Technical Analysis of Adjustments to Beam Trawl Fishing Gear and Methods

At present, beam trawl shrimp trawling has become one of the important operating methods of marine fishing in coastal fishing areas, and its output value as a proportion of the total marine fishery output value has risen year after year. Since this operating method belongs to single-boat bottom beam trawl operation, its gear design, rigging, operational adjustment, and fishing ground selection and other stages are all fairly highly technical, so even a slight oversight can affect shrimp trawling production. In particular, the adjustment techniques in beam trawl operation are the key to fishing production technology. For example, in production practice everyone often encounters this phenomenon: fishing boats of the same horsepower, using gear of the same specifications, producing in the same fishing ground, yet the net yields differ enormously, the highest being nearly a thousand jin, the lowest only a dozen jin or even an empty net. Investigating the cause, to a large extent it is closely related to the correct use of fishing gear and methods and to adjustment techniques. Therefore, we attach great importance to theoretical research and practical exploration in this area, constantly summing up experience to improve fishing technology, raising the catching effectiveness of beam trawls, and basically overcoming the past imbalance in production. Based on practice and summaries in recent years, the adjustment techniques for beam trawl fishing gear and nets and production experience are introduced as follows:

I. Selection of gear specifications and adjustment of gear weight

Since beam trawl operation mainly catches shrimp that inhabit lower water layers and have weak swimming ability, and incidentally catches crabs, flatfish and other fish, it has the operating characteristics of being active and flexible, actively chasing shrimp, and having a wide range of adaptability. In recent years, as shrimp trawling intensity has increased, the operating beam grounds have gradually extended to deep-water fishing grounds, and marine environmental conditions have become more complex. To improve safety at sea, correspondingly the tonnage and horsepower of fishing boats have continuously increased, and gear design also needs constant renewal and improvement to perfect its performance. Based on the above situation and comprehensive production experience, we believe the rationality of selecting beam trawl shrimp gear can be explained from the following aspects:

1. The vessel's power utilization rate should be raised as much as possible. Relevant experiments prove that when current shrimp trawl gear is in operation, the power consumed by the fishing boat (80--150HP) accounts for less than 10% of rated horsepower, whereas the power utilization rate of fishing boats in trawl operation is 30%--40%. Practice has proven that small-horsepower fishing boats operate more economically than large-horsepower fishing boats. Therefore, only by increasing gear specifications, making the gear width larger, expanding the swept sea area during towing, and raising the vessel's power utilization rate and yield. That is, the beam supporting horizontal spread must be lengthened and its specifications increased, but this is constrained by factors such as actual operating conditions. In actual production, we believe the beam should use seamless steel pipe with an outer diameter of 110mm--159mm, and the most suitable length is 0.9--1.1 times the length of the fishing boat.

2. The ratio of net mouth circumference to gear length must be reasonable. The determination of gear net mouth circumference is mainly based on the beam length used and the net mouth hanging coefficient, while also referring to the fishing boat's horsepower, and the gear length is determined by the net mouth circumference. As long as the ratio between gear length and net mouth circumference is reasonable, the gear will be both stable and balanced when towing, with low resistance and not prone to sucking sand and tearing the net. At present, we believe that for 80HP--150HP fishing boats, small-mesh multi-cod-end gear is better, and its main specifications and technical parameters are: mesh size change from net mouth to cod end 40mm--50mm, net mouth horizontal hanging coefficient about 0.25, gear length-to-circumference ratio between 0.07--0.10, net cover to total length ratio about 0.175, net body to total length ratio 0.32, cod end to total length ratio 0.485, with 3--10 cod ends varying. During operation this gear has good water filtration performance, the belly net is not easily prone to sucking sand, and it has a good shrimp trawling effect, while damage to shrimp bodies is small and the intact rate is high.

3. Because of the structural and operational particularities of beam trawls, accidents such as net tearing are often caused in production. Therefore, correctly equipping and adjusting gear weight is the key to raising yield and can prevent gear accidents. The buoyancy-sinking force configuration in beam trawl design is for general conditions. Under different tides and different sea conditions, its sinking and buoyancy forces must be appropriately adjusted. To judge whether gear weight is suitable, after the gear enters the water one can check whether the beam sinks at an angle, observe the degree of friction between the cement rollers of the sinkers and the seabed, and the various catch species and quantities and sand-sucking conditions. In production, the principle for configuring gear buoyancy-sinking force is: during towing, the sinker line should effectively startle and disturb shrimp, but must not be so heavy as to increase gear resistance and make the sinker line scrape the ground too much and suck sand. Nor should it be so light as to fail to startle and disturb shrimp and make gear towing unstable. The specific adjustment methods are as follows: (1) During spring and neap tides, neap tide currents are slow, shrimp inhabit the bottom layer or the sand-mud surface layer, and their active water-layer range is low; during spring tide currents are fast, shrimp easily rise, and their active range is higher than during neap tides. Therefore, during spring tides buoyancy should be appropriately increased to raise the net mouth and expand the swept sea area. (2) Under different sea conditions, when there is no wind or waves and the seawater is clear, shrimp often lurk in the surface layer of the sandy bottom or swim slowly in the bottom layer; while on windy and wavey days with turbid water quality, shrimp are frequently active and often emerge from the mud and swim slowly in the near-bottom or middle-lower layers of the water body. Therefore, buoyancy should be appropriately increased at this time to achieve effective catching. (3) With day-night changes, shrimp tend toward dim light at night and swim slowly in the bottom or lower layer, inhabiting a higher water layer than during the day. Therefore, buoyancy should be slightly increased compared with daytime. (4) When gear sand-sucking and net-tearing accidents are frequently encountered, it may be due to operating on soft mud bottom, or lifting bridles being too long, the sinker line being too heavy and scraping the ground too much, the net belly being loosely stressed, netting being bloated, towing speed being too slow and other reasons, and should be handled as circumstances require. (5) To avoid net-tearing accidents, the 8 bridles tied to the beam and corresponding to 2 lengths must be equal, so as to seek balanced force. In production, the bridles at both ends can be painted different colors so that after the gear enters the sea one can promptly judge whether sinking and floating are normal. The distribution of gear buoyancy-sinking force must be balanced to maintain the balance of the beam; otherwise the gear will create a tilted movement state in the sea and very easily cause the net to flip.

II. Determination and adjustment of towing speed

1. The towing speed of a fishing boat is not only directly related to factors such as hull type, engine type, and power, but also related to gear specifications, the length of the tow rope, and the gear's buoyancy-sinking force configuration. Towing speed includes absolute towing speed and relative towing speed. The former is the displacement produced by the boat or gear relative to the seabed per unit time, used to calculate the gear's swept sea area; the latter is the displacement produced by the boat or gear relative to the water flow per unit time, used to calculate gear resistance and analyze net shape, etc. Beam trawl towing speed generally refers to absolute towing speed, and speed can be calculated by using a positioning instrument to determine the boat's position.

2. When beam trawl is towing, adjustment of fast or slow towing speed should focus on adaptability to the main target species, fishing ground, and sea conditions. The fast or slow towing speed mainly depends on the fishing boat's main engine power and rotational speed. Crabs, shrimp, flatfish and others have weak swimming ability and generally inhabit the bottom layer or surface sand-mud, and during operation the cement rollers of the lower ground rope roll all the way to the bottom, causing shrimp to be startled and jump into the net. When mainly catching shrimp, the absolute speed should be controlled at about 1 knot. Production practice has proven that when towing speed is 1--1.5 knots the gear is relatively stable and has good bottom-hugging performance. If too slow, the swept sea area is small and catch is low; if too fast, the lower ground rope will jump, some sinkers leave the ground, the net mouth height drops sharply, and the per-haul shrimp yield will clearly decline.

3. In order to maintain a certain towing speed, the main engine rotational speed must be adjusted according to sea conditions. The method for determining fast or slow towing speed can be by observing the flow of water splashes along the side and the movement of splashes struck by the stern propulsion, and also by comparing speed with nearby vessels towing in the same direction. When we use gear specifications of 2300--2400 circumference, the main engine speed of a 120HP fishing boat is generally adjusted to above 1300 rpm, while the main engine speed of a 135HP fishing boat can be less than 1300 rpm. In actual production, adjustment of rotational speed is quite flexible: (1) When trawling against the current during spring tides and in sea areas with fast currents, the main engine speed should be faster, and when trawling with the current the main engine should be slower. (2) On windy and wavey days, when trawling with the wind the main engine speed should be slower, and when trawling against the wind the main engine speed should be faster. (3) In sea areas with fast currents during neap tides, trawling with the current should have a faster main engine speed than trawling with the current during spring tides and in fast-current sea areas. (4) When operating on relatively poor bottom conditions such as soft mud bottom or sandy bottom (coarse sand bottom), the towing speed is generally faster than on sand-mud bottom, because on soft mud bottom a slow towing speed may cause the tow rope and sinker line to sink into the mud, to the point of being unable to tow, while on sandy bottom and hard sand bottom a towing speed that is too slow easily causes sand-sucking and net-tearing accidents. Therefore, one should avoid operating on soft mud bottom as much as possible and appropriately raise the towing speed, but it is suitable to maintain 1--1.5 knots, and the towing time is generally about 3 hours.