**5. Conclusions**

In conclusion, we first found that the aquaculture ship was suitable for the breeding of the large yellow croaker, and its growth performance and nutrient composition were better than the cage. The result provides important basic support for the research and promotion of the aquaculture ship. However, the specific impact of environmental conditions on the large yellow croaker was unclear. Further research is necessary to evaluate the regulatory mechanisms of growth and provide a basis for the improvement of the management of fish in ships.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/jmse11010101/s1, Figure S1: Wave height (A) and wind scale (B) in the nearshore cages (denoted in orange) and offshore ship (denoted in blue) throughout the experiment period.

**Author Contributions:** Conceptualization, Y.Y., H.L. and M.C.; Data curation, W.H.; Funding acquisition, M.C.; Investigation, W.H.; Methodology, Y.Y., W.H. and H.L.; Project administration, H.L.; Software, M.C.; Validation, M.C.; Writing—original draft, Y.Y.; Writing—review and editing, F.Y., H.L. and M.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Key Research and Development Program of China (Grant No.2022YDF2401101), Program of Qingdao National Laboratory for Marine Science and Technology (Grant No.2021WHZZB1301), Key Research and Development Program of Shandong Province (Grant No.2021SFGC0701), and Central Public Interest Scientific Institution Basal Research Fund, YSFRI, Chinese Academy of Fisheries Science (Grant No.2021YJS005).

**Institutional Review Board Statement:** The animal study protocol was approved by the Ethics Committee of the Fishery Machinery and Instrument Research institute, Chinese Academy of Fishery Sciences (protocol code: YJS20210324NM and date of approval: 20210324).

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Thanks to Xuyang Jiang, Guangwei Meng, and Linlin Sun (Conson CSSC (Qingdao) Ocean Technology Co., Ltd., Qingdao, China) for help in the experiment. Special thanks to Shixian Huang for help in making the figures and tables.

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

#### **References**


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**Shengjie Zhou 1,2,†, Ninglu Zhang 1,2,†, Zhengyi Fu 1,2,3,†, Gang Yu 1,2, Zhenhua Ma 1,2,3,\* and Lei Zhao 4,\***


† These authors contributed equally to this work.

**Abstract:** To understand the impacts of salinity stress on the antioxidation of yellowfin tuna *Thunnus albacares*, 72 fishes (646.52 ± 66.32 g) were randomly divided into two treatments (32‰ and 29‰) and sampled at four time points (0 h, 12 h, 24 h, and 48 h). The salinity of the control group (32‰) was based on natural filtered seawater and the salinity of the stress group (29‰) was reduced by adding tap water with 24 h aeration to the natural filtered seawater. The superoxide dismutase (SOD), glutathione peroxidase (GSH-Px), and malondialdehyde (MDA) from liver, gill, and muscle tissues were used as the antioxidant indexes in this study. The results showed that the changes of SOD and GSH-Px in the gills were first not significantly different from the control group (*p* > 0.05) and finally significantly higher than the control group (SOD: 50.57%, GSH-Px: 195.95%, *p* < 0.05). SOD activity in fish liver was not significantly changed from 0 h to 48 h (*p* > 0.05), and was not significantly different between the stress group and control group (*p* > 0.05). With the increase in stress time, GSH-Px and MDA activities in the liver of juvenile yellowfin tuna increased first (GSH-Px: 113.42%, MDA: 137.45%) and then reduced (GSH-Px: −62.37%, MDA: −16.90%) to levels similar to the control group. The SOD activity in the white and red muscle of juvenile yellowfin tuna first decreased (white muscle: −27.51%, red muscle: −15.52%) and then increased (white muscle: 7.30%, red muscle: 3.70%) to the level of the control group. The activities of GSH-Px and MDA in white and red muscle increased first (white muscle GSH-Px: 81.96%, red muscle GSH-Px: 233.08%, white muscle MDA: 26.89%, red muscle MDA: 64.68%) and then decreased (white muscle GSH-Px: −48.03%, red muscle GSH-Px: −28.94%, white muscle MDA: −15.93%, red muscle MDA: −28.67%) to the level observed in the control group. The results from the present study indicate that low salinity may lead to changes in the antioxidant function of yellowfin tuna juveniles. In contrast, yellowfin tuna juveniles have strong adaptability to the salinity of 29‰. However, excessive stress may consume the body's reserves and reduce the body's resistance.

**Keywords:** low salinity stress; antioxidant index; superoxide dismutase; glutathione peroxidase; malondialdehyde
