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.

Intensive shrimp aquaculture systems often suffer from water quality deterioration and disease outbreaks. Biofloc technology is one of the sustainable management strategies for shrimp culture, while the choice of organic carbon source critically influences microbial community dynamics. This study investigated the effects of peanut shells (an abundant agricultural by-product) on the bacterial community in shrimp rearing water in Penaeus vannamei culture systems. Over a 28-day controlled experiment, the peanut shell–amended system exhibited significantly lower ammonia and nitrite concentrations than the control, indicating improved nitrogen removal. Overall bacterial diversity and community structure showed only moderate shifts with no significant change compared with the control, the relative abundances of Bacteroidetes , Flavobacteriaceae , and Saprospiraceae showed increasing trends, but were not statistically significant. Redundancy analysis identified phosphate and nitrate as key environmental factors associated with community succession. Functional prediction showed that peanut shells enhanced pathways related to quorum sensing, secretion systems, and organic matter degradation, while reducing biosynthetic activity. Neutral community modeling further revealed that peanut shells did not notably alter the assembly processes across the whole culture period, and these limited changes were mainly attributed to the selective enrichment of potential beneficial bacteria such as Rhodobacteraceae taxa. These findings indicate that peanut shells exert a gentler perturbation on the microbial ecosystem compared to readily degradable carbon sources like molasses or glucose, which are known to provoke more drastic community reassembly. This work highlights the potential of peanut shells as a sustainable carbon supplement for improving water quality in a sustainable manner in shrimp aquaculture.