Effects of Dietary Protein Levels on Growth Performance, Plasma Parameters, and Digestive Enzyme Activities in Different Intestinal Segments of Megalobrama amblycephala at Two Growth Stages

An 8-week rearing trial was designed to estimate the dietary protein requirement and evaluate the effects of dietary protein on growth performance, plasma parameters, and digestive enzyme activities of blunt snout bream at two growth stages. Six practical diets were prepared to feed two sizes of fish (larger fish: initial weight of 153.69 +/- 0.85 g; smaller fish: initial weight of 40.89 +/- 0.28 g) with graded protein levels (26%, 28%, 30%, 32%, 34%, and 36%). Our results show that the final weight, weight gain (WG), and specific growth rate (SGR) of the fish initially rose to peak values and then declined as the dietary protein levels increased. The higher WG and SGR were recorded in the larger fish fed diets containing 30%, 32%, and 34% protein, and in the smaller fish fed a 30% protein diet, all significantly higher than those in the control group (p < 0.05). No significant differences were observed in the feed conversion ratio (FCR), viscerosomatic ratio (VR), hepatosomatic index (HSI), condition factor (CF), or survival rate among the treatments at both growth stages (p > 0.05). The plasma total protein (TP) content was highest at both growth stages in fish fed a 30% protein diet (p < 0.05). As the dietary protein level increased, the plasma urea content of the larger fish increased, peaked in the 34% protein group (p < 0.05), and then remained stable. In contrast, no significant difference in the plasma urea content was seen among the treatment groups of the smaller fish (p > 0.05). Protease activity in the fish foregut at both growth stages peaked in the 32% protein group (p < 0.05). In the midgut of the larger fish, protease activity was higher in the control group, while in the smaller fish, it was higher in the 36% protein group (p < 0.05). In the larger fish, hindgut protease activity was higher in the 34% protein group (p < 0.05), while in the smaller fish, there was no significant difference in the hindgut protease activity among all groups (p > 0.05). The dietary protein levels had no significant effect on lipase activity in the foregut, midgut, or hindgut, or on amylase activity in the foregut or midgut of the fish at the two growth stages (p > 0.05). However, hindgut amylase activity was highest in the control group of the smaller fish (p < 0.05). Based on regression analysis, the optimal dietary protein levels for the larger and smaller fish were 30.45% and 29.95%, respectively. Overall, appropriate dietary protein levels (30%) could improve the growth performance, immune function, and health status of fish at two growth stages and promote the adaptive response of their digestive system, especially the spatial regulation of protease activity in different gastrointestinal regions.