Dietary Supplementation of Astragalus Polysaccharides Modulates Growth Physiology, Metabolic Homeostasis, and Innate Immune Responses in Rice Field Eels (Monopterus albus)

To investigate the dietary effects of Astragalus polysaccharides (APSs) on the growth performance, lipid metabolism, antioxidant activity, and non-specific immunity of Asian swamp eel (Monopterus albus) during the domestication stage, fish were randomly allocated into quadruplicate groups receiving Tenebrio molitor-based diets supplemented with Astragalus polysaccharides (APSs) at graded concentrations of 0 (CON), 700 (APS1), 1400 (APS2), and 2100 (APS3) mg/kg body weight for 28 days. The results showed that dietary APSs at 700–1400 mg/kg·bw significantly enhanced the weight gain rate (WG) and decreased the feed conversion ratio (FCR) of M. albus (p < 0.05). Concurrently, hematological analysis revealed that hemoglobin levels increased by 19.9% and 23.0% in the 700 and 1400 mg/kg APS groups, respectively (p < 0.05). In terms of lipid metabolism, supplementation with APSs significantly increased the serum high-density lipoprotein (HDL) content in all treatment groups (p < 0.05). Lower serum triglyceride (TG) levels were found in the APS2 group (p < 0.05), and decreased triglyceride (TG), cholesterol (CHO), and low-density lipoprotein (LDL) levels were displayed in the APS3 group (p < 0.05). Among the antioxidant parameters, the supplementation with 700 mg/kg·bw APSs significantly increased the glutathione peroxidase (GSH-Px) and catalase (CAT) activity levels of M. albus (p < 0.05). The APS2 group had a significantly increased total antioxidant capacity (T-AOC) and CAT activity levels (p < 0.05), and the APS3 group had significantly increased CAT activity levels (p < 0.05). In addition, the APS1 and APS3 groups had significantly reduced malondialdehyde (MDA) levels (p < 0.05). In terms of non-specific immunity, the APS1 and APS2 groups showed significantly increased superoxide dismutase (SOD) and lysozyme (LZM) activity levels of M. albus (p < 0.05), and the addition of 700 mg/kg·bw APSs significantly increased the levels of alkaline phosphatase (AKP) activity (p < 0.05). Furthermore, the levels of acid phosphatase (ACP) activity were significantly increased in all experimental groups (p < 0.05). In conclusion, the optimal APS addition for T. molitor as biocarrier bait is 700 mg/kg, corresponding to 352 mg/kg, which elicits improvements in the growth parameters, lipid homeostasis regulation, oxidative stress mitigation, and innate immune potentiation of M. albus during the domestication stage.