Detailed Explanation of the Fishing Techniques and Methods for Protosalanx hyalocranius
1. Introduction
In 1995, after the Ministry of Agriculture and the Ministry of Water Resources jointly issued the "Notice on Accelerating the Promotion of Icefish Transplantation and Proliferation Technology," a nationwide upsurge in large icefish transplantation was set off. Over the past decade, the transplantation and proliferation of large icefish has been widely developed in northern regions, especially in Inner Mongolia, and good transplantation results have been achieved in both lakes and reservoirs.
When the large icefish in the transplanted waters have reached fishery effectiveness, that is, when the large icefish have formed a high-quantity and relatively stable population, become naturalized, and formed fish catches, the question of how to catch these large icefish has been placed before us.
2. Introduction and Modification of Boats and Nets
2.1 Introduction of Boats and Nets
Since the south has a relatively long history and experience in catching large icefish, in the early stage of fishing, northern regions generally hired southern fishing teams to bring boats and nets here for fishing. In the south, there are many methods for catching large icefish, including gill nets, beach seines, and trawl fishing. After investigation, most northern reservoirs introduced the relatively practical Taihu double-boat single-trawl technique for catching large icefish.
2.2 Modification of Boats and Nets
2.2.1 Modification of Trawlers
A trawler is a boat that uses mechanical power to drive a trawl net. In the Taihu region, because the water surface is relatively shallow and large icefish are distributed relatively evenly, the trawler power is also relatively small. Generally, an 18-horsepower diesel engine with one stern-mounted propeller is sufficient to achieve good results. However, most reservoirs in the north where large icefish are transplanted are large and medium-sized reservoirs, which are not only deep but also large in area. The original trawlers clearly felt underpowered as soon as they entered the water. For this reason, we modified newly built trawlers to use two 15-horsepower diesel engines as power, with stern-mounted linked twin propellers, achieving very ideal results.
2.2.2 Modification of Trawl Nets
A trawl net is a mobile, filtering fishing gear. It relies on a trawler to drive it, towing it through the water, forcing large icefish within the trawl's operating range to enter the net, thereby achieving the purpose of catching them.
Trawl nets can be divided into many types according to different operating methods and structural characteristics. The trawl nets used for catching large icefish in northern reservoirs belong to surface trawls when divided by operating water layer position; belong to double-boat trawls when divided by the number of fishing boats used; and belong to wingless trawls when divided by net structure.
2.2.2.1 Changing multi-cod-end trawls to single-cod-end trawls. What was initially introduced in northern reservoirs was the wingless multi-cod-end trawl commonly used in Taihu. That is, the net had 2 long belt-shaped body nets and 2 cod ends. Later, because of its high resistance in water and cumbersome operation, it was not suitable for fishing operations in reservoirs, so it was changed to a wingless single-cod-end trawl, achieving relatively good fishing results.
2.2.2.2 Changing boat-deployed fish retrieval to trawler-based fish retrieval. In the introduced Taihu trawl operation, every hour a small boat (or sampan) had to be untied from the trawler to go into the cod end to retrieve fish, and then return to the main boat. In northern reservoirs, in order to reduce the number of "yellow-striped" large icefish and ensure the quality of large icefish, fishing operations are usually carried out at night. Therefore, to ensure production safety, we increased the tonnage of the fish-retrieval boat and directly connected it to the sweep line, towing it at the rear end of the cod end, so that fish could be retrieved at any time during the trawling process, and the catch was directly stored on the small boat, reducing the small boat's range of activity and making production safer.
2.2.2.3 Changing the net material from ordinary polyethylene netting to polyethylene monofilament knotless netting. Since a trawl is mobile fishing gear, its basic requirements for materials are extremely high strength, excellent wear resistance, relatively large elasticity and elongation. At the same time, it should have the minimum weight to reduce the resistance of the net and extend the service life of the net. The modified net basically possessed the above characteristics.
2.2.2.4 Changing sinkers to floats. The original net used moso bamboo as support poles, with relatively heavy sinkers attached at the lower end, which was both bulky and water-resistant, and was not conducive to adjusting the trawl's diving depth; on the new net we used steel (iron) support poles, with a small float barrel attached at the upper end, which solved the problems existing in the original net.
3. Technical Parameters, Structure, and Assembly of the New Net
The wingless trawl consists of sweep lines, bridles, float barrels, support poles, ropes, body net, and cod end. The body net consists of a back net, a belly net, and two side nets. The cod end consists of two rectangular net pieces of the same specifications, upper and lower, which after sewing form a cylindrical cod end. The degree of contraction of the body net from the net mouth to the end must both facilitate luring fish into the net bag and prevent them from escaping back out through the net mouth. The ropes include float lines, sinker lines, and upper and lower selvedge ropes. All are made of ethylene rope with a diameter of 10-20 mm. The support poles are used to increase the vertical expansion of the trawl.
3.1 Main Parameters of Trawl-Type Fishing Gear
3.1.1 Net mouth circumference The size of the net mouth circumference is related to the horsepower of the operating boat. Under the condition of maintaining a certain trawling speed, the greater the horsepower of the trawler, the correspondingly larger the net mouth circumference can be. The relationship between net mouth circumference and horsepower can be expressed by the following formula: C=KN1/2
Where C - net mouth circumference (m);
K - constant, value 4-6;
N - horsepower.
3.1.2 Net length The length of the body net is determined by the net mouth circumference, generally taken as 70%-80% of the net mouth circumference.
3.1.3 Net twine material It should have both strength and elasticity, and good wear resistance.
3.1.4 Mesh size 2a=10 mm.
3.1.5 Hanging coefficient A proper hanging coefficient can make the diamond shape of the mesh and the direction of force as consistent as possible, ensuring that the netting in each part of the net plays its due role during operation and maintains a relatively ideal line shape.
Upper middle rope of net mouth u1=0.45-0.50;
Lower middle rope of net mouth u1=0.35-0.40.
3.2 Net Structure
3.2.1 Main dimensions
Net mouth circumference 36 m, rectangular in shape. Length 14 m, height 4 m.
Body net length 25 m, cod end length 4 m.
3.2.2 Netting The body net is made of polyethylene monofilament heat-treated into knotless netting, mesh size 2a=10 mm, body net mouth circumference 36 m, and the junction circumference between the body net and the cod end is 4 m. The shape of the body net is achieved through gradual mesh reduction. The cutting slope is 4/1. The cutting method is: cutting cycle one single leg with two side edges or two single legs with three side edges.
3.2.3 Ropes
Upper rope ethylene rope, diameter 10 mm, length 14 m, 1 piece.
Upper selvedge rope ethylene rope, diameter 8 mm, length 14 m, 1 piece.
Lower rope ethylene rope, diameter 10 mm, length 14 m, 1 piece.
Lower selvedge rope ethylene rope, diameter 8 mm, length 14 m, 1 piece.
Side ropes ethylene rope, diameter 10 mm, length about 4 m, 1 piece each.
Bridles ethylene rope, diameter 12 mm, length 6 m, 2 pieces.
Sweep lines ethylene rope, diameter 20 mm, length 100 m, 2 pieces.
3.2.4 Accessories
Support poles steel pipes, diameter 4 cm, length 4.5 m, 2 pieces;
Floats 20 kg empty plastic barrels, 2 pieces.
Sinkers none, steel support poles also serve as sinkers.
3.3 Net Assembly
3.3.1 Netting sewing The various sections of netting are sewn transversely between each other using side seams, generally with a sewing ratio of 1:1. For longitudinal sewing between netting pieces, the wrapping stitch method is used, with a knot tied at certain intervals.
3.3.2 Assembly of netting and ropes
3.3.2.1 Float side assembly The float side structure of the net includes the upper rope, upper selvedge rope, and netting. After the upper selvedge rope passes through all the netting at the edge of the net, the netting is fixed to the upper selvedge rope at a horizontal hanging coefficient of u1=0.5, and then lashed together with the upper rope. Because the netting mesh is relatively small, the upper selvedge rope can be pre-threaded into a large-mesh selvedge net, and then the large-mesh selvedge net is increased in mesh and sewn to the body net. The two float barrels can be directly installed above the support poles.
3.3.2.2 Sinker side assembly The sinker side of the net includes the lower rope, lower selvedge rope, and netting. The sinker side assembly of the trawl is the same as the float side assembly.
3.3.3 Cod end assembly The cod end is the final fish-collecting location. During assembly, it is necessary both to ensure that the cod end has sufficient strength to withstand the weight and impact of the catch itself under maximum load, and also to pay attention to convenient operation when hauling the net. At the bottom of the cod end there is a cod end binding rope, used to tie the cod end tightly and prevent the catch already inside the cod end from escaping. At the junction between the cod end and the body net there is also a binding rope. When hauling the net, this binding rope is first lifted, the catch concentrated in the cod end is hoisted into the boat, the cod end bottom binding rope is untied, and all the catch in the cod end is poured out.
4. Brief Introduction to Fishing Methods
4.1 Preparation before setting the net Before setting the net, the net must be organized. The trawl should be stacked on the main boat in order from top to bottom, in the sequence of support poles, body net, and cod end. The edge of the main boat must be smooth to prevent the net from catching.
4.2 Selection of fishing grounds and fishing time Commercial large icefish are generally caught in September. At this time, large icefish are concentrated in reservoir bays and coastal zones to feed, so operating in reservoir bays and coastal zones yields better results.
Because large icefish caught during the day have more food in their intestines and are prone to the "yellow-striped" phenomenon, affecting the overall quality of the large icefish, most northern reservoirs adopt nighttime fishing operations.
4.3 Setting the net After selecting the fishing ground, the main boat and auxiliary boat come close together, and the bridles are connected to the sweep lines of the two boats respectively. The support poles and body net are put into the water, and the auxiliary boat carries the net forward, stretching the net shape open. The main boat checks whether the net is extending normally, then the main boat pays out the sweep lines, gradually expanding the distance between the two boats, and begins trawling.
4.4 Trawling and hauling During trawling, the horizontal distance between the two boats is maintained at 60-100 m, and the trawling speed is maintained at 5-8 km per hour. When retrieving fish, the motor boat maintains a constant speed, and the towed operating boat is responsible for hauling the net and retrieving fish. The personnel on the operating boat haul the cod end into the boat, untie the cod end bottom binding rope, pour out all the catch, then re-tie the cod end bottom binding rope tightly and throw the cod end back into the water, allowing continuous operation. After each fish retrieval, the personnel on the operating boat must communicate with the driver on the motor boat using signals, informing them of the amount of catch, in order to decide whether to change the operating location for the next haul.
4.5 Hauling in the net When hauling in the net, the main boat notifies the auxiliary boat to simultaneously come to the trawl. First, all the sweep lines are taken in, then the support poles are lifted onto the main boat, thus sealing the net mouth. Then the body net is taken in, down to the cod end, all the catch is retrieved, and the net is cleaned and organized for next use.