In the present study, the effects of dietary supplementation of garlic extract on growth performance, skin mucus immunological parameters and antibacterial activity of C. auratus were examined. Fish (17.12 g ± 2.44 g) were stocked in 100 L glass tanks (6 fish per tank) in triplicate. The fish were fed with diets (40.92 ± 1.37 protein and 3.72 ± 0.70 lipid) containing different garlic extracts [0 (control), 5, 10, and 15 ml/kg] for eight weeks. At the end of feeding period, the fish skin mucus was collected for evaluating the components of non-specific immune system (including lysozyme, complement, total immunoglobulin, dissolved protein, and alkaline phosphatase). Additionally, antimicrobial activity of the skin mucus against Aeromonas hydrophila, Yersinia ruckeri, Micrococcus luteus, Streptococcus faecium, and Streptococcus iniae was assessed. After the feeding trial, the fish fed diets containing garlic extract showed no significant difference in growth parameters. Significantly higher skin mucus lysozyme, complement, alkaline phosphatase activities, and total immunoglobulin and dissolved protein concentration were observed in the fish fed garlic extract-supplemented diets, compared to the control group (p < 0.0001). The antimicrobial activity of the skin mucus increased along with the increase in the dietary garlic extract levels (p < 0.0001). Moreover, garlic extract exhibited the antimicrobial activity against pathogenic bacterial species. The highest level of dietary garlic extract (15 ml/kg) led to significantly higher inhibition zones against pathogenic bacterial species, compared to the other garlic extract levels (p < 0.0001). The optimal administration of garlic extract at 15 ml/kg enhance skin mucus immune parameters and antimicrobial activity in goldfish.

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