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**ȦȂ DZ** ǰ ¢ Ȭ ǻǼ ǻǼ Ȧ ǻǼǯ Ȧ ǻǼ ¢ ¢ ¢ ¢ ¢ ǰ ǰ ǯ

**Zhenjia Qian 1,2, Jincheng Xu 2, Andong Liu 2, Jianjun Shan 2, Chenglin Zhang 2,\* and Huang Liu 2,\***

<sup>1</sup> College of Fisheries and Life Science, Shanghai Ocean University, Shanghai 201306, China


**Abstract:** This study aimed to investigate the effects of different water velocities on the growth performance, blood physiology, and digestive capacity of coral trout (*Plectropomus leopardus*) in a Recirculating aquaculture system (RAS). One hundred and twenty healthy, uniformly sized coral trout (body mass (92.01 ± 8.04) g; body length (15.40 ± 0.65) cm) were randomly assigned to three flow velocity groups (1 bl/s, 2 bl/s, and 2.5 bl/s) and one control group (0 bl/s). The results show that the weight gain rate (WGR) and specific growth rate (SGR) of coral trout in the 2.5 bl/s water flow velocity group were significantly lower than those in the control group and 1 bl/s water flow velocity group (*p* < 0.05), while their feed coefficient (FC) values were significantly higher than those of the control group and 1 bl/s water flow velocity group (*p* < 0.05). The blood glucose (GLU) concentration of coral trout in the 2 bl/s water flow velocity group and the 2.5 bl/s water flow velocity group significantly decreased compared to those in the control group (*p* < 0.05), while the lactic acid (LD) concentration increased. As the cortisol (COR) concentration and lipase (LPS) enzyme activity of coral trout did not significantly change (*p* > 0.05), the α- AMS enzyme activity significantly decreased (*p* < 0.05). Under 2.5 bl/s water flow velocity, the intestinal structure of coral trout changed, and the number of goblet cells decreased. High-water flow velocities affect the physiological homeostasis and intestinal digestion of coral trout, resulting in a decrease in their growth performance, indicating that coral trout is more sensitive to high-water flow velocities. In actual RAS aquaculture, the flow rate should be controlled within 1 bl/s.

**Citation:** Qian, Z.; Xu, J.; Liu, A.; Shan, J.; Zhang, C.; Liu, H. Effects of Water Velocity on Growth, Physiology and Intestinal Structure of Coral Trout (*Plectropomus leopardus*). *J. Mar. Sci. Eng.* **2023**, *11*, 862. https://doi.org/ 10.3390/jmse11040862

Academic Editor: Gabriella Caruso

Received: 6 March 2023 Revised: 17 April 2023 Accepted: 17 April 2023 Published: 19 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

**Keywords:** *Plectropomus leopardus*; water flow velocity; growth performance; blood biochemistry; intestinal digestibility
