**5. Conclusions**

In this study, the juvenile yellowfin tuna were subjected to a 48 h acute low-salt stress experiment to evaluate the physical response of the fish. The results showed that, in terms of antioxidant levels, the juvenile yellowfin tuna could adapt to 29‰ within 48 h. Under salinity stress, the antioxidant status in the liver, gills, and muscles of the fish recovered to average levels at 48 h, indicating that the antioxidant system was able to clear the excess free radicals and oxidation intermediates and maintain the dynamic balance of the antioxidant system in the cells in this salinity changing range. This study helps to understand the physiological changes of yellowfin tuna under low salinity, and provides guidance for the artificial culture of yellowfin tuna.

**Author Contributions:** Conceptualization, S.Z. and G.Y.; methodology, S.Z.; software, S.Z. and Z.F.; validation, Z.F.; formal analysis, S.Z.; investigation, S.Z. and N.Z.; resources, Z.F. and L.Z.; data curation, N.Z.; writing—original draft preparation, N.Z.; writing—review and editing, Z.M. and G.Y.; visualization, N.Z. and G.Y.; supervision, Z.M. and L.Z.; project administration, Z.M.; funding acquisition, Z.M. and L.Z. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Hainan Major Science and Technology Project (ZDKJ2021011); the Central Public-Interest Scientific Institution Basal Research Fund, CAFS (2020TD55), and the Central Public-Interest Scientific Institution Basal Research Fund South China Sea Fisheries Research Institute, CAFS (2021SD09).

**Institutional Review Board Statement:** The experiment complied with the regulations and guidelines established by the Animal Care and Use Committee of the South China Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences. The approval number is 2020TD55, which was approved on 5 January 2020.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
