The influence of dietary protein sources on tissue free amino acid levels of fingerling rainbow trout

Abstract

To estimate the quantitative adequacy of amino acids in practical dietary proteins, free amino acid (FAA) levels in various tissues of rainbow trout Oncorhynchus mykiss fed different protein sources were compared. Four diets, the proteins of which were derived mainly from fish meal (FM diet), soybean meal (SBM diet), malt protein flour (MPF diet), and the combination of SBM and MPF (COMB diet), were fed to duplicate groups (30 fish/group, 10g/fish) for 6 to 9 weeks at 15°C, until each treatment group reached the weight of the FM diet group attained in 6 weeks. The fish for FAA analysis were sampled 12 h after the last feeding. The body weight of the FM group was attained in 7 weeks by the COMB diet group, and in 9 weeks by the SBM and the MPF diet groups. Concentrations of free threonine, methionine, and lysine in most of the tissues of the COMB diet group, as well as the SBM and the MPF diet groups, were quite lower than those of the FM diet group. Levels of these amino acids between the four diets and most of the tissues examined were highly correlated. Although the threonine content in the COMB diet was slightly lower than the FM diet, these findings suggest that not only methionine and lysine but also threonine is still insufficient in the COMB diet.