Ragaa, N.M. et al. 2017. Effect of a Serine-protease on Performance Parameters and Protein Digestibility of Cultured Oreochromis niloticus Fed Diets with Different Protein Levels. Pakistan Journal of Nutrition. 16, 3 (Feb. 2017), 148–154. DOI:https://doi.org/10.3923/pjn.2017.148.154.