The increased use of protein sources other than fish meal can elevate the risk of unbalanced Met + Cys supply. High inclusion levels of legume proteins (e.g.: soy, pea, faba bean) can lead to Met + Cys deficiencies. For instance Figueiredo-Silva et al. (2015) showed in diets with high inclusion levels of soy in hybrid Tilapia (Oreochromis niloticus × O. mossambicus) an improved growth performance due to the supplementation of Met. Øverland et al. (2009) detected a lower digestibility of Cys in soy compared to fishmeal and pea protein in Atlantic salmon (Salmo salar). This also reduces the available amount of Met + Cys and increases the requirement of total Met + Cys.