The dietary lysine requirement for optimum protein retention differs with rainbow trout (Oncorhynchus mykiss Walbaum) strain

A 12-week feeding trial was conducted to compare the dietary lysine requirement of a fast-growing strain (FGS) and a slow-growing strain (SGS) of rainbow trout, Oncorhynchus mykiss. Five isonitrogenous (37% digestible protein) and isocaloric (17.6 MJ/kg of digestible energy) diets were formulated and supplemented with l-lysine HCl (98.5% lysine) in increments of 0.40% (1.80, 2.20, 2.60, 3.00 and 3.40% diet) resulting in analyzed lysine values of 1.83 (Lys₁.₈₃), 2.20 (Lys₂.₂₀), 2.67 (Lys₂.₆₇), 3.06 (Lys₃.₀₆) and 3.52 (Lys₃.₅₂) g/100 g diet, respectively. Juvenile rainbow trout from the FGS (initial weight 12.2 ± 0.3 g) and the SGS (initial weight, 8.54 ± 0.19 g) were randomly stocked to 145-L tanks, each containing 35 fish per tank, in a 2 by 5 factorial design. Each diet was fed to three replicate tanks of each strain to apparent satiation three times daily. After 12 weeks of feeding, the FGS had significantly higher final weight and daily feed intake (g/fish) than the SGS. No significant diet by strain interactions were detected. Dietary lysine significantly improved body weight gain, and calculated indices of weight gain in both strains up to 2.20% dietary lysine, but not higher. However, whole body crude protein and crude lipid levels significantly increased and decreased, respectively, in both strains with increasing dietary lysine supplementation up to 2.7%–2.8% using broken line regression. The only significant difference between strains was found in protein retention, indicating differences in lean body weight gain. Using broken-line regression analysis of protein retention, the optimum dietary lysine level for the FGS and SGS were 2.75 (6.18) and 2.23 (5.01) % of diet (% of dietary protein), respectively. Lysine retention decreased with increasing dietary levels in both strains. Whole body essential amino acid content, with the exception of arginine, showed significant increases with dietary lysine levels in both strains of rainbow trout. Liver aspartate transaminase activity significantly increased with increasing dietary lysine in both strains, but alanine transaminase activity did not. The results suggest that the optimum dietary lysine requirement of rainbow trout for lean-body growth and protein retention differs with fish strain and is independent of feed intake.