**5. Conclusions**

The results show that under water velocity stress, the growth indicators, serum biochemical indicators, intestinal digestive enzyme activity, and intestinal structure of coral trout have changed. High-water flow velocity (≥2 bl/s) stress can lead to a decrease in GLU levels and an increase in LD content in fish, disrupting their normal physiological homeostasis. The high-water flow velocity has an inhibitory effect on the α-AMS activity of the coral trout and reduces the number of goblet cells, altering the intestinal structure and weakening the digestive capacity of the gut, ultimately leading to a reduction in SGR and WGR. This study shows that coral trout are sensitive to water flow velocity, and high flow velocity can exacerbate physiological disorders, weaken digestive enzyme activity, and alter the intestinal structure in this species. Therefore, in actual RAS aquaculture, the water flow speed should be controlled within 1 bl/s, and the impact of high-water flow velocity stress on this species' physiology and intestinal digestion should be avoided as much as possible to achieve a better aquaculture environment.

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

**Funding:** The research is funded by Hainan Province Science and Technology Special Fund (ZDYF2022XDNY349); High-tech Ship Research Project of the Ministry of Industry and Information Technology, "Engineering Development and Key System Development of Movable Breeding Boat" (GXH (2019) No. 360).

**Institutional Review Board Statement:** The experiment conforms to the regulations and guidelines formulated by the Animal Protection and Use Committee of the Fishery Machinery and Instrument Research Institute, Chinese Academy of Fishery Sciences. The approval number is FMIRI-AWE-2023- 001, and approval was given on 30 September 2022.

**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.

**Acknowledgments:** The authors would like to thank Zhenhua Ma and He Zhang for their support and help in the experiment.

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