Analysis on the adjustment technology of truss trawl fishing gear and fishing method

At present, the production of truss trawling shrimp has become one of the important modes of marine fishing in coastal fishing areas, and the proportion of its output value in the whole marine fishery output value is increasing year by year. As this mode of operation is a single ship bottom truss trawl operation, its gear design, assembly, operation adjustment and fishing ground selection and other links are relatively technical, so a little negligence will affect the shrimp production. In particular, the adjustment technology in truss trawl operation is the key to fishing production technology, such as this phenomenon is often encountered in production practice: fishing vessels with the same horsepower, using the same specifications of net gear, are produced in the same fishing ground, but the net output is very different, the highest is nearly 1000 jin, the low is only more than ten jin or even empty net. To a large extent, the reason is closely related to the correct use of fishing gear, fishing methods and adjustment techniques. Therefore, we attach great importance to the theoretical research and practical exploration in this area, constantly sum up experience to improve the fishing technology, improve the fishing effect of truss trawl, and basically overcome the imbalance in the past production. According to the practice and summary in recent years, the adjustment technology and production experience of truss trawl fishing gear and fishing net are introduced as follows:

I. selection of gear specifications and adjustment of gear weight

As the truss trawl mainly catches shrimp with low perch and weak swimming ability, and also catches crabs, flounder and other fish, it has the characteristics of active flexibility, active pursuit of shrimp and wide adaptability. In recent years, with the increase of the intensity of shrimp trawling, the truss field has gradually been involved in deep-water fishing grounds, and the marine environmental conditions have become more complex. In order to improve the safety of marine production, the tonnage and horsepower of fishing vessels are constantly increasing. Net design also needs to be constantly updated and improved to improve its performance. According to the above situation and comprehensive production experience, we think that the rationality of choosing truss towing shrimp net can be explained from the following aspects:

1. The power utilization rate of fishing vessels should be improved as far as possible. Relevant experiments have proved that when the current trawler gear is in operation, the power consumption of fishing vessels (80--150HP) accounts for less than 10% of the rated horsepower, while the power utilization rate of trawlers is 30% Murray 40%. Practice has proved that the operation of small horsepower fishing vessels is more economical and economical, so only by increasing the gear specification, the width of the gear will be enlarged, the sweeping area will be enlarged, and the power utilization rate and output of fishing vessels will be improved. That is to say, the truss supporting horizontal expansion should be lengthened and the specification should be increased, but it is restricted by the actual operation conditions and other factors. In the actual production, we think that the truss bar should adopt seamless steel pipe with outer diameter 110mm--159mm, and the length is 0.9 Murray 1.1 times of the length of the fishing boat.

2. The ratio of the perimeter of the net mouth to the length of the net gear is reasonable. The determination of the perimeter of the net mouth is mainly based on the length of the truss bar and the shrinkage coefficient of the net mouth, and with reference to the horsepower of the fishing boat, while the length of the net gear is determined by the perimeter of the net mouth, as long as the ratio of the net length to the net mouth perimeter is reasonable, the net is stable and balanced when towing, and the resistance is low, so it is not suitable to absorb sand and break the net. At present, we think that the small mesh and multi-capsule type net gear selected by 80HP--150HP fishing boat is better, and its main specifications and technical parameters are as follows: mesh variation from net mouth to net capsule mesh, horizontal shrinkage coefficient of net mouth about 0.25, length-to-perimeter ratio of net gear to total length 0.10, ratio of net cover to total length about 0.175, ratio of net body to total length 0.32, ratio of net capsule to total length 0.485, net capsule 3UL 10. In the process of operation, the net has good filtering performance, the web is not easy to absorb sand, and has a good effect of dragging shrimp. At the same time, the shrimp has less damage and high integrity rate.

3. Due to the particularity of the structure and operation of the truss trawl, accidents such as net breaking are often caused in the production. Therefore, the correct allocation and adjustment of the weight of the gear is the key to increase the output, and can prevent the occurrence of the accident of the gear. The buoyancy and sinking force equipment in the truss trawl design is for the general situation. Its sinking and buoyancy should be properly adjusted in different tidal floods and different sea conditions. To judge whether the weight of the net gear is appropriate, we can check whether the truss rod sinks at an inclination angle after launching, observe the degree of friction between the sunken cement roller and the seafloor, the species and quantity of each catch and sand absorption. The equipment principle of net buoyancy and sinking force in production is: in the process of towing, sinking class can effectively disturb shrimp, but not too heavy to increase net resistance, sinking class too brake to absorb sand. It should not be too light to disturb shrimp and the net is unstable. The specific adjustment methods are as follows: during the ① tide, the small tide is slow, the shrimp lives in the bottom layer or the sand mud surface, and the range of active water layer is low: the spring tide is fast, the shrimp is easy to float, and the activity range is higher than that of the low tide. Therefore, the spring tide flood season should appropriately increase buoyancy to achieve the purpose of increasing the net opening and expanding the sea sweeping area. In different sea conditions of ⑦, there are no wind waves and sea water clarification, shrimp often lurk on the surface of the sand bottom or swim slowly in the bottom layer; while on windy and wavy days, shrimp activities are frequent and often drill out of the river mud to swim slowly near the bottom or the middle and lower layers of the water, so it is necessary to appropriately increase buoyancy at this time to achieve the purpose of effective fishing. During the diurnal change of ③, shrimp tend to dim light at night, swim slowly in the bottom or lower layer, and inhabit the water layer higher than in the daytime, so it should be slightly buoyant than in the daytime. When the ④ often encounters the accident of sand suction and breaking of the net, it may be dealt with at random because of the reasons such as soft mud bottom operation or overlong suspension, overweight and over-braking, loose force on the web, bloated net, slow towing speed and so on. ⑤ avoids the accident of breaking the net, and the lengths of the eight forks tied to the truss must be equal in order to balance the force. In production, the two ends of the fork can be painted with different colors, so as to judge whether the settlement is normal in time after the gear goes into the sea. The buoyancy and subsidence force distribution of the gear must be balanced and the balance of the truss must be maintained, otherwise the gear will cause inclined motion in the sea, which is very easy to cause the net to turn over.

2. Determination and adjustment of towing speed

The main results are as follows: 1. The towing speed of fishing vessels is not only directly related to the ship type, engine type, power and other factors, but also related to the specification of the gear, the length of the towing net, and the buoyancy and sinking force of the gear. There are two kinds of towing speed: absolute towing speed and relative towing speed, the former is the displacement of the ship or gear to the bottom of the sea in a certain unit of time, which is used to calculate the sweeping area of the gear, and the latter is the displacement of the current caused by the ship or gear in a certain unit of time. used to calculate gear resistance and net type analysis, etc. The towing speed of the truss trawl generally refers to the absolute towing speed, and the speed can be calculated by measuring the position of the ship with the locator.

2. During the towing operation of the truss trawl, the adjustment of the towing speed should focus on the adaptability to the main fishing target, fishing ground and sea condition, and the towing speed is mainly determined by the main engine power and rotational speed of the fishing vessel. Crab shrimp and flounder and other swimming ability is weak, generally perched in the bottom or surface sand mud, the next class cement roller rolls to the end, so that the shrimp is disturbed to jump into the net, the absolute speed of the main shrimp should be controlled at about 1 section. The production practice has proved that the gear is stable and the bottom sticking performance is good when the towing speed is 1.5 knots. If it is too slow, the catch will be low because of the small sweeping area; if it is too fast, the lower class will jump, part of the sunken seeds will leave the ground, and the height of the net mouth will drop sharply, and the net yield of shrimp will decrease obviously.

3. In order to maintain a certain towing speed, the speed of the main engine must be adjusted according to the sea conditions. The method of determining the speed of towing can not only observe the flow of the water spray on the side and the trend of striking the spray when the stern is advancing, but also compare the speed with the ships in the same direction around them. When we use the fishing gear specification for 2300 120HP 2400 weeks, the general main engine speed of the 120HP fishing boat is adjusted to more than 1300 rpm, and the main engine speed of the 135HP fishing boat can be less than 1300 rpm. The adjustment of rotational speed in actual production is quite flexible: ① spring tide flood and current sea area top current trawl, the main engine speed should be faster, the downstream trawl main engine should be slower. On the stormy day of ②, the speed of downwind trawl main engine is slower, and the speed of headwind trawl main engine should be faster. The main engine speed of the downstream trawl in the ③ neap tide flood area should be faster than that in the spring tide and rapids area. When ④ works on soft mud bottom or sand bottom (coarse sand bottom) with poor sediment quality, the towing speed is generally faster than that on sand bottom. Due to the slow towing speed on soft mud bottom, it may cause traps and sinkers to be trapped in the mud, so that they can not be dragged. When working on sand bottom and hard sand bottom, the towing speed is too slow, which is easy to cause accidents of sand absorption and net breaking. Therefore, the operation of soft mud bottom should be avoided as far as possible, and the towing speed should be increased appropriately, but it is appropriate to keep 1 / 5m / m / m, and the towing time is generally about 3 hours.