A device and method for culturing anguilla anguilla

Abstract

The application discloses a cultivation device and method for Anguilla marmorata, belonging to the field of aquatic breeding. The cultivation device comprises a tank body, an aeration pipe is arranged on the side wall of the tank body, the tank body is sequentially connected by straight side walls on the front and back sides and arc walls on the left and right sides, a plurality of straight side light strips are arranged on the two straight side walls respectively, a top light strip and a bottom light strip are arranged on the two arc walls respectively, a partition is arranged on the bottom surface of the tank body, a V-shaped shallow groove is arranged around the partition on the bottom surface of the tank body, water inlet pipes are arranged on the straight side walls respectively, the water inlet pipes respectively inlet water in the tangential direction along the circulating direction, and the water inlet pipes jointly form a one-way closed circulation, a sewage outlet is arranged on the bottom of each arc wall, and the cultivation method comprises a feeding process, a sewage discharge process and a drifting habitat. The application solves the problems of aggregation stress and insufficient nutrition supply in the artificial breeding of Anguilla marmorata, improves the survival rate and metamorphic development quality of Anguilla marmorata at the stage, and meets the needs of large-scale breeding.

Description

Technical Field

[0001] This invention relates to the field of aquaculture technology, specifically to a cultivation device for the eel species *Eriocheir sinensis*, and also to a cultivation method using the cultivation device. Background Technology

[0002] In the fully artificial breeding of eel species, the cultivation of the phyllota stage is a crucial step determining the success or failure of the breeding process. Wild willow eels inhabit continuously flowing ocean currents, passively drifting with the current. Their thin, transparent bodies make them highly susceptible to infection from abrasions. They are also highly specialized in morphology and physiology: their retinas are predominantly composed of rod cells, exhibiting a strong negative phototaxis response to blue-green light in the 470–480 nm wavelength range, while showing almost no sensitivity to visible light above 500 nm; they have extremely weak mobility, possessing almost no independent swimming ability, and are highly sensitive to water turbulence; their feeding behavior involves probing with their snout and scraping suspended soft particles in the water with their needle-like teeth, lacking the ability to actively hunt or filter feed. These characteristics place special demands on the flow field, light environment, and food morphology in the breeding apparatus for willow eels.

[0003] Currently, facilities used for raising willow eels are mostly circular tanks or traditional racetrack-style tanks. Circular tanks rely on the overall rotation of the water to create circulation, with uniform curvature throughout. This centripetal compression continuously acts on the larvae, easily disrupting the stable distribution of willow eels within the tank. Traditional racetrack-style tanks have right-angle transitions between straight and curved sections, resulting in significant flow shear in the bend areas and the potential for creating low-speed dead zones. Neither of these types of facilities can provide a smooth, stable, unidirectional drifting environment for the extremely weak willow eels.

[0004] In terms of lighting management, existing facilities usually adopt uniform top lighting or overall shading, without considering the impact of the willow eel's avoidance response to specific wavelengths of light on the spatial distribution of the willow eel. Under uniform lighting conditions, willow eels are prone to abnormal behaviors such as clinging to the wall, stacking stress, and hitting the bottom of the pool.

[0005] In terms of feed supply and water quality maintenance, the traditional method mainly involves laying paste-like feed on the bottom or directly scattering pellet feed. However, the feed is difficult to maintain in the water in a suspended and slowly sinking form suitable for the willow eel to scrape and feed. After feeding, most of it settles at the bottom of the tank, making it difficult for the willow eel to feed effectively. The uneaten feed degrades continuously at the bottom of the tank, and the ammonia nitrogen and nitrite in the water increase accordingly. In order to maintain water quality, the breeding process often requires frequent water changes or tank emptying. Frequent human intervention further aggravates the stress response of the willow eel.

[0006] In addition, artificial breeding of eels usually needs to be carried out in near-isotonic brackish water. Existing schemes mostly only provide salinity ranges and do not set the relative relationship between the apparent density of the breeding water and the apparent density of the feed as a physical constraint for facility operation. The sedimentation and accumulation behavior of feed in the water is difficult to control, which further exacerbates the difficulty of fixed-point feeding and uneaten feed removal.

[0007] In summary, existing cultivation devices and methods cannot simultaneously solve the problems of stress aggregation and insufficient nutrient supply in the fully artificial breeding of eels. The survival rate and metamorphosis quality of eels at the eel stage are still difficult to meet the needs of large-scale breeding. Summary of the Invention

[0008] The technical problem to be solved by this invention is to provide a cultivation device and method for *Eriocheir sinensis*. First, the cultivation device forms a unidirectional closed loop through a tank and a flow field guide barrier. A time-division multiplexing aquaculture light system is used to construct light field partitions, defining suitable habitat and feeding areas. Combined with shallow V-shaped feeding ditches and slow-sinking active flocculents (which settle in the water due to the apparent density difference between themselves and the water and accumulate in the feeding ditches to form ridges), controlled aggregation, reduced stress, and adequate nutrient supply are achieved. Second, through the feeding, wastewater discharge, and drifting habitat procedures of the cultivation method, stress caused by human intervention is reduced, improving the survival rate and metamorphic development quality of the *Eriocheir sinensis*.

[0009] The technical solution of the present invention is as follows: A cultivation device for the eel species *Eriocheir sinensis* includes a tank with several aeration pipes on its side walls. The tank is composed of straight side walls on the front and rear sides and curved walls on the left and right sides connected sequentially. Several straight light strips are vertically arranged on the inner sides of the two straight side walls, and these strips are arranged sequentially along the length of the straight side walls. Top light strips are provided on the inner top sides of the two curved walls, and bottom light strips are provided on the inner bottom sides of the two curved walls. The straight light strips, top light strips, and bottom light strips are all used to emit blue-green light with a wavelength of 470–480 nm. The bottom surface of the tank is vertically provided with a partition, which is located in the center of the tank and is arranged in the left and right direction. There are water passages between the left and right ends of the partition and the arc-shaped wall at the end of the partition. The bottom surface of the tank is surrounded by a V-shaped shallow ditch, which includes a sludge collection ditch located inside the straight side wall and a feeding ditch located inside the arc-shaped wall. The two sludge collection ditches and the two feeding ditches are connected in sequence. Each straight wall is equipped with a water inlet pipe, which tangentially introduces water along the circulation direction to form a unidirectional closed circulation; each curved wall is equipped with a drain outlet at its bottom.

[0010] Preferably, each sewage outlet is connected to a sewage pipe, and a connecting pipe is provided vertically upward on the side of the sewage pipe near the trough; the top of the connecting pipe is open, and the top edge of the connecting pipe is lower than the top edge of the trough; a valve is also provided on the sewage pipe, and the valve is located on the outside of the connecting pipe; the sewage outlet is located at the tip of the bottom of the feeding ditch.

[0011] Preferably, a plurality of the aeration pipes are arranged circumferentially around the inside of the tank, and the aeration direction of the aeration pipes is obliquely downward at 30° to 45° along the circulation direction inside the tank.

[0012] Preferably, the ratio of the major axis to the minor axis of the groove is 3:1 to 8:1.

[0013] Preferably, the angle between the wall of the V-shaped shallow ditch and the horizontal plane is 5° to 10°, and the ditch depth is 2 to 4 cm; the bottom tip of the feeding ditch is provided with a soft layer.

[0014] A cultivation method using the cultivation device includes the following steps: Brackish water with a salinity of 15‰ to 20‰ is injected into the tank through the inlet pipe; S1. Feeding process: First, turn on the straight-edge light strip and set its illuminance to 5-15 lx. Then, turn off the top and bottom light strips, close the aeration pipe, and adjust the water inlet flow rate to maintain the circulation velocity in the tank at 0.3-0.5 cm / s. After the willow eels gather in the feeding area surrounded by the two curved walls, close the water inlet pipe and feed them slow-sinking active flocculents into the feeding ditch. The apparent density of the slow-sinking active flocculents is higher than that of the brackish water. After the slow-sinking active flocculents reach the bottom of the tank, turn on the top light strip and set its illuminance to 5-10 lx to drive the willow eels to avoid the light and sink away from the top of the tank. The willow eels use their snouts to probe and scrape the slow-sinking active flocculents with their needle-like teeth above the feeding ditch and put them into their mouths and throats to feed. At this time, the willow eels are in a feeding state. The feeding process is repeated 5 to 6 times a day, with the interval between two consecutive feeding processes being evenly distributed. S2. Sewage discharge process: After the feeding process is completed, turn on the straight edge light strip and set its illuminance to 0.1-1 lx, while keeping the top light strip closed; turn on the bottom light strip and set its illuminance to 5-15 lx to drive the willow eels to avoid the light and float to the surface, away from the bottom of the tank. After the eels gather at the top of the tank, open the valve. The difference in liquid level between the tank and the connecting pipe serves as the power source for natural drainage. The waste in the feeding ditch is discharged from the drain pipe with the water flow, and the waste in the collection ditch is collected and discharged from the tank. S3, Drifting Habitat: During the remaining time period, the willow eels are in a resting state. During this period, the straight edge light strip and the bottom light strip are turned on, and their illuminance is set to 0.1-1 lx. The top light strip is turned off, the aeration pipe is turned on, and the water inlet pipe is adjusted to maintain the circulation velocity in the tank at 0.8-1.2 cm / s.

[0015] Preferably, the brackish water with a salinity of 15‰ to 20‰ has an apparent density of 1.0113 to 1.015 g / cm³ at 25°C, and the apparent density of the slow-settling active flocs is 1.016 to 1.021 g / cm³.

[0016] Preferably, the feeding process lasts for 45 to 60 minutes each time, and the filling height of the slow-settling active flocculent in the feeding ditch is 10 to 40 mm from the bottom tip of the ditch; the sewage discharge process lasts for 10 to 15 minutes each time.

[0017] Preferably, when manual operation is required, a red light working light with an external ambient light intensity of 620–630 nm is turned on.

[0018] Preferably, the method further includes the following steps: S4. Use of compound microbial agents: Throughout the entire cultivation cycle, including the feeding, wastewater discharge, and drifting habitat processes described above, a compound microbial agent used to maintain low ammonia nitrogen levels in the aquaculture water is diluted and added to the water. The ammonia nitrogen content in the water is monitored in real time. The frequency and dosage of addition must meet the following requirements: maintaining the ammonia nitrogen concentration below 0.05 mg / L and the nitrite nitrogen below 0.01 mg / L. The compound microbial agent consists of *Rhodopseudomonas palustris* strain P-3 and *Bacillus amyloliquefaciens* strain B4. The *Rhodopseudomonas palustris* strain P-3 is numbered CCTCC NO: M 2020808, and the *Bacillus amyloliquefaciens* strain B4 is numbered CCTCC NO: M 2019853. The ratio of viable *Rhodopseudomonas palustris* strain P-3 to viable *Bacillus amyloliquefaciens* strain B4 in the compound agent is (2-3):1.

[0019] Compared with the prior art, the present invention has the following advantages: First, the breeding device integrates the flow field, light field, trench structure, and physical properties of the feed into a unified design. This allows the willow eels to stably distribute in suitable habitats in the water during non-feeding periods and to gather in suitable feeding areas above the feeding trench during feeding periods to scrape and feed at specific points. This fundamentally avoids the problems of willow eels sticking to the wall, stacking stress, insufficient feeding, and elimination of weak seedlings that occur with traditional devices. As a result, the survival rate and metamorphosis quality of the willow eels are improved.

[0020] Second, the tank and the central partition work together to form a unidirectional closed circulation within the device, with the flow connected end to end and the direction constant. The circulation path transitions with a circular arc tangent, without right-angle deflections, resulting in low water shear force and no low-speed dead zones. This aligns with the physiological characteristics of the willow eel, which has almost no autonomous swimming ability and is averse to violent turbulence. This circulation flows continuously without beginning or end, simulating the environment of the willow eel's natural habitat where it passively drifts with ocean currents. The willow eel can move with the current without actively swimming, thus reducing maintenance stress.

[0021] Third, the straight-edge light strip, top light band, and bottom light band all utilize 470-480nm blue-green light, which is most sensitive to the negative phototaxis of the willow eel. By switching on and off and adjusting the illuminance under different operating conditions, a light wall is formed in the straight-edge section, a relatively dark area is formed in the curved section, and a repelling light field is formed at the bottom of the tank. The spatial distribution of the willow eels is guided to the designated area, achieving controlled aggregation without the need for manual catching or driving. The bottom light band is activated during the waste disposal process to drive the fish away from the bottom, ensuring that the willow eels are not damaged throughout the process. The same light source completes the feeding, waste disposal, and resting processes at different times, simplifying the device structure.

[0022] Fourth, the angle of the shallow V-shaped feeding ditch wall, the depth of the ditch, and the soft layer at the tip of the ditch bottom together form a feed-bearing structure adapted to the scraping and feeding habits of the willow eel. The slow-sinking active flocculents accumulate in the ditch to form a wedge-shaped ridge, and the soft layer reduces the risk of abrasion when the willow eel touches the bottom. The slow-sinking active flocculents have a porous and loose structure and are biologically active. Their filling height and the covering layer that is always maintained during the feeding period ensure that the willow eel can continuously scrape and feed within the feeding window, avoiding bottom exposure stress and compression of the feeding layer.

[0023] V. The apparent density relationship between brackish water and slow-settling active flocs determines the settling behavior of flocs in water bodies. The apparent density of brackish water at 25℃ with a salinity of 15‰~20‰ is 1.0113~1.015 g / cm³, while the apparent density of the slow-settling active flocs is 1.016~1.021 g / cm³. The apparent density of the slow-settling active flocs is higher than that of the brackish water. The static water settling rate is 1.5~2.5 mm / s, which allows the slow-settling active flocs to slowly sink and stably accumulate at the bottom of the feeding ditch in the circulation. This prevents them from suspending and spreading throughout the pond, thus avoiding rapid settling and hardening. The difference in apparent density alone is sufficient to achieve targeted feeding. In the sewage discharge process, the waste (including uneaten feed) in the feeding ditch is discharged with the water flow through the sewage pipe, and the waste (including uneaten feed) in the collection ditch is manually collected and discharged, improving the efficiency of waste removal.

[0024] VI. Cultivation Method: Following the behavioral patterns of the willow eel, the feeding, wastewater discharge, and drifting / resting processes are sequentially linked, ensuring that each function of the apparatus operates effectively under its respective conditions. During the feeding phase, the influent volume is reduced and the circulation slowed to create a weak-current feeding environment. Once the straight-edged light strips drive the willow eels into the feeding area surrounded by the curved wall, a measured amount of slow-sinking active flocculent material is then fed. During the wastewater discharge phase, the bottom light strips are used to drive the fish off the bottom before the valves are opened to flush out the waste. During the drifting / resting phase, circulation and aeration are restored to maintain drifting with the current. Throughout the entire cultivation cycle, the willow eels do not require transfer to different ponds, maintain a stable operating rhythm, significantly reduce the number of human interventions, and correspondingly decrease the stress response caused by operational disturbances. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the overall structure of the cultivation device involved in this invention.

[0026] Figure 2 yes Figure 1 A front and rear sectional view.

[0027] Figure 3 This is a top view of the bottom of the cultivation tank of the cultivation device involved in this invention.

[0028] Figure 4 yes Figure 1 Cross-sectional view of the central curved wall section from left to right.

[0029] In the diagram: 1. Straight wall; 2. Aeration pipe; 3. Water inlet pipe; 4. Straight light strip; 5. Curved wall; 6. Top light strip; 7. Connecting pipe; 8. Valve; 9. Sewage pipe; 10. Tank; 11. Partition; 12. Sewage collection ditch; 13. Feeding ditch; 14. Sewage outlet; 15. Bottom light strip. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Example 1: Device Example

[0032] The cultivation device of this invention is used for the fully artificial breeding of *Eriocheir sinensis*, a species of eel. The retina of *Eriocheir sinensis* is mainly composed of rod cells, exhibiting a strong negative phototaxis response to blue-green light in the 470–480 nm wavelength range, but almost no sensitivity to light above 500 nm. Its mobility is extremely weak, possessing almost no autonomous swimming ability, and it is highly sensitive to water turbulence. Its feeding method involves probing with its snout and scraping suspended soft particles in the water with its needle-like teeth. Based on these biological characteristics, this invention achieves low-stress cultivation of *Eriocheir sinensis* through the coupling design of the tank's geometry (10), unidirectional closed-loop circulation, embedded light field, and the apparent density relationship between slowly settling active flocculents and brackish water.

[0033] like Figure 1 As shown, the cultivation device includes a tank 10, which is composed of straight side walls 1 on the front and rear sides and curved walls 5 on the left and right sides connected sequentially. The ratio of the major axis to the minor axis of the tank 10 is 3:1 to 8:1, and the depth of the tank 10 is 0.6 to 1.2 m. As an example, a small test tank can be 4m × 1.2m × 0.8m, with a major axis to minor axis ratio of approximately 3.3:1; the mass production tank can be geometrically enlarged to 20m × 4m × 1.0m, while maintaining the ratio of the major axis to the minor axis and the proportional relationships of the following geometric parameters. The curvature of the straight section of the straight side wall 1 approaches zero, making it suitable as a water transport section; the curvature of the semi-circular arc section of the curved wall 5 is the largest, and the water flow naturally decreases, making it suitable as a feeding area. The top edge of the tank 10 is higher than the normal water surface, and cultivation water is injected into the tank 10.

[0034] A partition 11 is vertically provided on the bottom surface of the tank 10. The partition 11 is located in the center of the tank 10 and is arranged in the left-right direction. Water passages are provided between the left and right ends of the partition 11 and the arc-shaped wall 5 at their respective ends. For example... Figure 2 and Figure 3 As shown, the water used for cultivation forms a counterclockwise closed circulation within the tank 10. This unidirectional closed circulation simulates the continuous drifting environment of the willow eel's natural habitat, allowing the willow eel to move with the current without actively swimming. At the same time, the circulation path transitions with a circular arc tangent, without right-angle bends, resulting in low water shear force, which aligns with the willow eel's aversion to violent turbulence.

[0035] The bottom surface of the tank 10 is provided with a V-shaped shallow ditch surrounding the partition 11. The V-shaped shallow ditch includes a sludge collection ditch 12 located inside the straight side wall 1 and a feeding ditch 13 located inside the curved wall 5. The two sections of sludge collection ditch 12 and the two sections of feeding ditch 13 are connected sequentially to form a closed channel surrounding the partition 11, such as... Figure 3As shown. The V-shaped shallow ditch has an angle of 8° with the horizontal plane and a depth of 3cm. The collection ditch 12 is used to collect sedimented waste in the water; the feeding ditch 13 is used to collect slowly settling active flocculent matter for the willow eel to scrape and feed on. The bottom tip of the feeding ditch 13 is provided with a dark blue matte soft layer. The soft layer can be made of food-grade silicone strips with a thickness of 2-3mm (preferably 2.5mm) and a Shore hardness of no more than 50A, to avoid the willow eel scratching its body surface when it touches the bottom; the dark blue matte surface color also reduces the intensity of reflected light at the bottom of the ditch, which helps to maintain a relatively dark environment above the feeding ditch 13.

[0036] Several straight-edge light strips 4 are vertically arranged on the inner sides of the two straight-edge walls 1, and these strips 4 are arranged sequentially along the length of the straight-edge walls 1 to form a continuous light wall along the straight-edge walls 1. Top light bands 6 are provided on the inner top sides of the two curved walls 5, and bottom light bands 15 are provided on the inner bottom sides of the two curved walls 5. The straight-edge light strips 4, top light bands 6, and bottom light bands 15 all emit blue-green light with a wavelength of 475 nm, which is the wavelength range in which the willow-leaf eel is most sensitive to negative phototaxis.

[0037] During feeding, the straight-edge light strip 4 forms a light wall on the straight-edge wall 1, driving the willow eels away from the straight-edge wall 1 and guiding them to gather towards the curved wall 5; the top light strip 6 provides illumination to the upper part of the curved wall 5, driving the willow eels to avoid the light and sink to the top of the feeding ditch 13. During the sewage discharge process, the bottom light strip 15 provides illumination to the bottom of the curved wall 5 to drive the fish away from the bottom. The same set of embedded light sources completes the regulation of the three conditions of feeding, sewage discharge and habitat in a timed manner, without the need for manual catching or driving. The light wall formed by the straight-edge light strip 4, the relatively dark area formed by the curved wall 5, and the unidirectional closed circulation jointly define the suitable habitat area and the suitable feeding area. The suitable habitat area is located in the dark water area, and the suitable feeding area is located above the curved feeding ditch 13 in the tank 10. The spatial distribution of the willow eels is thus stably guided by the optical flow coupling field.

[0038] Several aeration pipes 2 are provided on the side walls of the tank 10. The aeration pipes 2 are vertically arranged in a circumferential ring inside the tank 10, and their opening and closing are used to regulate the circulation velocity. There are two aeration pipes 2 on each straight side wall 1, which are interspersed with the straight side light strips 4 and are symmetrical about the central axis of the straight side wall 1. There are three aeration pipes 2 on each arc-shaped wall 5, which are vertically arranged in the middle of the arc-shaped wall 5 and on both sides where they meet the straight side wall 1, and are symmetrical about the central axis of the arc-shaped wall 5. The aeration direction of the aeration pipes 2 is obliquely downward at 45° along the circulation direction inside the tank 10, which can both push the water to help maintain a unidirectional closed circulation and prevent air bubbles from directly hitting the water surface and disturbing the willow leaf eels.

[0039] Each straight wall 1 has a water inlet pipe 3 located at the top center. The two water inlet pipes 3 allow water to enter tangentially along the circulation direction, forming a unidirectional counterclockwise closed circulation. The circulation speed within the tank 10 can be adjusted by regulating the water flow rate through the water inlet pipes 3.

[0040] Each of the curved walls 5 has a drain outlet 14 located at the bottom center of the feeding ditch 13. The drain outlet 14 is equipped with a perforated cover, the diameter of which is smaller than the body diameter of the eel larvae. The perforated cover can catch the flocculent feed without leakage. The eel larvae and intact flocculent feed are trapped within the trough 10, while the eel feces and dispersed flocculent particles can be discharged with the water flow through the perforations. Figure 4 As shown. Each drain outlet 14 is connected to a drain pipe 9, which extends outward from the drain outlet 14. A connecting pipe 7 is vertically installed on the side of the drain pipe 9 near the tank 10. The top of the connecting pipe 7 is open, and its top edge is lower than the top edge of the tank 10, ensuring that the liquid level in the connecting pipe 7 remains consistent with the liquid level in the tank 10. This prevents the cultivation water from overflowing from the tank 10 and facilitates observation of the liquid level in the tank 10. A valve 8 is also installed on the drain pipe 9, located outside the connecting pipe 7. The valve 8 is used to open and close the drain pipe 9. During the draining process, the valve 8 is opened, and the deposited waste is discharged with the water flow through the drain outlet 14 and the drain pipe 9. The connecting pipe 7 ensures that the liquid level in the tank 10 remains stable when the valve 8 is opened, preventing a sudden drop in liquid level from disturbing the eels.

[0041] The cultivation water described in tank 10 is discharged into tank 10 from the outlet of inlet pipe 3. The cultivation water is specifically brackish water with a salinity of 15‰ to 20‰ and an apparent density of 1.0113 to 1.015 g / cm³ at 25°C. Willow eels need to be cultivated in near-isotonic brackish water. This salinity and apparent density range is compatible with the osmotic pressure of the willow eel's body fluids, maintaining the willow eel's body fluid balance. The brackish water maintains a pH of 7.5 to 8.0.

[0042] In the cultivation device, slow-sinking active flocs were fed to the willow eels. These flocs had a porous and loose structure, possessed biological activity, and had an apparent density of 1.016–1.021 g / cm³, which was higher than that of brackish water. Given this apparent density difference, the slow-sinking active flocs exhibited a settling rate of 1.5–2.5 mm / s in still water.

[0043] Preferably, when the salinity of the brackish water is 18‰, the apparent density at 25℃ is 1.0136 g / cm³, and its pH is maintained at 7.8; the apparent density of the slow-settling active flocs is 1.0185 g / cm³, and the difference between the two apparent densities is 1.0185-1.0136=0.0049 g / cm³. At this time, the settling rate of the slow-settling active flocs in still water is 2.1 mm / s.

[0044] After the slow-sinking active flocs are fed into the feeding ditch 13, they slowly sink in the still water environment and preferentially deposit and accumulate at the bottom tip of the feeding ditch 13, forming wedge-shaped flocculent ridges. This prevents them from suspending and spreading throughout the pond, thus avoiding rapid settling and compaction. The flocs can be concentrated at a specific point simply by relying on the apparent density difference. If the apparent density difference between the slow-sinking active flocs and the brackish water is too small, the flocs will settle too slowly and be carried by the circulation, becoming suspended and spreading without forming ridges in the feeding ditch 13. If the apparent density difference between the slow-sinking active flocs and the brackish water is too large, the flocs will rapidly sink to the bottom and compact, making it impossible for the eels to scrape them off and causing the uneaten food to rot.

[0045] The slow-settling active flocs can be obtained using methods known in the art for preparing active flocs, such as the method described in the applicant's published patent application (publication number CN122623785A). This application does not limit the specific components, preparation process, or source of the slow-settling active flocs, but only requires that they possess the aforementioned physical properties and biological activities.

[0046] Example 2 Method Example

[0047] When implementing the cultivation method, operate in the following order.

[0048] S1. Feeding process: First, turn on the straight-edge light strip 4 and set its illuminance to 10 lx. Then, turn off the top light strip 6 and the bottom light strip 15, and close the aeration pipe 2. Adjust the water inlet flow rate of the inlet pipe 3 to maintain the circulation velocity within the tank 10 at 0.4 cm / s. The straight-edge light strip 4 forms a light wall on the straight edge section. Under negative phototaxis, the willow eels leave the straight-edge wall 1 and gather towards the arc-shaped walls 5 on both sides.

[0049] After the willow eels have gathered in the feeding area surrounded by the two curved walls 5, turn on the 625nm red ambient light for the operators to use during feeding. The red light is in a wavelength that the willow eels are almost imperceptible to, and will not interfere with their feeding behavior. Simultaneously, close the inlet pipe 3 and quantitatively feed slow-sinking active flocculent material into the feeding ditch 13, ensuring that the filling height of the flocculent material in the feeding ditch 13 is 25 mm from the bottom tip. Within the feeding window, a flocculent material covering layer of no less than 10 mm must always be maintained above the bottom tip of the feeding ditch 13 to prevent exposure of the bottom.

[0050] After the slowly settling active flocs reach the bottom of the tank 10, the top light strip 6 is turned on and its illuminance is set to 8 lx. This drives the willow eels to avoid the light and sink away from the top of the tank 10, gathering above the feeding ditch 13 at the bottom of the arc-shaped wall 5. Above the feeding ditch 13, the willow eels use their snouts to probe and scrape the slowly settling active flocs with their needle-like teeth, then feed them into their mouths and pharynx. At this time, the willow eels are in a feeding state.

[0051] The feeding process is repeated 6 times a day, each lasting 45 to 60 minutes, with the interval between two consecutive feeding processes evenly distributed.

[0052] S2. Sewage discharge process: After each feeding cycle, turn on the straight edge light strip 4 and set its illuminance to 0.5 lx, while keeping the top light strip 6 closed; turn on the bottom light strip 15 and set its illuminance to 10 lx, shining upwards from the bottom of the curved wall 5 to drive the willow eels to avoid the light and float away from the bottom of the tank 10. Keep the aeration pipe 2 and the water inlet pipe 3 in the same state as in S1.

[0053] After the eels have gathered at the top of tank 10, turn on the 625nm red ambient light for the operators to perform waste removal. Simultaneously, open valve 8 for 10-15 minutes. The liquid level difference between tank 10 and connecting pipe 7 serves as the natural drainage source. Debris settled in feeding ditch 13 is discharged with the water flow through drain pipe 9. Debris settled in collection ditch 12 needs to be manually collected and discharged from tank 10. The settled debris mainly consists of slow-settling, active flocculent uneaten food scattered in the water. Because its apparent density is greater than that of brackish water, it settles and falls into the V-shaped shallow ditches (i.e., collection ditch 12 and feeding ditch 13).

[0054] S3, Drifting Habitat: During the remaining time periods, turn on the straight-edge light strip 4 and the bottom light strip 15, and set their illuminance to 0.5 lx. Turn off the top light strip 6, turn on the aeration pipe 2, and adjust the water inlet flow rate of the inlet pipe 3 to maintain the circulation velocity within the tank 10 at 1.0 cm / s.

[0055] Under this condition, the dim light of the straight edge light strip 4 can prevent the willow eel from sticking to the wall, and the dim light of the bottom light strip 15 can prevent the willow eel from touching the bottom. The circulation provides a drifting environment, so that the willow eel is evenly distributed in the tank 10 and drifts with the current. At this time, the willow eel is in a resting state.

[0056] S4. Use of compound microbial agents: Throughout the entire rearing cycle, including the feeding, wastewater discharge, and migratory habitat processes described above, a compound microbial agent used to maintain low ammonia nitrogen levels in the aquaculture water will be diluted and added to the water. The ammonia nitrogen content in the water will be monitored in real time. The frequency and dosage of addition will meet the following requirements: maintaining the ammonia nitrogen concentration below 0.05 mg / L and the nitrite nitrogen concentration below 0.01 mg / L.

[0057] The compound microbial agent is composed of Rhodopseudomonas palustris P-3 strain and Bacillus amyloliquefaciens B4 strain. The Rhodopseudomonas palustris P-3 strain is numbered CCTCC NO: M 2020808, and the Bacillus amyloliquefaciens B4 strain is numbered CCTCC NO: M 2019853. In the compound microbial agent, the ratio of viable Rhodopseudomonas palustris P-3 strain to Bacillus amyloliquefaciens B4 strain is (2-3):1.

[0058] Note: The Rhodopseudomonas palustris P-3 strain and Bacillus amyloliquefaciens B4 strain mentioned above are both biological materials that are publicly available.

[0059] By simultaneously implementing the use of compound microbial agents, the light field, water specific gravity, feed density, water flow and probiotics are coupled to maintain a low-stress breeding environment for the willow eel.

[0060] Effect verification

[0061] Using the above-mentioned cultivation method for fully artificial breeding of willow eels, the fixed-point feeding rate of willow eels at feeding ditch 13 reached 86.5%, and the tail abrasion rate was controlled at around 3.8%. This indicates that by time-division control of the straight-edge light stripe 4, the top light band 6, and the bottom light band 15, combined with unidirectional closed circulation, the formation of ridges by feeding in feeding ditch 13, and the density difference between the slowly settling active flocculent material and the brackish water, willow eels can stably scrape at fixed points in the designated area, maintaining a low level of body surface damage. The breeding indicators are improved compared to existing technologies.

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A cultivation device for *Eriocheir sinensis*, comprising a tank (10), wherein a plurality of aeration pipes (2) are provided on the side wall of the tank (10), characterized in that: The tank (10) is composed of straight side walls (1) on the front and rear sides and arc-shaped walls (5) on the left and right sides connected in sequence; several straight side light strips (4) are vertically arranged on the inner side of the two straight side walls (1), and the several straight side light strips (4) are arranged in sequence along the length direction of the straight side walls (1); top light strips (6) are provided on the inner side of the top of the two arc-shaped walls (5), and bottom light strips (15) are provided on the inner side of the bottom of the two arc-shaped walls (5); the straight side light strips (4), top light strips (6) and bottom light strips (15) are all used to emit blue-green light with a wavelength of 470-480nm; The bottom surface of the tank (10) is vertically provided with a partition (11). The partition (11) is located in the center of the tank (10) and is arranged in the left and right direction. The left and right ends of the partition (11) have water passages between them and the arc wall (5) at the end. The bottom surface of the tank (10) is provided with a V-shaped shallow ditch around the partition (11). The V-shaped shallow ditch includes a sludge collection ditch (12) located inside the straight side wall (1) and a feeding ditch (13) located inside the arc wall (5). The two sections of sludge collection ditch (12) and the two sections of feeding ditch (13) are connected in sequence. Each straight wall (1) is provided with a water inlet pipe (3), and the water inlet pipe (3) is tangentially fed into the circulation direction to form a unidirectional closed circulation; each arc wall (5) is provided with a sewage outlet (14) at its bottom.

2. The cultivation device for *Eriocheir sinensis* as described in claim 1, characterized in that: Each sewage outlet (14) is connected to a sewage pipe (9), and a connecting pipe (7) is provided vertically upward on the side of the sewage pipe (9) near the tank (10); the top of the connecting pipe (7) is open, and the top edge of the connecting pipe (7) is lower than the top edge of the tank (10); a valve (8) is also provided on the sewage pipe (9), and the valve (8) is located outside the connecting pipe (7); the sewage outlet (14) is located at the bottom tip of the feeding ditch (13).

3. The cultivation device for *Eriocheir sinensis* as described in claim 1, characterized in that: Several aeration pipes (2) are arranged circumferentially inside the tank (10), and the aeration direction of the aeration pipes (2) is obliquely downward at 30° to 45° along the circulation direction inside the tank (10).

4. The cultivation device for *Eriocheir sinensis* as described in claim 1, characterized in that: The ratio of the major axis to the minor axis of the groove (10) is 3:1 to 8:

1.

5. The cultivation device for *Eriocheir sinensis* as described in claim 1, characterized in that: The angle between the wall of the V-shaped shallow ditch and the horizontal plane is 5° to 10°, and the ditch depth is 2 to 4 cm; the bottom tip of the feeding ditch (13) is provided with a soft layer.

6. A cultivation method using the cultivation apparatus of claim 2, characterized in that, Includes the following steps: Using the inlet pipe (3), brackish water with a salinity of 15‰ to 20‰ is injected into the tank (10); S1. Feeding process: First, turn on the straight edge light strip (4) and set its illuminance to 5-15 lx. Then, turn off the top light strip (6) and the bottom light strip (15), and turn off the aeration pipe (2). Adjust the water inlet flow rate of the inlet pipe (3) to maintain the circulation velocity in the tank (10) at 0.3-0.5 cm / s. After the willow eels gather in the feeding area surrounded by the two arc-shaped walls (5), close the water inlet pipe (3) and feed the slow-sinking active flocs into the feeding ditch (13). The apparent density of the slow-sinking active flocs is higher than that of brackish water. After the slow-sinking active flocs reach the bottom of the tank (10), turn on the top light strip (6) and set its illuminance to 5-10 lx to drive the willow eels to avoid the light and sink away from the top of the tank (10). The willow eels use their snouts to probe and scrape the slow-sinking active flocs with their needle-like teeth above the feeding ditch (13) and put them into their mouths and throats to eat. At this time, the willow eels are in a feeding state. The feeding process is repeated 5 to 6 times a day, with the interval between two consecutive feeding processes being evenly distributed. S2. Sewage discharge process: After the feeding process is completed, turn on the straight edge light strip (4) and set its illuminance to 0.1-1 lx, and keep the top light strip (6) closed; turn on the bottom light strip (15) and set its illuminance to 5-15 lx to drive the willow eels to avoid the light and float up away from the bottom of the tank (10); After the eels gather at the top of the tank (10), open the valve (8). The difference in liquid level between the tank (10) and the connecting pipe (7) serves as the power source for natural drainage. The dirt in the feeding ditch (13) is discharged from the sewage pipe (9) with the water flow. The dirt in the collection ditch (12) is collected and discharged from the tank (10). S3, Drifting Habitat: During the resting period, the willow eel is in a resting state. During this period, the straight edge light strip (4) and the bottom light strip (15) are turned on and their illuminance is 0.1 to 1 lx. The top light strip (6) is turned off, the aeration pipe (2) is turned on, the water inlet pipe (3) is adjusted, and the circulation speed in the tank (10) is kept at 0.8 to 1.2 cm / s.

7. The cultivation method as described in claim 6, characterized in that: The brackish water with a salinity of 15‰ to 20‰ has an apparent density of 1.0113 to 1.015 g / cm³ at 25°C, and the apparent density of the slow-settling active flocs is 1.016 to 1.021 g / cm³.

8. The cultivation method as described in claim 6, characterized in that: The feeding process lasts for 45 to 60 minutes each time, and the filling height of the slow-settling active flocculent in the feeding ditch (13) is 10 to 40 mm from the bottom tip of the ditch; the sewage discharge process lasts for 10 to 15 minutes each time.

9. The cultivation method as described in claim 6, characterized in that: When manual operation is required, turn on the external environment red light operation lamp at 620-630nm.

10. The cultivation method as described in claim 6, characterized in that, It also includes the following steps: S4. Use of compound microbial agents: Throughout the entire cultivation cycle, including the feeding, wastewater discharge, and drifting habitat processes described above, a compound microbial agent used to maintain low ammonia nitrogen levels in the aquaculture water is diluted and added to the water. The ammonia nitrogen content in the water is monitored in real time. The frequency and dosage of addition must meet the following requirements: maintaining the ammonia nitrogen concentration below 0.05 mg / L and the nitrite nitrogen below 0.01 mg / L. The compound microbial agent consists of *Rhodopseudomonas palustris* strain P-3 and *Bacillus amyloliquefaciens* strain B4. The *Rhodopseudomonas palustris* strain P-3 is numbered CCTCC NO: M 2020808, and the *Bacillus amyloliquefaciens* strain B4 is numbered CCTCC NO: M 2019853. The ratio of viable *Rhodopseudomonas palustris* strain P-3 to viable *Bacillus amyloliquefaciens* strain B4 in the compound agent is (2-3):1.

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