**2. Materials and Methods**

#### *2.1. Experimental Fish and Design*

A batch of juvenile coral trout was obtained from Delin Chengxin Aquaculture Co., Ltd. (Lingshui, China) with an initial body weight of 92.01 ± 8.04 g and a body length of 15.40 ± 0.65 cm. The culture experiments were carried out in a recirculating water culture tank at the pilot plant of the Institute of Fisheries Machinery and Instruments of the Chinese Academy of Fisheries Sciences (Figure 1). The system consists of a culture tank, a push-flow water pump, a wet and dry separation filter, a moving bed, a UV sterilization device, and a reservoir. The mobile bed has an oxygenating and aeration device to ensure the presence of sufficient dissolved oxygen in the experimental setup. Each tank is equipped with a push-flow water pump, and the intended water flow velocity is provided by a pump (Pond Spring, ABB-200), pushing the water at a speed of 0.5–7.5 bl/s (10–120 cm/s) in the tank.

**Figure 1.** Circulating water culture tank.

The experiment was divided into three water flow velocity groups, with water flow velocities of 1 bl/s, 2 bl/s, and 2.5 bl/s. In addition, a control group was set up with a water flow velocity of 0 bl/s. Each group contained 3 tanks, and the experimental fish were evenly allocated to each tank. There were 10 fish in each tank and a total of 120 fish in the experiment. After one week of acclimatization, the experiment started and then lasted for 42 d. During the experimental period, the fish were fed at 2% of their body mass daily. The fish were fed three times daily at 8:00, 13:00, and 18:00, and the amount of food and residual bait was recorded. Body weight and length measurements were conducted on the 7th, 14th, 21st, 28th, 35th, and 42nd days of the experimental period to adjust the water flow velocity and feeding rate. A total of 6 fish (2 fish per tank) were collected from the control group and the different water flow velocity groups. They were weighed with water and put back into the original tank after weighing. We ensured that the feeding amount was always maintained at 2% body weight and that the water flow velocity was maintained at 1 bl/s, 2 bl/s, and 2.5 bl/s. The water temperature was maintained at 26.5 ± 1.5 ◦C, salinity 22–24‰, pH 8.07–8.31, dissolved oxygen 8.08 ± 0.26 mg/L, ammonia nitrogen 0.07 ± 0.05 mg/L, and nitrite 0.18 ± 0.12 mg/L.

#### *2.2. Sample Collection and Processing*

#### 2.2.1. Sample Collection

On day 42, the experimental fish were sampled for blood samples, and intestinal and liver tissue. Before sampling, the experimental fish were starved for 24 h, and a total of 6 fish were collected from each experimental group (2 per tank). They were put into MS-222 water with a concentration of 120 mg/L for rapid anesthesia, and their body length was measured and weighed. First, blood was collected from the caudal vertebral vein of the experimental fish, and then the fish's body was dissected. The visceral mass was taken out and weighed along with the liver. The intestines were separated from the visceral mass, and the foregut, midgut, and hindgut were collected and transferred to a 10% concentration of formalin for preservation, which was used for the observation of intestinal tissue sections. The rest of the intestines were placed directly into a cryopreservation tube and stored in liquid nitrogen.
