Supplementing Nile Tilapia (Oreochromis niloticus) (Linnaeus, 1758) Larvae With Dietary Beta-Glucan Could Improve Their Growth, Survival, Immune Function, Intestinal and Liver Histomorphology

β-glucan exerts a positive impact and is considered as a functional feed additive to enhance fish growth, immunity, control disease outbreaks and pathogen infections by increasing functional, immune and decreasing harmful responses. The present study aimed to determine the most effective dietary level of beta-glucan (βG) to improve Nile tilapia Oreochromis niloticus (L.) fry growth performance, feed utilization, and hematological indicators. Seven isonitrogenous (32% crude protein) and isocaloric diets (18 MJ/kg) were formulated. All diets were identical except for the variation in βG levels. The basal experimental diet (control diet) had no βG added. Diets 2–7 each contained βG at levels of 250, 500, 1000, 2000, 4000, and 8000 mg/kg diet, respectively. A total of 840 fish with an average body weight of 0.2±0.01 g were allocated into the seven experimental treatments (in triplicate). The trial lasted for 120 days, Nile tilapia fry growth performance and feed utilization were significantly (P≤0.05) higher in all treatments receiving βG diets than in the control diet. The fish fed with ßG_0.1 diet showed the highest significant growth indices and the best FCR values. The survival (S%) of the fish also showed a significant increase in the βG diets up to ßG_0.8 levels, when compared to the other experimental diet groups. All hematological parameters increased (P≤0.05) in fish fed with a diet supplemented up to ßG_0.8 compared to the control group. Histological examination of the intestine and liver in the control group revealed histopathological alterations in the villi. On the other hand, the groups treated with βG had longer and structurally normal villi. The most well-preserved intestinal tissue and the tallest villi were observed in ßG_0.1, followed by ßG_0.050 and ßG_0.8 groups. In the liver, the control group exhibited fatty degeneration, necrosis and pyknosis whereas the ßG_0.1 group displayed the most preserved hepatic tissue, followed by ßG_0.8 and ßG_0.2 groups. The results indicate that according to FBW-based broken-line model analysis, the optimal dietary level of βG for Nile tilapia fry to exhibit superior growth and diet utilization efficiency associated with the best FCR for Nile tilapia should be 0.12% βG/kg diet. To increase the immunity of fish and improve the properties of hematological and histopathological indices, the dose can be increased to 0.8% βG without any adverse effects.