The acrylic pressure-resistant chamber is divided into two parts: the inner cylinder and the outer one. The inner cylinder is a cylindrical structure with an internal height of 80 cm and an internal diameter of 30 cm. It is equipped with an inlet valve, an outlet valve, and a pressure sensor at the top. The outer one is a rectangular structure with an internal height of 50 cm and internal dimensions of 35 cm by 35 cm, the experimental space can ensure the normal activities of experimental fish. The chamber is constructed from transparent acrylic sheets, providing excellent pressure resistance and visibility. To avoid refraction of the water surface during observation, an equal height of seawater is added to both the outer and inner cylinders, allowing for clear and detailed observation of the behavioral indicators of the experimental fish to pressure changes within the chamber.

The gas passage is composed of an air compressor, air hose, air filter pressure regulator valve, and proportional valve. The air compressor (Dongcheng DQE800–24 L) can reach a maximum pressure of 7 bar and has a gas capacity of 24 L, ensuring the stability of pressurization during the experiment. The air hose (Dongcheng EVA5*8) connects the experimental equipment in the gas passage. The air filter pressure regulator valve (AW20–02BCG) maintains the pressure in the gas pipeline within a safe and stable range through pressure settings, achieving the regulation of gas settings, ensuring stable pressure, and providing a safe pressurization environment during the experiment. The proportional valve (SMC voltage type ITV1030–312CL) continuously and proportionally controls the airflow, pressure, and direction of the air based on the input analog signal. By opening the air compressor for pressurization, the air flows through the air hose, undergoes filtration and stabilization through the air filter pressure regulator valve, and then enters the acrylic pressure-resistant chamber by controlling the pressurization through the proportional valve based on changes in the valve opening.

The control system consists of a DC power supply, pressure transmitter, PLC controller, and control panel. The DC power supply (Delta CKDU-S150 W/24 V/6.5 A) provides stable regulated DC output for the entire system. The pressure transmitter (CYYZ11A) monitors real-time changes in pressure inside the acrylic pressure-resistant chamber. The differential voltage signal generated by the pressure resistance reaction is amplified through a dedicated amplifier, converting the signal corresponding to the full scale into a standard analog signal, which is then sent back to the PLC controller. The PLC controller (SK2N PLC), based on the input signals and the program written in Gx Works 2 software, executes instructions and outputs controls. The control panel (FLEXEM control panel), programmed with FStudio 2 software on the host computer, allows the setting of pressure and speed for pressurization and depressurization. It accurately displays the internal pressure of the acrylic pressure-resistant chamber and provides manual or automatic control of the proportional valve’s opening. The control panel visually presents all the equipment of the control system in both modes.

The recording system consists of two cameras and a computer, providing real-time monitoring of the activity of experimental fish inside the acrylic pressure-resistant chamber. Cameras are installed on the front and side of the water tank, connected to the computer via a switch. By adjusting the eight vertices of the water tank to the internal video image, the selected camera (Hanbang High-tech HB772S) with a resolution of 1960×1280 pixels and a frame rate of 25 frames/s captures the activity of the fish.

Before the experiment, set the pressurization pressure and pressurization speed through the control panel, then open the intake valve and the air compressor to start pressurization. During the experiment, monitor the real-time pressure on the pressure transmitter and control panel. Once the internal pressure reaches the preset value, close the intake valve to stabilize the pressure inside the chamber. A high-definition camera synchronously records the entire experimental process, capturing the behavioral states of the experimental fish in the chamber. The recorded videos are then imported for image processing and analysis.

The L.crocea used in this experiment were selected from the East China Sea area of Zhejiang Province, China, with a length of 90–110 mm and width of 4–8 mm. The fish were in good health with intact scales. Based on the actual effective avoidance depth of typhoon submersion in the submerged cage, the pressure range of the experiment was set at 0–200 kPa, corresponding to the submerged depth of the cage from 0 m (standard atmospheric pressure) to -20 m water depth pressure. Half of the acrylic pressure-resistant chamber’s inner cylinder was filled with seawater (depth of 40 cm), and the upper half was filled with air. The pressure inside the chamber was adjusted by the control system to simulate various pressure changes during the descent process of fish in a real submerged cage under sea conditions.

This experiment investigated the behaviors of L.crocea under different pressurization speeds (corresponding to the respective test for the descent speed of the cage) and different pressure peaks (corresponding to the respective test for the descent depth of the cage). The experimental conditions are shown in Table 1 . The control group represents L.crocea under normal atmospheric pressure. In the experiment, ΔP refers to the actual value of the pressure in the box minus one atmosphere. H-number represents different pressure peaks, where H1, H2, H3, and H4 correspond to pressures of 50, 100, 150, and 200 kPa, respectively, corresponding to descent depths of 5, 10, 15, and 20 m in the cage, the descent speeds adopted was uniform at 1 m/min. K-number represents different pressurization speeds, where K1, K2, K3, K4, and K5 represent pressurization speeds and cage descent speeds of 0.6,1.0, 1.3,2.0 and 4.0, m/min, respectively. Thirty L.crocea were used in this experiment, and Each set of conditions was repeated with three L.crocea.

Figure 4 illustrates the impact of different pressure values resulting from the submersion depth on the swimming speed of L.crocea. As shown in Figure 4 , at a submersion depth of 0 m (Control group H0—no pressure applied), the average swimming speed of L.crocea is approximately 0.024 ± 0.003 m/s. At submersion depths of 5 m (H1—50 kPa), 10 m (H2—100 kPa), 15 m (H3—150 kPa), and 20 m (H4—200 kPa), the average swimming speeds of L.crocea are 0.029 ± 0.006, 0.039 ± 0.009, 0.055 ± 0.015, and 0.058 ± 0.016m/s, respectively, coefficient of variation <0.3. With increasing pressure, the swimming speed of L.crocea significantly increases. Clearly, the variation in submersion depth of the net cage, causing changes in environmental pressure, holds a certain impact on the physical behavior of L.crocea—swimming speed. Particularly, at submersion depths of 15 m and 20 m, corresponding to environmental pressures of 150 kPa and 200 kPa, the average swimming speed of L.crocea increases significantly, exceeding 0.05 m/s. This represents an increase of over 50% in comparison to L.crocea freely swimming in a comfortable environment (Control group H0—no pressure applied).

Figure 5 presents the impact of different pressure values resulting from the submersion depth on the tilt angle of L.crocea. A tilt angle of 0° indicates that L.crocea inside the net cage maintains a horizontal swimming state. Tilt angles of ±90° indicate that L.crocea inside the net cage is in a state of vertical ascent or descent. As shown in Figure 5 , in the control group H1, where the net cage has no submersion, and thus, L.crocea experiences no additional external pressure, L.crocea is predominantly distributed in a ‘clustered’ pattern, with tilt angles mostly falling within the range of 0°- ± 15°. As the submersion pressure increases, the tilt angles of L.crocea show a ‘forked’ pattern, and with increasing pressure, the range of forked distribution gradually widens, and tilt angles increase with pressure. When the pressure transitions from 100 kPa to approximately the midpoint value of 125 kPa, L.crocea in a horizontal steady state, i.e., 0° tilt angle, is almost non-existent. When the submersion depth reaches 20 m, corresponding to a pressure of 200 kPa, it is evident that the tilt angles of L.crocea approach 90° (vertical state). This indicates that if the pressure is above 125 kPa, L.crocea exhibits an unstable state of either floating or sinking, and at 200 kPa, the discomfort in L.crocea, as reflected by the tilt angle indicator, is most pronounced.

Figure 6 depicts the impact of different net cage submersion depths on the gasping frequency of L.crocea under corresponding pressure values. As shown in Figure 6 , at submersion depths of 0 m (free state), 5 m (50 kPa), and 10 m (100 kPa), the gasping frequency is 0, indicating no occurrence of gasping behavior in L.crocea. However, as the pressure increases to 150 kPa at a depth of 15 m and 200 kPa at 20 m, frequent gasping occurrences are observed, with frequencies of 11 ± 3.21 times/min and 21 ± 6.57 times/min, respectively. The gasping frequency is positively correlated with submersion depth. The research results suggest that L.crocea can adapt to the pressure inside the net cage without the need for gasping behavior to replenish oxygen at submersion depths of 0–10 m. As the submersion depth continues to increase, excessive environmental pressure prevents the exchange of gases within the bodies of fish, necessitating gasping behavior to maintain pressure. Additionally, there is a close correlation between the gasping and tilt angle, especially when a 90° tilt angle is often accompanied by gasping behavior. The trends in tilt angle from Section 3.1.2 and gasping from Section 3.1.3 are consistent.

Figure 7 illustrates the impact of different net cage submersion depths on the tail-beat frequency of L.crocea under corresponding pressure values. Tailbeat frequency is closely related to the behavioral stress response of L.crocea. As shown in Figure 7 , the pressure generated by the submersion depth of the net cage significantly influences the tail-beat frequency of L.crocea. In the control group, where the aquaculture net cage is not equipped with lifting and submersion (resulting in no additional environmental pressure), the tail-beat frequency of L.crocea remains approximately 1 time/s. With increasing pressure, corresponding to the submersion of the net cage, the tailbeat frequency of L.crocea increases. The trend in tailbeat frequency aligns with the results of swimming speed, tilt angle, and gasping frequency with pressure, demonstrating consistent outcomes across the experimental results.

Figure 8 depicts the impact of different net cage submersion speeds on the instantaneous swimming speed of L.crocea under varying pressure change speeds. In Figure 8 , the solid line represents the instantaneous swimming speed of L.crocea, while the dashed lines represent the average swimming speed of L.crocea under different pressure change speeds corresponding to different net cage submersion speeds. In the control group, the instantaneous swimming speed of L.crocea fluctuates relatively smoothly within the range of 0.05 m/s, and the average swimming speed is maintained at 0.0239 m/s. When the submersion speeds, corresponding to pressure change speeds, are set to 0.6, 1.0, 1.3, 2.0, and 4.0 m/min, the instantaneous swimming speed exhibits significant fluctuations. Specifically, at pressure change speeds of 0.6 and 1.0 m/min, the average swimming speed increases to 0.0570 and 0.0590 m/s, respectively. When the pressure change speeds are 1.3, 2.0, and 4.0 m/min, the average swimming speed of L.crocea increases from that of the control group 0.0239 m/s to 0.0742, 0.0801, and 0.0902 m/s, representing a respective increase of 210.5%, 235.1%, and 277.4%. The instantaneous swimming speed of L.crocea is significantly influenced by the net cage submersion speed. As the submersion speed increases (corresponding to an increase in pressure change speed), the swimming speed of L.crocea increases accordingly, especially when the submersion speed reaches 1.3 m/min and beyond.

The excessive submersion speed prevents aquaculture fish from rapidly adjusting their buoyancy by introducing air into their swim bladders. This results in negative buoyancy, and fish often compensate for or adjust this negative buoyancy by continuously changing their swimming direction and angle. Figure 9 presents the distribution of the tilt angles of L.crocea under different pressure change speeds at various net cage submersion speeds. As shown in Figure 9 , firstly, with an increase in pressure change speed, the distribution of tilt angles of L.crocea becomes more scattered. The tilt angles of normal swimming L.crocea are relatively concentrated, showing stable and slight variations within the range of 0 ± 10° (as seen in Figure 5 ). As the pressure change speed increases, the tilt angles of L.crocea vary within the range of 90°, and with a further increase in pressure change speed, the tilt angles of L.crocea significantly increase. Secondly, as the pressure change speed increases, the distribution of tilt angles of L.crocea begins to disperse at smaller pressure values. This indicates that as the pressure change speed increases, the tilt angles of L.crocea become larger at the same pressure, showing that at faster net cage submersion speeds, L.crocea feel more discomfort at the same net cage depth, adjusting their bodies to cope with the rapid changes in pressure.

Precise quantitative analysis of the distribution of tilt angles under different net cage submersion speeds can provide accurate insights into the impact of pressure changes on fish behavior during the submersion process. Figure 10 illustrates the probability distribution of tilt angles for L.crocea under different net cage submersion speeds. Observations from Figure 10 include: (1) With decreasing submersion speed, the frequency of tilt angles within range of 0°- ± 15° gradually increases. (2) At the maximum submersion speed of 4.0 m/min, tilt angles are primarily distributed within the range of ±30°- ± 45°, with less frequent occurrences in the range of 0°- ± 15°.

Due to the varying rates of pressure changes caused by different submersion speeds, the rapid decline in dissolved oxygen content in the net cage due to environmental pressure and pressure changes leads L.crocea to float at the water surface to gulp air due to oxygen deficiency. Mild floating may impact the growth rate of farmed fish, while severe floating can result in mass fish mortality. Figure 11 illustrates the frequency of air gulping by L.crocea under different net cage submersion speeds corresponding to various pressure change rates. As shown in Figure 11 : First, the frequency of air gulping generally increases with the rise in submersion speed. Second, at submersion speeds of 0.6, 1.0, and 1.3 m/min, the air gulping frequency changes are not significant, fluctuating around 8 times/min. When the submersion speed increases to 2.0 m/min, the air gulping frequency significantly increases by a magnitude. At the maximum submersion speed of 4.0 m/min, the air gulping frequency of L.crocea approaches 30 times/min, due to the rapid increase in pressure inside the net cage leading to a sharp decrease in oxygen levels, prompting L.crocea to gulp air frequently to maintain possibly oxygen supply.

Based on the experiment videos recorded by the video system and manually counted, the tail-beating frequency of L.crocea under different pressure change rates is shown in Figure 12 . As depicted in the figure, the tail-beating frequency generally increases with the rise in the speed of environmental pressure. The trend of tail-beating frequency under different net cage submersion speeds corresponding to various pressure change rates maintains a good correlation with the trend of swimming speed changes.