Influence of Stocking Density on Growth Performance, Intestinal Antioxidant Status, and Immune‐Related Gene Expression of Endangered Megalobrama pellegrini Reared in a Flow‐Through System

Abstract A 42‐d feeding trial was conducted to evaluate the effects of stocking density on the growth and health status of juvenile Megalobrama pellegrini . Fish (initial body weight = 0.45 ± 0.01 g) were reared in a flow‐through system under five stocking densities: 0.15, 0.24, 0.34, 0.42, and 0.50 kg/m 3 . Significantly lower growth performance (weight gain and specific growth rate) and a higher feed conversion ratio were recorded in fish reared at the highest stocking density (0.50 kg/m 3 ). Similarly, intestinal oxidative stress analyses indicated that activities of the antioxidant enzymes superoxide dismutase and catalase were reduced in M. pellegrini reared at the highest stocking density; the highest malondialdehyde content was also recorded in this group. In addition, intestinal glutathione content was significantly increased in higher‐density groups compared with the lowest density group (0.15 kg/m 3 ), and the highest value appeared in the 0.34‐kg/m 3 group. Intestinal CYP1A * (cytochrome P450 family 1 subfamily A gene) expression was significantly lower in the 0.15‐kg/m 3 group than in the 0.42‐kg/m 3 group. Furthermore, the 0.50‐kg/m 3 group had significantly higher gene expression of Nrf2 * (nuclear factor [erythroid‐derived 2]‐like 2), NF‐ κ B * (nuclear factor kappa beta), and MyD88 * (myeloid differentiation factor 88) than the lower‐density groups. This study indicated that 0.45–1.36‐g M . pellegrini could be cultured at a density of 0.34 kg/m 3 for maximum growth in a flow‐through system without negative effects on the measured physiological indicators of stress and immunity.