feed consumption (g tank(-1)) increased gradually, but that for the Met-def diet seemed nearly stable. In the complete-Lys-def diet group, feed consumption when the complete diet was offered increased, while it then decreased after the complete diet was replaced with the Lys-def diet. These results indicate that trout discriminate sufficiency of lysine in diets and regulate their feed consumption.