Tilapia mossambica fishing net and fishing method

Abstract

The invention provides a tilapia mossambica fishing net which comprises a net body, a net bag, a net plate and dragging ropes, the dragging ropes are fixedly connected to the two ends of the net body, a net sleeve is fixedly connected to one end of each dragging rope, a ship is fixedly connected to the end, away from the net sleeve, of each dragging rope, the net bag is fixedly connected to the rear side of the net body, and the middle of each dragging rope is sleeved with the net plate. A hand rope penetrates through the front side of the net body, and the hand rope is movably connected with the dragging rope; the tilapia mossambica fishing method comprises the following specific fishing modes: step S1, gill net fishing; s11, a netting gear structure is constructed; s12, the mesh size is determined; s13, a fishing scheme. Fish schools within the range of 3km are scanned in all directions through a horizontal fish detector, the directions and the distribution water depth of the fish schools are determined, and the scale and the cluster density of the fish schools are judged according to the shape and the color of detection images. The image plaque size represents the fish school scale, the image color represents the cluster density, and the cluster density is increased along with the increasing of the color.

Description

Technical Field

[0001] The present invention relates to the technical field of tilapia fishing, and specifically to a fishing net and method for Coptodon zillii. Background Art

[0002] Coptodon zillii belongs to Perciformes, Cichlidae, Coptodon, also known as red-bellied tilapia, Coptodon zillii, which has invaded the main water systems in South China. It is currently the most harmful tilapia species in China, and also the species with the widest range of natural water diffusion, the fastest diffusion speed, and the largest stock scale. It was included in the List of Key Managed Alien Invasive Species in China in 2022.

[0003] Coptodon zillii is a substrate-spawning fish, monogamous, with parental care behavior. Water temperature is the main environmental influencing factor for reproduction. In tropical waters with water temperature above 20°C, reproduction can occur throughout the year. During the breeding period, male fish establish territories and clean the nests. The nest-building process is completed by both male and female fish, and the nests are usually built in shallow waters with a water depth of no more than 2.5m (mostly 1.5m).

[0004] However, for existing tilapia, it is not convenient to explore the location of the fish school, resulting in the inability to accurately catch the fish school during the fishing process, lacking the function of determining the orientation and distribution depth of the fish school, thus affecting the fishing efficiency. Summary of the Invention

[0005] (1) Technical Problems to be Solved

[0006] In view of the deficiencies of the prior art, the present invention provides a fishing net and method for Coptodon zillii, which solves the problem of inconveniently judging the scale and cluster density of the fish school.

[0007] (2) Technical Solutions

[0008] To achieve the above object, the present invention is realized through the following technical solutions: A fishing net for Coptodon zillii includes a net body, a net bag, a net board and a towing rope. Both ends of the net body are fixedly connected with towing ropes. One end of the towing rope is fixedly connected with a net sleeve, and the end of the towing rope away from the net sleeve is fixedly connected with a ship. The net bag is fixedly connected to the rear side of the net body. The middle of the towing rope is sleeved with a net board. A main rope passes through the front side of the net body, and the main rope is movably connected to the towing rope;

[0009] Preferably, the gillnet fishing: A gillnet is a passive fishing gear that mainly catches target objects through fishing methods such as piercing, cutting, and entanglement; The net structure: The gillnet is composed of several net panels connected in series. The working water layer is adjusted by the ratio of floats and sink chains. The height of the net panel can be adjusted as needed. The net panel material is polyethylene monofilament with a wire diameter of 0.2 mm; The mesh size: It is generally considered that there is the following linear relationship between the mesh size of the gillnet and the body length of the fish,

[0010] a = kL where: a is the mesh opening length (mm), L is the body length of the fish, and K is a dimensionless coefficient, usually 20% of the ratio of the maximum body circumference to the body length. After actual measurement, the body length and body circumference of Oreochromis niloticus are similar, that is, the K value is about 0.2; The fishing plan: Since Oreochromis niloticus is mainly distributed in shallow waters (1 - 7 m) or the surface layer of open waters, and the individuals with the minimum sexually mature body length are about 10 cm, various mesh surface gillnets can be selected for fishing targeting the breeding population. The suitable height of the net panel is 5 - 7 m, and the mesh opening length (a) is greater than 2 cm.

[0011] Preferably, the single - boat single - sleeve single - bag trawl: The single - boat single - sleeve single - bag trawl is an active fishing gear, consisting of a net sleeve, a net body, a net bag, steel cables, and accessories; The design drawing of the fishing gear: The specifications of the fishing gear in this design are 184 meshes × 56 cm (114 m × 34 m - 18 m), and the lengths of the float line and the sinker line are each 18 m, and the lengths of the two side lines are each 17 m.

[0012] Preferably, the estimation of the towing force of the fishing gear:

[0013] The resistance of the mid - layer trawl fishing gear is calculated according to the following formula:

[0014]

[0015] where: R - the resistance of the fishing gear, N;

[0016] L - the total straightened length of the fishing gear, m;

[0017] C - the straightened circumference of the fishing gear, m;

[0018] - the wire area coefficient (average value) of the fishing gear;

[0019] V - the towing speed, kn.

[0020] Then: R = 8×114×34×0.017×9 = 4744 N. When the towing speed is 3 kn, the resistance of the fishing gear in this design is 4744 N; The manufacturing process of the fishing gear: This net is made by combining hand - knitting and machine - knitted net panel cutting. The specific requirements are as follows and should be strictly implemented during the manufacturing process; The knitting part of the fishing gear: 1. The hand - knitting part: The sewing and the connection parts of the front and rear net panels are hand - knitted; 2. The machine - knitted net panel cutting part: The main netting is cut symmetrically from machine - knitted net panels.

[0021] Preferably, for the setting treatment of the netting: after the netting is separately knitted and completed, the net sheets should be respectively set. During the setting treatment, the pre-tension for setting the net sheets and the consistency of the setting lengths of the four netting panels should be mastered; the longitudinal stitching between the netting panels is divided into: a. The longitudinal stitching between the back and ventral net and the two side nets adopts the method of winding stitching; b. For the longitudinal stitching between the four netting panels in the middle and rear parts of the net body, 1.5 meshes are respectively wound in on each side, and the thickness of the winding thread is the same as that of each section of the cord thread.

[0022] Preferably, for the assembly of the net gear and the steel cable: after the four-panel net is stitched together, the power cables should be assembled first according to the requirements of the shortening knot; for the assembly of the power cables: 4 power cables are assembled for the whole roll of net, and the material is PE Φ5mm braided rope. The assembly position starts from the middle of the front end of the swallowtail to the net bag. The power cables are assembled with a shortening coefficient of 0.97, and each section is firmly tied evenly according to the shortening coefficient. For the assembly of the upper and lower cables: according to the dimensions marked on the net diagram, a marginal cable made of a PE braided rope with a diameter of 6mm is first assembled on the cable-edge of the upper and lower net sleeves, and then tied to the upper and lower cables. A double dead knot is tied every 20 cm for the assembly length to ensure the symmetry of the assembly of the upper and lower cables.

[0023] Preferably, for the design of the net board: the area of the net board is determined. According to the dynamic similarity method, the area of the newly designed mid-water trawl net board is calculated by the following formula:

[0024]

[0025] In the formula:

[0026] S1, S2 - the areas of the master net board and the newly designed net board, m²;

[0027] Fn1, Fn2 - the resistances of the master net and the designed net, N;

[0028] V1, V2 - the towing speeds of the master ship and the survey ship, m / s.

[0029] When the towing speed of the survey ship is the same as that of the master ship, the area of the newly designed net board:

[0030] For the manufacture of the net board, the materials for making the net board: This design is a vertical rectangular V-shaped double-arc net board, mainly composed of an arc panel, a towing cable board and a spreader cable board, with a net board area of 1 m². This net board is suitable for mid-water and upper-water fish fishing.

[0031] Preferably, the requirements for making the net board are as follows: The towing wire board is the fixed connection point of the main stress center of the net board, with a plate thickness of 3 mm. Weld the towing wire pull plate to connect the towing wire at the stern of the ship. The method for connecting the net board and the fishing gear: There are 5 adjustment holes longitudinally designed on the towing wire board of this design net board, which are mainly used to adjust the inner and outer inclination of the net board. The net position control: The net position refers to the vertical distance from the middle of the float line of the fishing gear to the sea surface during trawling. The key to the midwater trawling fishing technology is to always control the net position well during the trawling movement, accurately place the net mouth at the position of the fish school, so that the fish school enters the net, thereby achieving the purpose of fishing.

[0032] Preferably, the fishing operation on the deck is as follows: The important thing in the midwater trawling deck operation is to pay attention to controlling the ship speed. When releasing the net, the ship speed stops at about 2.5 knots, and the fishing gear is gradually released during the inertial navigation of the ship. After the fishing gear is normally opened, prepare to release the net board. When releasing the net board, the ship speed should be appropriately increased. The fishing strategy - aiming fishing: Use a horizontal fish finder to scan the fish school within a range of 3 km in all directions, determine the orientation and distribution depth of the fish school, and judge the scale and aggregation density of the fish school according to the shape and color of the detection image.

[0033] A method for fishing Oreochromis niloticus includes the following specific fishing methods:

[0034] Step S1: Gillnet fishing; Step S11: Net structure; Step S12: Mesh size; Step S13: Fishing plan;

[0035] Step S2: Single-boat single-sleeved single-bag trawling; Step S21: Fishing gear design drawing; Step S22: Estimation of the towing force of the fishing gear; Step S23: Manufacturing process of the fishing gear; Step S231: Braiding part of the fishing gear; Step S232: Setting treatment of the netting; Step S233: Longitudinal stitching of the netting; Step S234: Assembly of the fishing gear and the steel cable; Step S235: Assembly of the power cable; Step S236: Assembly of the upper and lower cables; Step S24: Net board design; Step S241: Determination of the main dimensions of the net board; Step S242: Manufacturing of the net board; Step S243: Requirements for making the net board; Step S244: Connection method between the net board and the fishing gear; Step S245: Net position control; Step S246: Fishing operation on the deck; Step S247: Fishing strategy - aiming fishing.

[0036] (III) Beneficial effects

[0037] The present invention provides a fishing gear and a fishing method for Oreochromis niloticus. It has the following beneficial effects:

[0038] 1. The present invention is provided with: a horizontal fish finder that scans the fish schools within a range of 3 km in all directions to determine the orientation of the fish schools and the distribution water depth, and judges the scale and aggregation density of the fish schools according to the shape and color of the detection image. The size of the image patch represents the scale of the fish school, and the image color represents the aggregation density; the image color ranges from light white, grayish white, green, dark green, yellow, orange to red, and the increasing color indicates the increasing aggregation density.

[0039] 2. After determining the orientation of the fish school, the present invention aligns the bow of the ship with the fish school and adjusts the spatial distance between the ship and the fish school to leave sufficient space for net laying and net position adjustment.

[0040] 3. After adjusting the orientation, the present invention lays the net and controls the net position through the towing rope and the net position instrument to keep the central position of the net mouth consistent with the central position of the fish school.

[0041] 4. The present invention observes the situation of the fish school entering the net through the net position instrument. When the fish school approaches the lower or upper net, the net position should be adjusted in real time according to the change of the fish school shape. Description of the Drawings

[0042] Figure 1 It is a schematic diagram of the catch rate curves of different mesh sizes (mesh a and mesh b) of the present invention;

[0043] Figure 2 It is a schematic diagram of the gillnet operation of the present invention;

[0044] Figure 3 It is a schematic diagram of the single-boat sleeve-type purse seine of the present invention;

[0045] Figure 4 It is a schematic diagram of the deployment of the midwater trawl net of the present invention;

[0046] Figure 5 It is a schematic diagram of the fixed connection points of the crosstree plate and the towing rope plate of the present invention;

[0047] Figure 6 It is a schematic diagram of the fish school detection by the horizontal fish finder of the present invention;

[0048] Figure 7 It is a schematic diagram of the net position instrument showing the situation of the fish school entering the net of the present invention.

[0049] Among them, 1. Net body; 2. Net purse; 3. Net sleeve; 4. Net board; 5. Bridle; 6. Towing rope. Detailed Embodiment

[0050] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0051] Embodiment:

[0052] As Figure 1-7 shown, an Oreochromis niloticus fishing gear provided by an embodiment of the present invention includes a net body 1, a net bag 2, a net board 4, and a hauling line 6. Hauling lines 6 are fixedly connected to both ends of the net body 1. One end of the hauling line 6 is fixedly connected to a net sleeve 3. The end of the hauling line 6 away from the net sleeve 3 is fixedly connected to a ship. A net bag 2 is fixedly connected to the rear side of the net body 1. A net board 4 is sleeved in the middle of the hauling line 6. A main line 5 penetrates through the front side of the net body 1, and the main line 5 is movably connected to the hauling line 6;

[0053] An Oreochromis niloticus fishing method includes the following specific fishing methods:

[0054] Fishing plan

[0055] In view of the geographical and hydrological environmental characteristics of the Wujiang River source and the behavioral habits of Oreochromis niloticus, while taking into account the protection of native fish species, the following fishing plan is proposed for Oreochromis niloticus.

[0056] Gillnet fishing

[0057] A gillnet is a passive fishing gear that mainly catches target objects through fishing methods such as piercing, cutting, and entanglement. It has low usage costs and is relatively simple technically, and is suitable for operations under different water depths, complex geological conditions, and other environmental conditions.

[0058] Gillnets can be divided into single-layer gillnets and multi-layer gillnets.

[0059] A single-layer gillnet has strong selectivity for the fishing target. When the size of the fish body differs from the optimal fishing specification of the mesh by more than 20%, the probability of the fish being caught is very small. For example, Figure 3 in Figure a, the optimal fishing body length of the mesh is L1. When the fish body is less than 80% of L1 or greater than 120% of L1, theoretically the probability of the fish body being caught is small. There are also differences in the optimal fishing specifications for different mesh sizes ( Figure 1 ).

[0060] A multi-layer gillnet is usually composed of single-layer net sheets of various mesh sizes. The fishing ability of a multi-layer gillnet is higher than that of a single-layer gillnet, and the selectivity of the fishing target will also be affected.

[0061] 1.1 Net structure

[0062] The gillnet is formed by connecting several net sheets in series( Figure 2 ). The working water layer is adjusted by the ratio of floating balls and sink chains, and the height of the net sheet can be adjusted as needed. The net sheet material is polyethylene monofilament with a wire diameter of 0.2mm.

[0063] 1.2 Mesh size

[0064] It is generally considered that there is the following linear relationship between the mesh size of the gillnet and the body length of the fish,

[0065] a = kL(1)

[0066] In the formula: a is the mesh foot length (mm), L is the body length of the fish, and K is a dimensionless coefficient, usually 20% of the ratio of the maximum body circumference to the body length. After actual measurement, the body length and body circumference of Oreochromis niloticus are similar, that is, the K value is about 0.2.

[0067] 1.3 Fishing plan

[0068] Since Oreochromis niloticus is mainly distributed in shallow waters (1-7m) or the surface layer of open waters, and the individuals with the minimum sexually mature body length are about 10cm, various mesh surface gillnets can be selected for fishing targeting the breeding population. The suitable net sheet height is 5-7m, and the mesh foot length (a) is greater than 2cm. When fishing, the net gear is deployed in the main distribution areas such as spawning grounds and foraging grounds, and can be deployed in multiple rows. Usually, the net is set in the evening and retrieved in the morning.

[0069] Single-boat sleeve single-bag trawl

[0070] The single-boat sleeve single-bag trawl is an active fishing gear, which consists of a net sleeve, a net body, a net bag, steel cables and accessories, etc. During operation, a single boat drags the fishing gear and uses the otter board as the horizontal expansion device of the net gear to drag the fishing target into the net in the waters it passes through to achieve the fishing purpose.

[0071] 2.1 Design drawing of the net gear

[0072] The designed net gear has a specification of 184 meshes × 56cm (114m × 34m - 18m). The lengths of the float line and the sinker line are each 18m, and the lengths of the two side lines are each 17m.

[0073] Eight kinds of mesh length changes are adopted. Among them, for the 5 kinds of mesh length changes in the front part, the number of meshes of the back-and-forth net width and the number of meshes of the two-side net width remain the same up and down, naturally forming a slope change and simplifying the knitting process.

[0074] According to factors such as the physical structure performance of the net gear material, the resistance of the net gear, the force conditions of different parts of the net gear, and the service time of the net gear, all the designed nets adopt PE fishing net lines, and the material specification performance is shown in the following table.

[0075]

[0076] 2.2 Estimation of the Trawl Force of the Fishing Gear

[0077] The resistance of the midwater trawl fishing gear is calculated according to the following formula:

[0078]

[0079] Where: R - the resistance of the fishing gear, N;

[0080] L - the total straightened length of the fishing gear, m;

[0081] C - the straightened perimeter of the fishing gear, m;

[0082] - the line area coefficient of the fishing gear (average value);

[0083] V - the towing speed, kn.

[0084] Then: R = 8×114×34×0.017×9 = 4744 N. When the towing speed is 3 kn, the resistance of the fishing gear in this design is 4744 N.

[0085] 2.3 Manufacturing Process of the Fishing Gear

[0086] This fishing gear is made by combining hand knitting and cutting of machine - knitted mesh. The specific requirements are as follows and should be strictly implemented during the manufacturing process.

[0087] 2.3.1 Knitting Part of the Fishing Gear

[0088] (1) Hand - knitting Part

[0089] The sewing and the connection parts of the front and rear mesh are hand - knitted.

[0090] (2) Cutting Part of Machine - Knitted Mesh

[0091] The main netting is made by symmetrically cutting machine - knitted mesh. The net material is PE braided rope. The netting is also of a four - panel structure. The transverse mesh number of the netting is the net mesh number, and during construction, pay attention to adding 1.5 meshes for the sewing edge on each side.

[0092] 2.3.2 Setting Treatment of the Netting

[0093] After the netting is knitted separately, the netting pieces should be set respectively. During the setting treatment, the pre - tension for setting the netting pieces and the consistency of the setting length of the four netting pieces should be mastered. After the netting pieces are set, the two parts of the netting are sewn together and then the whole net is sewn.

[0094] 2.3.3 Longitudinal Sewing of the Netting

[0095] a. The longitudinal stitching between the dorsal-ventral net and the side nets adopts the method of circumferential stitching. The longitudinal stitching of the middle and front parts of the net body and the net sleeves adopts half-eye circumferential stitching, that is, 1 eye is circumferentially wound in the middle of the joint of the four net panels. The thickness of the circumferential stitching line is the same as that of each section of the rope. The circumferential line is double-stranded. The circumferential stitching starts from the swallowtail opening and first makes a double dead knot, then makes a double dead knot at a certain interval, and makes a clove hitch at each half-eye to strengthen the strength of the stitching edge.

[0096] b. For the longitudinal stitching between the four net panels in the middle and rear parts of the net body, 1.5 eyes are circumferentially wound on each side. The thickness of the circumferential stitching line is the same as that of each section of the rope. The circumferential line is double-stranded. Double fisherman's knots and single fisherman's knots are made at a certain interval, and the knotting distance from the four sides should be the same. For the assembly of the 2.3.4 net gear and the steel cable, after the four-piece net is stitched together, the power cable is assembled first according to the requirements of the reduction knot.

[0097] 2.3.5 Power cable assembly

[0098] A total of 4 power cables are assembled for the whole net. The material is PE Φ5mm braided rope. The assembly position starts from the middle of the front end of the swallowtail to the net bag. The power cable is assembled with a reduction coefficient of 0.97. Each section is firmly tied according to the reduction coefficient. The technological process is strictly implemented. A tie is made every 20 cm, and the consistency of the assembly process of the 4 power cables is ensured.

[0099] 2.3.6 Upper and lower cable assembly

[0100] According to the dimensions marked on the net diagram, first assemble a marginal cable made of a 6mm diameter PE braided rope on the cable installation edges of the upper and lower net sleeves, and then tie it to the upper and lower cables. A double dead knot is made every 20 cm for the assembly length to ensure the symmetry of the upper and lower cable assembly.

[0101] 2.4 Net board design and manufacturing

[0102] 2.4.1 Net board design

[0103] (1) Determination of net board area

[0104] According to the dynamic similarity method, the net board area of the newly designed midwater trawl net board is calculated by the following formula:

[0105]

[0106] In the formula:

[0107] S1, S2 - the areas of the net board prototype and the newly designed net board, m2;

[0108] Fn1, Fn2 - the resistances of the prototype net and the designed net, N;

[0109] V1, V2 - the towing speeds of the prototype ship and the survey ship, m / s.

[0110] When the towing speed of the survey ship is the same as that of the prototype ship, the area of the newly designed net board:

[0111]

[0112] (2) Determination of the main dimensions of the net panel

[0113] The aspect ratio (λ) of the rectangular vertical curved surface "V"-type double-slit net panel is about 2.7 (L / b). Then Rectangular net panel

[0114] Then:

[0115]

[0116] Then the initial designed area of the net panel is about 1 m², the span (L) of the net panel is 1.6 m, and the chord (b) of the net panel is 0.66 m.

[0117] 2.4.2 Manufacture of the net panel

[0118] (1) Materials for making the net panel

[0119] This design is a vertical rectangular V-type double-arc surface net panel, which is mainly composed of an arc panel, a towing wire plate, and a bridle wire plate. The area of the net panel is 1 m². This net panel is suitable for fishing for middle and upper layer fish.

[0120] The arc panel is divided into a main panel and a guide plate. The material is Q345B steel with a thickness of 2 mm. This design can reduce the water body vortex of the net panel, increase the lift of the net panel, and reduce the resistance of the net panel.

[0121] The towing wire plate is a connecting component of the trawl towing wire, which bears all the resistance of the net panel and the fishing gear during operation. Its design strength and material are determined according to the total resistance of the trawl fishing gear. The material of the towing wire plate is Q345B steel with a thickness of 3 mm.

[0122] The bridle wire plate is a component used to connect the two hands of the trawl. It bears all the resistance of the fishing gear during towing and is composed of upper and lower plates. Its design strength and material are determined according to the trawl resistance calculation. The material of the steel plate is Q345B steel, and the thickness is: 2 mm for the upper bridle wire plate and 3 mm for the lower bridle wire plate.

[0123] 2.4.3 Requirements for making the net panel

[0124] The towing wire plate is the fixed connection point of the main force center of the net panel, with a plate thickness of 3 mm. Weld the towing wire plate to connect the towing wire at the stern of the ship.

[0125] A single net panel is divided into upper and lower bridle wire plates, which are the fixed connection points for the resistance of the cable fishing gear and connect the trawl bridle. The plate thicknesses of the upper and lower bridle wire plates are 2 mm and 3 mm respectively.

[0126] Main panel of the net board: The front main panel has a thickness of 1 mm, a curvature of 12%, an angle d of 6°, and there are 2 pieces in total, symmetrically arranged up and down. The rear main panel has a thickness of 1 mm, a curvature of 14%, an angle c of 18°, and there are 2 pieces in total, symmetrically arranged up and down. The front main panel and the rear main panel are parallel plates, with an inclined cutting angle of 6°.

[0127] The thickness of the towing wire plate is 2 mm, which is used for connecting the towing wire on the ship and the front end of the net board.

[0128] 2.4.4 Connection method of the net board and the fishing gear

[0129] See 3 for the connection method of the fishing gear system.

[0130] There are 5 adjustment holes longitudinally designed on the towing wire plate of this designed net board, which are mainly used to adjust the internal and external inclination of the net board. For the vertical V-shaped hyperbolic net board, the towing wire is generally connected to the middle hole G2c of the towing wire plate.

[0131] There are 3 adjustment holes transversely designed on the base of the towing wire plate of this design, which are also used for adjusting the attack angle of the net board. Due to the cumbersome operation, the towing wire plate is not used to adjust the attack angle of the net board. The attack angle adjustment is generally implemented through the adjustment holes of the spreader wire plate.

[0132] There are also 5 adjustment holes on the spreader wire plate of the net board, which are mainly used to adjust the attack angle of the net board. When the hole position moves forward from the back, the attack angle increases, and accordingly the horizontal expansion of the net board increases, and the horizontal expansion of the net sleeve also increases; conversely, when the hole position moves backward, the attack angle decreases, and the horizontal expansion of the net board and the net sleeve decreases. Generally, the upper and lower spreader wires (upper and lower handlines) are respectively connected to holes A3 and E3.

[0133] 2.4.5 Net position control

[0134] The net position refers to the vertical distance from the middle of the float line of the fishing gear to the sea surface during trawling.

[0135] The key to the midwater trawling technology is: during the trawling movement, always control the net position well, accurately place the net mouth at the position of the fish school, so that the fish school enters the net, thereby achieving the purpose of fishing. The common methods for adjusting and controlling the midwater trawl net position are: adjusting the length of the towing wire, adjusting the trawling speed, or using both.

[0136] Adjusting the net position during trawling: One is to increase or decrease the trawling speed; the other is to increase or decrease the length of the towing wire. When the fish school deviates from the net position relatively close (a few meters or more than ten meters), the trawling speed can be used for adjustment. For example, if the fish school deviates above the net position, increase the trawling speed to raise the net position, and vice versa, decrease the trawling speed; if the fish school deviates from the net position relatively far (more than twenty meters), it is still advisable to adjust the length of the towing wire. It should be noted that in the method of adjusting the net position by changing the length of the towing wire, the length of the towing wire should not be adjusted too frequently. Because the process of hauling in the towing wire takes a certain amount of time and will cause deformation of the fishing gear, which is not conducive to targeted fishing, so it should be carefully selected.

[0137] 2.4.6 Deck fishing operations

[0138] An important matter in the deck operation of midwater trawling is to pay attention to controlling the ship speed. When paying out the net, the ship stops at a speed of about 2.5 knots, and the net is gradually paid out during the inertial navigation of the ship. After the net is normally opened, prepare to release the otter boards; when releasing the otter boards, the ship speed should be appropriately increased. After the otter boards enter the water, when the towing line is paid out 40 - 50 m, the brakes should be applied once to allow the otter boards to fully open. Thereafter, at regular intervals, the towing line should be braked to expand the left and right otter boards and prevent them from getting entangled with each other. After the otter boards enter the water, the ship speed when paying out the towing line is controlled at about 3 knots; when hauling in the net, the ship speed is controlled at 1 - 2 knots. When the otter boards are close to the stern (about 50 m), the ship speed should be appropriately increased to straighten the net and prevent it from entangling the propeller.

[0139] The length of the towing line paid out is related to factors such as the towing force of the fishing vessel, the specifications of the fishing gear and its floating and sinking forces, the water flow, and the towing speed. The general rule is that at a certain towing speed, when different lengths of the towing line are paid out, the net position is at different locations. As the length of the towing line changes, the difference in the net position becomes greater. Therefore, during the operation, the instrument and equipment on the ship can be used to test the relationship between the length of the towing line paid out and the net position.

[0140] The fish schools are often not distributed at the same water layer, sometimes high, sometimes low, sometimes large, and sometimes small. The operator must pay full attention to the height of the fish finder image and its shape and color changes to correctly adjust the net position.

[0141] 2.4.7 Fishing strategy - Targeted fishing

[0142] (1) Use a horizontal fish finder to scan the fish schools within a range of 3 km in all directions, determine the azimuth and distribution depth of the fish schools, and judge the scale and aggregation density of the fish schools according to the shape and color of the detected image. The size of the image patch represents the scale of the fish school, and the image color represents the aggregation density; the image color ranges from light white, grayish white, green, dark green, yellow, orange to red, and as the color increases, it indicates an increase in the aggregation density.

[0143] (2) After determining the azimuth of the fish school, align the bow of the ship with the fish school and adjust the spatial distance between the ship and the fish school, leaving enough space for net laying and net position adjustment.

[0144] (3) After adjusting the azimuth, lay the net and control the net position through the towing line and the net position instrument to keep the center position of the net mouth consistent with the center position of the fish school.

[0145] (4) Observe the situation of the fish school entering the net through the net position instrument. When the fish school approaches the lower or upper netting, the net position should be adjusted in real time according to the change in the shape of the fish school.

[0146] Although embodiments of the present invention have been shown and described, those of ordinary skill in the art will appreciate that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention. The scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A tilapia nilotica zillii fishing net, comprising a net body (1), a net bag (2), net boards (4) and a towing rope (6), characterized in that: Both ends of the net body (1) are fixedly connected with hauling ropes (6). One end of the hauling rope (6) is fixedly connected with a net sleeve (3). The end of the hauling rope (6) far from the net sleeve (3) is fixedly connected with a ship. A net bag (2) is fixedly connected to the rear side of the net body (1). A net board (4) is sleeved in the middle of the hauling rope (6). A main rope (5) passes through the front side of the net body (1), and the main rope (5) is movably connected with the hauling rope (6).

2. The Qishi tilapia fishing gear according to claim 1, characterized in that: The gillnet fishing: The gillnet is a passive fishing gear, mainly catching target objects through fishing methods such as stabbing, cutting, and entangling; The net structure: The gillnet is composed of several net sheets connected in series. The working water layer is adjusted by the ratio of floats and sink chains. The height of the net sheet can be adjusted as needed. The net sheet material is polyethylene monofilament with a wire diameter of 0.2 mm; The mesh size: It is generally considered that there is the following linear relationship between the mesh size of the gillnet and the body length of the fish, a = kL 3. The fishing net for Tilapia chinensis according to claim 1, characterized in that: Where: a is the mesh foot length (mm), L is the body length of the fish, and K is a dimensionless coefficient, usually 20% of the ratio of the maximum body circumference to the body length. After actual measurement, the body length and body circumference of Oreochromis niloticus are similar, that is, the K value is about 0.2; The fishing plan: Since Oreochromis niloticus is mainly distributed in shallow waters (1 - 7 m) or the surface layer of open waters, and the individuals with the smallest sexually mature body length are about 10 cm, various mesh surface gillnets can be selected for fishing for the breeding population. The suitable net sheet height is 5 - 7 m, and the mesh foot length (a) is greater than 2 cm.

4. A fishing net and fishing method for Tilapia zebulii according to claim 1, characterized in that: The single - ship with - sleeve single - bag trawl: The single - ship with - sleeve single - bag trawl is an active fishing gear, consisting of a net sleeve, a net body, a net bag, steel cables and accessories; The design drawing of the fishing gear: The specifications of the fishing gear in this design are 184 meshes × 56 cm (114 m × 34 m - 18 m). The lengths of the float line and the sinker line are each 18 m, and the lengths of the two side lines are each 17 m. The estimation of the towing force of the fishing gear The resistance of the mid - water trawl fishing gear is calculated by the following formula Where: R - the resistance of the fishing gear, N L - the total straightened length of the fishing gear, m - Netting wire area coefficient (average value); C - the straightened circumference of the fishing gear, m V - the towing speed, kn 5. A Qi's tilapia fishing net according to claim 1, characterized in that: Then: R = 8×114×34×0.017×9 = 4744 N. When the towing speed is 3 kn, the resistance of the fishing gear in this design is 4744 N; The manufacturing process of the fishing gear: This net is made by combining manual knitting and machine - knitted net sheet cutting. The specific requirements are as follows and should be strictly implemented during the manufacturing process; The knitted part of the fishing gear:

1. The manual knitting part: The sewing part and the connection part of the front and rear net sheets are knitted by hand; 2. The machine - knitted net sheet cutting part: The main netting is cut symmetrically by machine - knitted net sheets. The setting treatment of the netting: After the netting is knitted separately, the net sheets should be set respectively. During the setting treatment, the pre - tension for setting the net sheet and the consistency of the setting lengths of the four net sheets should be mastered; The longitudinal sewing between the netting is divided into: a. The longitudinal sewing between the dorsal - ventral net and the two - side nets uses the winding sewing method; b. The longitudinal sewing between the four net sheets in the middle and rear parts of the net body, each side winds in 1.5 meshes, and the thickness of the winding thread is the same as the thickness of each section of the rope thread.

6. The fishing net for Tilapia chinensis according to claim 1, characterized in that: Assembly of the fishing gear and wire ropes: After the four-piece net is sewn together, the power cables are assembled first according to the requirements of the reduction knot. Assembly of the power cables: Four power cables are assembled for the whole roll of net, with the material being PE Φ5mm braided ropes. Their assembly positions start from the middle of the front end of the swallowtail to the net bag. The power cables are assembled with a reduction knot of 0.97, and each section is firmly tied evenly according to the reduction coefficient. Assembly of the upper and lower cables: According to the dimensions marked on the net diagram, a marginal cable made of a 6mm diameter PE braided rope is first assembled on the cable edges of the upper and lower net sleeves, and then tied to the upper and lower cables. A double dead knot is tied every 20 cm for the assembly length to ensure the symmetry of the upper and lower cable assemblies.

7. The fishing net for Tilapia zebulii according to claim 1, characterized in that: Design of the otter board: The area of the otter board is determined. According to the dynamic similarity method, the area of the newly designed midwater trawl otter board is calculated by the following formula: Where: S1, S2 - the areas of the otter board prototype and the newly designed otter board, m²; Fn1, Fn2 - the resistances of the prototype net and the designed net, N; V1, V2 - the towing speeds of the prototype ship and the survey ship, m / s. When the towing speed of the survey ship is the same as that of the prototype ship, the area of the newly designed otter board: For the manufacturing of the net panel, the materials for making the net panel: This design is a vertical rectangular V-shaped double-curved surface net panel, mainly composed of arc panels, towing line plates, and spreader line plates, with a net panel area of 1 m2. This net panel is suitable for fishing mid- and upper-layer fish.

8. A kind of Qishi tilapia fishing gear according to claim 1, characterized in that: Requirements for manufacturing the otter board: The towing cable plate is the fixed connection point of the main stress center of the otter board, with a plate thickness of 3 mm. The towing cable pull plate is welded to connect the towing cable at the stern. Connection method between the otter board and the fishing gear: Five adjustment holes are longitudinally designed on the towing cable plate of this designed otter board, which are mainly used to adjust the inward and outward inclination of the otter board. Net position control: The net position refers to the vertical distance from the middle of the float cable of the fishing gear to the sea surface during trawling. The key to midwater trawl fishing technology is to always control the net position well during the trawling movement, accurately place the net mouth at the position of the fish school, so that the fish school enters the net, thereby achieving the purpose of fishing.

9. A Qishi tilapia fishing net according to claim 1, characterized in that: Deck fishing operations: An important matter in midwater trawl deck operations is to pay attention to controlling the ship speed. When paying out the net, the ship speed stops at about 2.5 knots, and the fishing gear is gradually paid out during the inertial navigation of the ship. After the fishing gear is normally opened, prepare to release the otter board; the ship speed should be appropriately increased when releasing the otter board. Fishing strategy - aiming fishing: Use a horizontal fish finder to scan the fish school within a range of 3 km in all directions, determine the azimuth and distribution water depth of the fish school, and judge the scale and aggregation density of the fish school according to the shape and color of the detected image.

10. A method for catching Oreochromis niloticus, based on the Oreochromis niloticus fishing gear according to any one of claims 1-9, characterized in that; Including the following specific fishing methods: Step S1: Gillnet fishing; Step S11: Net structure; Step S12: Mesh size; Step S13: Fishing plan; Step S2: Single-boat single-pocket trawl with sleeves; Step S21: Fishing gear design drawing; Step S22: Estimation of the towing force of the fishing gear; Step S23: Manufacturing process of the fishing gear; Step S231: Braided part of the fishing gear; Step S232: Setting treatment of the netting; Step S233: Longitudinal stitching of the netting; Step S234: Assembly of the fishing gear and wire ropes; Step S235: Assembly of the power cables; Step S236: Assembly of the upper and lower cables; Step S24: Otter board design; Step S241: Determination of the main dimensions of the otter board; Step S242: Manufacturing of the otter board; Step S243: Requirements for manufacturing the otter board; Step S244: Connection method between the otter board and the fishing gear; Step S245: Net position control; Step S246: Deck fishing operations; Step S247: Fishing strategy - aiming fishing.

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