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Dietary supplementation with royal jelly (RJ) may improve growth, antioxidant gene expression, and intestinal antioxidant capacity, and possibly mitigate effects of stress induced by high stocking density (HSD) in zebrafish (Danio rerio). Effects of HSD and RJ supplementation on growth performance, antioxidant gene expression, and intestinal antioxidant capacity in zebrafish were investigated. A total of 240 four-month-old zebrafish were used in a 2 x 2 factorial design, with four treatments and three replications. The treatments consisted of normal stocking density (NSD) (1 fish/L) and high stocking density (HSD) (3 fish/L) and an unsupplemented commercial diet (0RJ) and commercial diet supplemented with 10% RJ (10RJ). The experiment lasted 25 days. Daily weight gain, final body length, specific growth rate, and survival were recorded. On day 25, six fish per treatment were euthanised and their intestines analysed for antioxidant capacity, and superoxide dismutase 2 (SOD2) and catalase (CAT) gene expression. There were no effects of diet or SD and diet interaction on growth. High stocking density decreased daily weight gain by 25.31%, body length by 2.70%, and survival by 16.85%. Stocking density (SD) and diet interaction influenced intestinal SOD2 and CAT expression and antioxidant capacity. The HSD/0RJ group had lower antioxidant gene expression and antioxidant capacity than the HSD/10RJ and NSD/0RJ groups. Supplementation with RJ did not mitigate the harmful effects of stocking stress. These results indicate that HSD promotes transcriptional suppression of gene encoding enzymes that form the first line of antioxidant defence, resulting in reduced elimination of toxic substances and occurrence of intestinal oxidative stress.