Effects of dietary protein level on protein apparent digestibility and digestive enzyme activities of Litopenaeus vannamei at different sizes

We investigated the effects of 7 dietary protein levels (32%, 34%, 36%, 38%, 40%, 42% and 44%) on protein apparent digestibility, hepatopancreas protease and amylase activity of Litopenaeus vannamei at 3 sizes (Ⅰ: 0.6~4.0 g; Ⅱ: 4.0~10.0 g; Ⅲ: 10.0~20.0 g). The apparent protein digestibility of Size Ⅰ fed with 36% dietary protein was the highest (85.61%), and that of Size Ⅱ and Ⅲ fed with 40% diet was the highest (84.19% and 84.67%, respectively), significantly higher than that of the other diets. For Size Ⅰ, the protease activity of 40% diet was the highest (26.67 U·mg^-1), significantly higher than that of 44% diet; the amylase activity of 32% diet was significantly higher than that of the other diets, and there was no significant difference among 38%~44% diets. For Size Ⅱ, the protease activity of 36% diet was the highest (25.84 U·mg^-1) but no significant difference was observed among all groups; the amylase activities of 32%~38% diets were significantly higher than those of 40%~44% diets. For Size Ⅲ, the protease activity of 44% diet was significantly higher than that of 32% diet; the amylase activity of 42% and 44% diets was significantly higher than that of 32% diet.