| Summary: | This study investigated the effects of added eugenol on water quality and the metabolism and antioxidant capacity of the liver and gills of the greater amberjack (<i>Seriola dumerili</i>) during simulated transport. The juvenile fish (10.34 ± 1.33 g) were transported in sealed plastic bags containing different eugenol concentrations at a density of 24.79 kg/m<sup>3</sup> for 8 h. The different eugenol concentrations were divided into five groups: 0 μL/mL (control group), 0.0125 μL/mL, 0.025 μL/mL, 0.0375 μL/mL, and 0.05 μL/mL, with three replicates of each. The results showed that 0.05 μL/mL of eugenol could significantly increase dissolved oxygen, but 0.025 μL/mL–0.0375 μL/mL resulted in a significant decrease in dissolved oxygen and significant increases in NH<sub>4</sub><sup>+</sup>-N and NO<sub>2</sub><sup>−</sup>-N. It was found that 0.05 μL/mL of eugenol caused significant up-regulation of the relative expression of CPT-1 in the liver, significant down-regulation of the relative expression of FAS and PK in the liver and gills, a significant increase in glycogen concentration, and a significant decrease in glucose concentration. This suggests that 0.05 μL/mL of eugenol could reduce the metabolic capacity of fish. In addition, 0.05 μL/mL of eugenol caused significant up-regulation of the relative expression of CAT and a significant decrease of MDA concentration in the liver. Meanwhile, the gills showed significant up-regulation of CAT relative expression, significant down-regulation of Keap1 relative expression, and a significant increase in GSH activity, resulting in a significant increase in MDA concentration when the concentration of eugenol reached or exceeded 0.025 μL/mL. This suggests that 0.05 μL/mL eugenol could improve the antioxidant capacity of fish and lipid peroxidation levels in the gills. In conclusion, the addition of 0.05 μL/mL eugenol could improve water quality, and the metabolic and antioxidant capacities of liver and gills, but it could also increase lipid peroxidation levels in the gills under transport conditions.
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