The role of β-glucan in the growth, intestinal morphometry, and immune-related gene and heat shock protein expressions of Nile tilapia (Oreochromis niloticus) under different stocking densities

This study aimed to evaluate the growth, intestinal morphometry, immune-related genes, and heat shock protein in Nile tilapia (Oreochromis niloticus) reared at a rate of 10 (low density-LD, 200/m(3)), 20 (medium density-MD, 400/m(3)), and 30 (high density-HD, 600/m(3)) fish per aquarium (50 L). The fish were fed two diets [control and beta-glucan (BG)] (2 x 3, factorial design) for 60 days. The growth parameters were significantly decreased by increasing the rearing density of Nile tilapia fed the control diet, whereas the fish reared in LD and MD and fed the BG diet showed increased growth (P < .05). However, the feed conversion ratio (FCR) was significantly increased in the fish reared in HD without the BG diet, whereas the fish reared in LD and MD and fed the BG diet significantly had the lowest FCRs (P < .05). Intestinal morphometry revealed decreased villus length, villus width, and goblet cell count in the fish reared in MD and HD and fed the control diet (P < .05), whereas the fish reared in LD and MD and fed the BG diet showed the highest values for these parameters (P < .05). Both hemoglobin (Hb) levels and red blood cell (RBC) count were lower in fish reared in MD and HD and fed the control diet, whereas the fish reared in in LD and MD and fed the BG diet showed comparatively higher Hb levels and RBC counts (P < .05). The fish reared in HD had higher cortisol and glucose levels than those reared in LD and MD and fed the BG diet (P < .05). The fish fed the control diet showed similar lysozyme activity, phagocytic activity, and phagocytic index at all stocking densities (P > .05), whereas those fed the BG diet showed relatively higher values (P < .05). The BG diet resulted in increased transcription of INF-gamma in the fish reared in MD and HD, whereas TNF-alpha and IL-1 beta were upregulated in the fish reared in LD and fed the BG diet. The fish reared in HD had higher HSP70 transcription levels than those reared in LD and MD, irrespective of their diet. In conclusion, we recommend to feed fish with the BG diet to counteract the potential negative impacts of crowding stress on Nile tilapia aquaculture by improving their growth, intestinal morphometry, stress resistance, and immunity.