*3.1. Change of Gill State of Juvenile Yellowfin Tuna under Salinity Stress*

The SOD activity in the fish gills in the stress group increased with time from 0 to 48 h and reached the maximum value at 48 h (830.86 ± 80.66 U·mgprot−1, Figure 2a). The SOD activity in the fish gills of the stress group was significantly higher than those in the control group at 24 h and 48 h (*p* < 0.05, Figure 2a). The GSH-Px activity in the stress group was not significantly different from 0 h to 24 h (*p* > 0.05), and reached the maximum value of 5.25 ± 1.01 U·mgprot−<sup>1</sup> at 48 h (*<sup>p</sup>* < 0.05, Figure 2b). In the stress group, the MDA content in the gills of the juvenile yellowfin tuna was not significantly different from 0 h to 24 h (*<sup>p</sup>* > 0.05), and the highest value (2.87 ± 0.12 nmol · mgprot<sup>−</sup>1) was observed at 48 h (Figure 2c).

#### *3.2. Changes in Liver Status of Juvenile Yellowfin Tuna under Salinity Stress*

The SOD activity in the liver of juvenile yellowfin tuna in the stress group fluctuated slightly from 0 h to 48 h with the extension of the stress time, and no significant differences were observed between the sampling times and between the control and stress groups in the same sampling time (*p* > 0.05, Figure 3a). The GSH-Px activity in the liver of fish in the control group and the experimental group experienced a change trend of first increasing (113.42%) and then decreasing (−19.69%), and reached the maximum value at 24 h (1.50 ± 0.27 U·mgprot<sup>−</sup>1, Figure 3b). Before 24 h, the GSH-Px activity of the fish liver in the stress group was significantly higher than that observed in the control group (*p* < 0.05, Figure 3b), and the lowest GSH-Px activity in the fish liver was observed in the stress group at 48 h. The content of MDA in the liver of the fish in the stress group increased first (137.45%) and then decreased (−97.33%, Figure 3c). At 24 h, the content of MDA reached the highest value of 2.92 ± 0.23 nmol·mgprot<sup>−</sup>1. There was a significant difference between the stress group at each time point (*p* < 0.05). The MDA content in the stress group at 6 and 48 h was significantly lower than in the control group (*p* < 0.05, Figure 3c).

**Figure 2.** Effect of acute low salt stress on the antioxidation of gills of juvenile yellowfin fish (*Thunnus albacares)*, (*n* = 9). (**a**), Superoxide dismutase activity; (**b**), Glutathione peroxidase activity; (**c**), Malondialdehyde. Different letters indicate that there is a significant difference in the same salinity group at different time points (*p* < 0.05). 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.

**Figure 3.** Effect of acute low salt stress on the antioxidation of liver 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.
