Fishing techniques of large silverfish

1. Preface

In 1995, the Ministry of Agriculture and the Ministry of Water Resources jointly issued the notice on accelerating the Promotion of Silver Fish Transplantation and Proliferation Technology, which set off an upsurge of large silverfish transplantation in the whole country. In the past ten years, the cause of transplantation and proliferation of large silverfish has been widely developed in the north, especially in Inner Mongolia, and good results have been achieved in lakes and reservoirs.

When the large silverfish in the transplanted waters achieve the fishery effect, that is, the large silverfish form a high number and stable population, naturalize and form the catch, the problem of how to catch these big silverfish is placed in front of us.

2. Introduction and transformation of ships and nets.

2.1 introduction of ships and nets

As the fishing of large silverfish in the south has a long history and experience, in the early stage of fishing, the northern area generally hired the southern fishing team to come here with boats and nets. In the south, there are many ways to catch large silverfish, including Gill net, trawl, trawl and so on. After investigation, most of the northern reservoirs have introduced the more practical technology of fishing large silverfish with double boats and single towing in Taihu Lake.

2.2 Transformation of ships and nets

2.2.1 Transformation of tugboat

Tugs drive trawls with the help of mechanical power. In the Taihu Lake area, due to the shallow water surface and the uniform distribution of large silverfish, the power of the tugboat is also small. Generally, an 18 horsepower diesel engine with a tail paddle can achieve better results. On the other hand, most of the reservoirs transplanting large silverfish in the north are large and medium-sized reservoirs, which are not only deep, but also have a large area, and the original tugboat obviously felt lack of power when it was in the water. For this reason, we changed the newly-built tugboat to be powered by two 15-horsepower diesel engines and hung double slurries at the tail, and achieved very ideal results.

2.2.2 Transformation of trawl

Trawl is a kind of mobile and filtering fishing gear. It is driven by tugboats, towing in the water, forcing large silverfish within the scope of trawling operations into the net, so as to achieve the purpose of fishing.

Trawls can be divided into many kinds according to different operation modes and structural characteristics. the trawls used to catch large silverfish in northern reservoirs belong to surface trawls according to the position of operating water layer, double-boat trawls according to the number of fishing vessels, and wingless trawls according to the structure of nets.

2.2.2.1 change from multi-bag trawl to single-bag trawl. The northern reservoir began to introduce the wingless polycystic trawl commonly used in Taihu Lake. That is, the net has two long belt-shaped body nets and two capsule nets. Later, because of its high resistance in the water and annoying operation, it was not suitable for fishing in the reservoir, so it was changed to wingless single bag trawl, and a good fishing effect was obtained.

2.2.2.2 change the boat to fish for tugboat fish. The introduction of Taihu Lake trawling operation, every hour from the tugboat (or sampan) to the bag net to pick up fish, and then return to the main ship. In the northern reservoir, fishing operations are usually carried out at night in order to reduce the number of "yellow strips" of large silverfish and ensure the quality of large silverfish. Therefore, in order to ensure the safety of production, we will increase the tonnage of the fishing boat, connect it directly to the drag, tow it at the back end of the bag net, take the fish at any time during the tugboat towing, and store the catch directly on the boat, thus reducing the range of activities of the boat and making production safer.

2.2.2.3 the net material is changed from ordinary polyethylene mesh to polyethylene monofilament nodular-free mesh. As the trawl is a mobile fishing gear, its basic requirements for materials are extremely high strength, excellent wear resistance, large elasticity and elongation. At the same time, it should have the minimum weight to reduce the resistance of the gear and prolong the service life of the gear. The reformed gear basically has the above characteristics.

2.2.2.4 change sunk to float. The original net gear uses bamboo as a strut, and the lower end is equipped with a heavier sinker, which is not only bulky, but also water-blocking, and is not conducive to adjusting the depth of the trawl; we use steel (iron) struts on the new net. The upper end is equipped with a small floating bucket to solve the existing problems of the original net.

3. Technical parameters, structure and assembly of new net gear.

The wingless trawl is composed of drag, fork, floating bucket, strut, cable, body net and capsule net. The body net consists of a back net, a ventral net and two side nets. The capsule net is composed of the upper and lower rectangular mesh of the same specification, and the suture of the capsule net is cylindrical. The shrinkage degree of the body net from the net mouth to the end should not only facilitate the fish to enter the net bag, but also can not make it escape from the net mouth again. There are floats, sunken classes and upper and lower marginal classes. All are made of ethylene fiber rope with a diameter of 10-20mm. Struts are used to increase the vertical expansion of the trawl.

3.1 main parameters of trawl fishing gear

3.1.1 the perimeter of the net mouth is related to the horsepower of the operating vessel. In the case of maintaining a certain towing speed, the greater the horsepower of the tugboat, the perimeter of the net mouth can be increased accordingly. The relationship between net mouth perimeter and horsepower can be expressed by the following formula: C=KN1/2

Where Cmure-net mouth perimeter (m)

Kmuri-constant, numeric 4mur6

Nmura-horsepower.

3.1.2 the length of the net is determined by the perimeter of the net, which is generally 70% and 80% of the perimeter of the net.

3.1.3 the mesh material has both strength and elasticity, and has good wear resistance.

3.1.4 mesh size 2a=10mm.

3.1.5 proper shrinkage coefficient can make the prismatic shape and force direction of the mesh consistent as far as possible, ensure the role that each part of the net clothing should play in the operation and maintain an ideal linetype.

On the net mouth, the middle class U1 is 0.45, the color is 0.50.

Under the net mouth, the middle class U1 is 0.35, and the color is 0.40.

3.2 net structure

3.2.1 Principal scale

The perimeter of the net mouth is 36m, which is rectangular and open. It is 14m long and 4m high.

The length of body net is 25m and the length of capsule net is 4m.

3.2.2 the net is made of polyethylene monofilament heat treatment into nodular-free mesh, the mesh size is 2a=10mm, the perimeter of the net mouth is 36m, and the perimeter of the interface between the net and the capsule net is 4m. The shape of the body net is achieved by gradually decreasing the eye. The clipping slope is 4 stroke 1. The clipping method is: the clipping cycle is next to both sides of one foot or to the three sides of two feet.

3.2.3 outline

The rope is made of ethylene fiber, 10mm in diameter and 1 in length.

The upper edge is made of ethylene fiber rope, 8mm in diameter and 1 piece in length.

The lower ethylene rope is 10mm in diameter and 1 piece in length.

The lower edge of ethylene fiber rope, diameter 8mm, length 14m 1.

Side class ethylene fiber rope, diameter 10mm, length about 4m, 1 each.

Fork class ethylene fiber rope, diameter 12mm, length 6m 2 pieces.

Drag ethylene rope, diameter 20mm, 100m long 2 pieces.

3.2.4 Appliances

Brace steel pipe, diameter 4cm, length 4.5m 2

Float 20kg empty plastic buckets, 2.

Sunken son no, steel strut is also used as sunken son.

3.3 net assembly

3.3.1 the net clothing is stitched horizontally between the segments of the net clothing, using side seams, and the general stitching ratio is 1 ∶ 1. The net closet is sewn longitudinally, with a knot for each stitch at a certain distance.

3.3.2 Assembly of net clothing and cable

3.3.2.1 the float square structure of float side assembly net includes upper class, upper edge class and net clothing. After the upper edge class runs through all the net clothes at the edge of the net, the net is fixed on the upper edge according to the horizontal shrinkage coefficient U1x 0.5, and then tied with the upper class. Because the mesh of the net is small, the upper edge can be worn into a large edge net in advance, and then the large edge net can be stitched with the body net. Two floating buckets can be installed directly above the strut.

3.3.2.2 the sinker side of the sunken side assembly net includes the lower class, the lower edge class and the net clothing. The sinker assembly of the trawl is the same as the float assembly.

3.3.3 the bag net assembles the bag net as the final fish collection place, and when assembling, it is necessary to ensure that the bag net has sufficient strength to withstand the weight and impact of the catch itself under the maximum load, and pay attention to the convenience of operation when lifting the net. There is a capsule net class at the bottom of the capsule net, which is used to fasten the capsule net and prevent the catch that has entered the capsule net from escaping. There is also a class at the interface between the capsule net and the body net. when raising the net, pick up this class first, lift the catch concentrated in the capsule net into the boat, untie the bottom bundle class of the capsule, and pour out all the catch in the capsule net.

4. Brief introduction of fishing method

4.1 the gear should be arranged before the net is released, the trawl should be arranged from top to bottom in order, and the trawl should be stacked on the main ship in the order of strut, body net and bag net. the edge of the main ship must be smooth to prevent hanging the net.

4.2 the selection of fishing grounds and fishing time to catch commercial large silverfish is generally carried out in September. At this time, the big silverfish are concentrated in the reservoir bay and coastal areas to find food, so the operation effect is better in the reservoir bay and coastal areas.

Because the large silverfish caught in the daytime have more food in the intestines, it is easy to "yellow strip" phenomenon, which affects the overall quality of the big silverfish, so most of the reservoirs in the north adopt the operation mode of night fishing.

4.3 after the fishing ground is selected, the main ship and the secondary ship are close together, and the fork class is connected to the two ships respectively. Put the strut and body net into the water, the auxiliary ship carries the net forward, the net shape is extended, and the main ship checks whether the net is stretched normally, and then the main ship sends out the tractor, gradually expands the distance between the two ships, and begins to tow the net.

4.4.When the towing net and the towing net are raised, the horizontal distance between the two ships shall be maintained at 60 5-8km 100m, and the speed of the towing net shall be maintained at 100m per hour. When taking fish, the motor-driven boat keeps the speed of the boat constant, and the towed operation boat is responsible for netting the fish, and the personnel on the operation ship raise the capsule net into the ship, untie the capsule bottom bundle class, pour out all the catch, then re-fasten the bottom bundle class of the capsule, and throw the bag net into the water to carry out continuous operation. After each fish collection, the personnel on the operating ship should use the signal to contact the driver on the motor ship to inform the catch of the catch in order to decide whether to change the place of operation in the next round.

4.5 when the net is closed, the main ship notifies the secondary ship to go to the trawl at the same time, first put away all the trawlers, and then put the strut on the main ship, that is, seal the net mouth. Then pull in the net until the bag net, take all the catch, clean and arrange the net gear for next use.