Effects of Different Stocking Densities on Growth Performance, Stress Resistance, Antioxidant Capacity and Immunity of Grass Carp

With the rapid development of intensive aquaculture, unreasonable stocking density has become a major factor restricting the healthy growth of grass carp (Ctenopharyngodon idella). This study aimed to evaluate the effects of three stocking densities (0.57, 1.13, and 2.27 kg/m³) on the growth performance, stress response, antioxidant capacity, and immunity of grass carp. Grass carp with an initial body weight of 81.76 ± 17.69 g were randomly assigned to three groups with three replicates. After 75 days of cultivation, we randomly sampled and measured their growth performance. Reagent kits were used to detect serum biochemical indicators, kidney immune enzyme activity, and liver antioxidant indicators in each treatment group. The expression of spleen immune-related genes was detected using real-time quantitative PCR (RT-qPCR). Results showed that the final body weight, weight gain rate, specific growth rate, and condition factor were significantly higher in the medium-stocking-density group (p < 0.05). High stocking density significantly increased serum cortisol, glucose, transaminases, creatinine, and urea nitrogen, and decreased cholesterol and triglyceride levels (p < 0.05). For immune parameters, the activities of immunoglobulin M (IgM), lysozyme (LZM), antimicrobial peptide (AMP), alkaline phosphatase (AKP), and acid phosphatase (ACP) in the kidneys decreased with increasing density. The mRNA levels of IL-1β, IL-6, TNF-α, and IL-10 in the spleen were significantly upregulated, while IgM was downregulated in the high-density group (p < 0.05). Regarding antioxidant capacity, hepatic total antioxidant capacity (T-AOC), catalase (CAT), and glutathione (GSH) levels increased initially and then decreased with increasing density, while malondialdehyde (MDA) content increased continuously. Collectively, these findings suggest that high stocking density induces growth inhibition, oxidative stress, and immune dysfunction in grass carp. The medium density of 1.13 kg/m³ was found to be optimal for the growth and physiological health of grass carp in this study, providing a scientific basis for the optimization of intensive farming strategies.