Dietary Salidroside Preparation and Astaxanthin Mitigate High Biomass Density Stress in Pearl Gentian Grouper: Effects on Growth, Antioxidant Status, and Immune Responses

We examined whether dietary salidroside and astaxanthin preparations could alleviate stress in pearl gentian grouper (Epinephelus fuscoguttatus ♀ × E. lanceolatus ♂). Pearl gentian grouper (initial body weight: 56.2 ± 2.0 g) were reared for 42 days in a recirculating aquaculture system with four treatments (three replicates each): medium biomass density (MD, 15.17 kg/m3), high biomass density (HD, 20.61 kg/m3), high biomass density supplemented with 10 mg/kg commercial salidroside preparation (HS, 20.41 kg/m3), and high biomass density supplemented with 50 mg/kg astaxanthin preparation (HA, 20.58 kg/m3). Experimental diets were based on commercial extruded compound feed, with supplementation levels selected from preliminary trials. Compared with MD, HD significantly decreased weight gain rate (WGR), specific growth rate (SGR), and condition factor (CF), while increasing feed conversion ratio (FCR) (p < 0.05). Both HS and HA significantly improved WGR and SGR and reduced FCR compared with HD (p < 0.05). Serum biochemical analysis revealed decreased alkaline phosphatase activity (ALP) and glucose (GLU) concentration and increased aspartate aminotransferase activity (AST) in HD compared with MD. Both HS and HA reduced serum AST and alanine aminotransferase activities (ALT) and increased ALP and high-density lipoprotein cholesterol concentration (HDL-C), while HA also decreased low-density lipoprotein cholesterol concentration (LDL-C) (p < 0.05). In the liver, HD decreased glutathione peroxidase activity (GSH-Px) and increased malondialdehyde (MDA) content, whereas both HS and HA increased GSH-Px and reduced MDA and interleukin-1 beta (Il1b) expression (p < 0.05). Furthermore, HS increased pepsin activity (PEP) and downregulated intestinal nuclear factor kappa B p65 (P65) and Il1b expression, whereas HA upregulated intestinal toll-like receptor 2 (Tlr2) expression (p < 0.05). These findings suggest that both supplements may improve feed utilization efficiency and selected physiological responses under high biomass stocking density conditions.