*3.3. Changes in Red Muscle Status of Juvenile Yellowfin Tuna under Salinity Stress*

The SOD activity in the red muscle of fish was not significantly different at 0, 24, and 48 h in the stress group (*<sup>p</sup>* > 0.05), and the lowest value of 813.13 ± 39.81 U·mgprot−<sup>1</sup> was observed at 6 h (*<sup>p</sup>* < 0.05, Figure 4a). The highest GSH-Px activity (15.71 ± 1.65 U·mgprot<sup>−</sup>1) in the red muscle of the fish from the stress group was observed at 24 h, and the lowest value (3.35 ± 0.10 U·mgprot−1) was recorded at 48 h (*<sup>p</sup>* < 0.05, Figure 4b). In the stress group, the highest MDA value of 3.74 ± 0.36 nmol·mgprot−<sup>1</sup> was observed in the red muscle of fish at 24 h (*p* < 0.05, Figure 4c), while no significant difference was observed at other sampling times (*p* > 0.05). At 6 and 24 h, the MDA value of the fish in the stress group was significantly higher than those in the control group (*p* < 0.05).

**Figure 4.** Effect of acute low salt stress on the antioxidation of red muscle of juvenile yellowfin fish (*Thunnus albacares*), (*n* = 9). (**a**), Superoxide dismutase activity; (**b**), Glutathione peroxidase activity; (**c**), Malondialdehyde. Different letters indicate the difference between the experimental group and the control group over time, and \* indicates the difference between the experimental group and the control group.
