*4.2. Changes in Oxidative Stress Parameters and Immune Indexes of Juvenile Yellowfin Tuna under an Acute Temperature Rise*

Superoxide dismutase can maintain the superoxide anion free radicals produced under normal conditions and maintain balance [47]. The research showed that [48] the changes of oxidative stress and related enzyme activities in goldfish tissues caused by temperature rise, indicating that the activity of SOD in the liver has increased by about twice, showing a similar trend in the muscle, which is consistent with the results of this study. The activity of SOD in the liver decreases at 48 h, this may result in the decreased antioxidant capacity in the liver of juvenile yellowfin tuna due to the prolonged stress time. The research showed that [49] the antioxidant response of carp liver under temperature stress. The results showed that under the same conditions, the activity of SOD in the liver decreased with the increase in temperature, under the same salinity, SOD in gills gradually decreased with the temperature increase. This study may differ from the results of our study because the heat stressors previously studied by researchers hinder intestinal digestion and local immunity, thereby inhibiting systemic immunity. In this study, heat stress improved the antioxidant capacity of yellowfin tuna, but did not inhibit it. The results showed that the time after the high temperature had little effect on the SOD in gills, and the SOD in gills tended to be stable after gradually adapting. The change trend of SOD in the red and white muscle of the experimental group was the same, but the red power was more sensitive to temperature. In this study, the muscle content is the highest, which may be because different experimental times have different effects on each organ or other species.

Malondialdehyde (MDA) is a substance with strong toxicity to organisms after the decomposition of lipid peroxide. It can directly reflect the degree of lipid peroxidation and cell damage, and indirectly reflect the ability of cells to remove free radicals [50]. The research showed [51] the tissue oxidative stress response of Nile tilapia under temperature shock. The results showed that the level of malondialdehyde in gills increased significantly. In this study, the concentration of MDA in the gills of the experimental group gradually increased and then decreased, which is similar to the previous study results. Dawood, MAO [51] found that under the same salinity, MDA in gills gradually increased with the increase in temperature, which is also similar to the results of this study. With the extension of time, fish gradually adapt to the environment, SOD increases, and MDA decreases. The MDA had no significant change at the 6th hour in the liver. It may be that the gills are more sensitive to changes in environmental conditions than the liver, so the MDA content

in the gills is significantly increased. In this study, the change of MDA in red muscle is more obvious than that in white muscle, and the MDA content in white muscle gradually decreases and tends to be stable because red muscle is more sensitive to temperature changes than white muscle. According to the analysis of MDA concentration in various organs, acute warming affects the content of the liver, and MDA in gills and red muscles is more sensitive to temperature.

The research showed [52] the effect of different temperature stress on the immune indexes of crucian carp. The results showed that the alkaline phosphatase activity first increased and then decreased with the temperature increase, which was different from the results of this study. This may be because acute warming causes yellowfin tuna to consume a large amount of alkaline phosphatase for non-specific immunity. Additionally, it increases over 48 h, this is because the fish adapt to the temperature change for 48 h, and the non-specific immune function is enhanced. This shows that acute temperature stress significantly changes the non-specific immunity of juvenile yellowfin tuna. The research showed [53] the effect of temperature on the immunity of juvenile carp. The results showed that the activities of C3 and C4 in the liver increased significantly with the temperature increase. This study showed that C3 and C4 decreased first and then increased after high-temperature stress, which was different from previous studies. This may be because heat stress causes cannot be synthesized in a short time, and with the extension of non-specific immune time, the addition of complement gradually increases. These results indicate that the ability of yellowfin tuna to synthesize complement is weak in a short time.

#### **5. Conclusions**

The result shows that the serum ion concentration and osmotic pressure, biochemical indicators of blood glucose, and lactate were changed after acute temperature stress. Acute temperature stress can cause excessive free radicals in the body and reduce immune indicators (e.g., ALP in the liver and the complement C3 and C4 in serum). Under acute temperature rise stress, antioxidant enzymes and metabolic indicators of the juvenile yellowfin tuna changed significantly. The triglycerides, cholesterol, and alkaline phosphatase in serum have changed significantly and gradually adapted to the environment with time. The gills and liver also improve the activities of SOD to eliminate free radicals, but still could not stop the increase in MDA, which may cause peroxidation damage to the body. In this study, the juvenile yellowfin tuna is sensitive to temperature rise, and the tendency of physiological activity disorder was aggravated over time within 48 h. Therefore, in actual production and large-scale intensive aquaculture, it is necessary to avoid sharp temperature changes as much as possible, reduce the frequency and duration of acute temperature stress, and make it a suitable breeding and growing environment.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/jmse10121857/s1, Table S1: Statistical values of HT group (including the squares of F, df, P and eta). Table S2: Statistical values between the control group and experimental group (including the squares of df, P and Sig).

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

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

**Institutional Review Board Statement:** The animal study protocol was approved by the Institutional Review Board (or Ethics Committee) of Animal Care and Use Committee of South China Sea fisheries Research Institute, Chinese Academy of Fishery Sciences (BIOL5346, 9 May 2022).

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

**Acknowledgments:** The authors would like to thank Jicai Liu for his support and help in sample determination.

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