*3.4. Changes in White Muscle Status of Juvenile Yellowfin Tuna under Salinity Stress*

The SOD activity of the white muscle of the juvenile yellowfin tuna in the stress group decreased (−28.56%) at 6 h (*p* < 0.05, Figure 5a) and gradually stabilized after 24 h (*p* > 0.05). The SOD activity in the white muscle of the fish at 6 and 48 h was significantly higher than that observed in the control group (*p* < 0.05). However, the SOD activity in the stress group was significantly lower than in the control group at 24 h (*p* < 0.05, Figure 5a). In the stress group, the highest GSH-Px activity (5.97 ± 0.14 U·mgprot<sup>−</sup>1) in the white muscle of the fish was observed at 24 h, and the lowest GSH-Px activity (1.71 ± 0.16 U·mgprot−1) was observed at 48 h (*p* < 0.05, Figure 5b). At 6 and 24 h, the GSH-Px activity of the fish from the stress group was significantly higher than that observed in the control group (*p* < 0.05, Figure 5b). At 48 h, the GSH-Px activity of the fish from the stress group was

significantly lower than that recorded in the control group (*p* < 0.05). The MDA value in the white muscle of the fish was not significantly different between the sampling times and treatments (*p* > 0.05, Figure 5c).

**Figure 5.** Effect of acute low salt stress on the antioxidation of white 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.
