**1. Introduction**

*Plectropomus leopardus*, commonly known as coral trout, is one of the most important species cultured in China, with such advantages as delicious meat, bright color, and rich nutritional value [1]. The limitations of the marine fishing industry and its high commercial value have promoted the development of the artificial breeding of coral trout [2]. China is a late adopter in the captive breeding of coral trout, and the production in this country is far from sufficient to meet the growing demand. There are currently two mainstream farming methods for coral trout, flowing water culturing, and cage culturing [3]. Flowing water culturing entails a serious waste of water and does not meet today's interests in water and energy conservation. Cage culturing may lead to waves and typhoons, which can cause some pollution to offshore waters. Recirculating aquaculture system (RAS) is developed for intensive aquaculture, which can save water resources and land resources, protect the environment, and help regulate the aquaculture water quality to provide the best aquaculture environment for fish [4]. The fish cultured under RAS have significant advantages in terms of food quality and safety. Coral trout is a warm-water reef fish that prefers to live near coral reefs with low flow velocities [5]. However, the influence of water flow velocity is often ignored in the artificial cultivation of coral trout.

As an environmental factor, water velocity also significantly affects the growth performance, morphological characteristics, and physiological changes of fish [6]. Bl/s is the body length per second. The tolerable water flow velocity of fish is related to their body length [7]. A previous study has reported that high-velocity and high-intensity continuous exercise training (2.0 bl/s) will reduce the growth rate and food conversion rate of juvenile tinfoil barbs (*Barbodes schwanenfeldi*) (15.12 ± 1.35 cm) [8]. Conversely, largemouth bass (*Micropterus salmoides*) (4.50 ± 0.36 cm) can show improved growth performance, digestion, and immunity at a high-water flow velocity (4.0 bl/s) [9]. *Salmo salar* (19.26 ± 0.08 cm) can obtain the best growth performance at 1 bl/s water flow velocity [7]. *Scophthalmus maximus* juvenile (161.9 ± 8 g) shows a reduced feed coefficient and improved specific growth rate at medium water flow velocity [10]. However, there are few reports on the appropriate water flow velocity for breeding coral trout. To optimize the aquaculture facilities and improve the aquaculture environment, it is essential to determine the appropriate water flow velocity for this species.

In this study, the growth performance, blood physiological changes, intestinal digestive enzyme activity, and digestive system structure of fish at different water flow velocities were used as indicators to explore the appropriate water flow velocities for coral trout in RAS. The results from the present study will provide a theoretical basis for the circulation of water culturing of coral trout and promote the healthy and sustainable development of aquaculture.
