Chinese reservoirs are primarily stocked with silver carp and bighead carp; therefore, harvest methods for these species are pivotal to the development of reservoir fisheries in China. Since the late 1960s, seines and trawl nets have been operated in reservoirs. A fishing method that combines of blocking, driving, gill-netting, and seining has been developed based on an understanding of the habits of silver carp and bighead carp. This technology has, to some extent, solved the technic al problems of harvesting pelagic species and has laid the foundation for the further development of reservoir fisheries.

Migration habits of silver carp and bighead carp

Like most other species stocked in reservoirs, silver carp and bighead carp school. However, their movements vary depending on the developmental stage of the fish and on environmental conditions. Annual movements of silver carp and bighead carp can be cla ssified into the three types of migration: spawning, overwintering, and feeding. An understanding of the migration behaviour of fish schools helps locate suitable fishing grounds.

Silver carp and bighead carp naturally reach maturity in some reservoirs. In May or June, a large number of brooders, which are stimulated by the increasing water level and run-off, migrate upstream and spawn in groups in shallow areas. These are good fis hing grounds. In addition, the brooders sometimes aggregate adjacent to the outlets of irrigation channels that have a slight water flow. These locations also present good opportunities for harvests.

Water temperature is an important environmental factor that stimulates overwintering migration of fish. The optimum water temperature for silver carp and bighead carp is between 12 and 30^oC; below 12^oC, the fish are ready to overwinter. Under normal conditions, they do not feed until spring. During the overwintering period, silver carp and bighead carp usually inhabit deeper water and become sluggish. Therefore, the efficiency of harvesting is low because the fish are less se nsitive to the operation of fishing nets, particularly in larger and deeper reservoirs. However, in some medium and small reservoirs that have flat bottoms, harvest occurs principally in the winter.

Under normal conditions, silver carp and bighead carp overwinter from January to March. During the remainder of the year, they feed heavily and always aggregate in groups to feed in places where plankton are abundant. These locations are good fishing grounds.

Joint fishing method

The joint fishing method, which combines blocking, driving, gill netting, and seining, is a large-scale operation used specifically to catch silver carp and bighead carp. This fishing method was developed in the mid-1960s and has been constantly improved. It is now widely applied in most of the large and medium reservoirs in China.

Principles and practice

The joint fishing method is a large-scale operation that uses three or more types of fishing gears. The fish are first blocked and then chased into a fixed filter net for harvesting.

The method has several operational characteristics:

• Several types of fishing gear are jointly operated in the same water body. In this way, the fishing gear can be used for different functions and passive fishing gears can be turned into active gear. The joint fishing method can be applied and adapted to reservoirs with complicated bottom topography, large surface areas, and scattered fish populations.

• In actual operation, the blocking and driving operations are conducted simultaneously with capture so that all of the silver carp and bighead carp are forced to aggregate in a certain place and are caught. The joint fishing method is such a large-scale op eration that it can be used for a fishing area ranging from a hundred to thousands of hectares.

• The joint fishing method also captures some grass carp, bream, Elopichthys bambusa, and Erythroculter spp. Moreover, some bottom fishes, such as common carp and crucian carp, can also be harvested using this method.

• Because it is a large-scale operation, the method involves a lot of gear, boats, and labour and investment is high. It is used in large and medium reservoirs.

Types of fishing gear

The joint fishing method uses several types of fishing gear.

Driving gear
The driving gear includes the net and non-net driving gear. They give the best results when they are used jointly; however, they can be used separately.

• Net-driving gear. Net-driving gear is mostly gill nets, such as trammel nets, frame gill nets, and simple gill nets. But, beach seines are sometimes used to drive fish in shallow, flat-bottom lacustrine reservoirs. The trammel net is used mainly to drive the fish schools. The frame gill net and simple gill net are the most common fishing gears for capture, but are not popular for driving operations. When operated in combination with trammel nets, the driving effect is greatly increased and other species c an also be caught.

• Non-net driving gear. The commonly used devices include white boards, air curtains, and electricity. White boards tied to ropes are towed by a motor boat back and forth in the fishing ground to drive the fish. This is the simplest of the non-net driving g ear. The white boards are made of wood and are 400 mm x 50 mm x 5 mm in size. The white boards are tied at the distance of 1 1.5 m to a manila rope that is 18 20 mm in diameter (Figure 82). An iron, lead, or stone weight (3 5 kg) is attached to the end of the rope. The fishes are driven in a given direction by the swaying white boards. The results are best when the fish are driven from shallow to deep water. When the boat is driven 200 500 m, the area should be blocked with an impounding net to prevent th e return of the fish.

Figure 82. Structure of the white boards (not to scale).

A compressor is used to introduce air into the water through several tubes to form an air curtain. The fish are frightened by the bubble sound and low frequency oscillations and move rapidly in the desired direction. This device includes an engine-driven compressor that forces compressed air into a steel manifold (8 m in length and 20 cm in diameter) and then through nine rubber pipes (8 mm in diameter). The rubber pipes are held in place with wire and weighted with iron sinkers. The rubber pipes should b e 3 5 m longer than the water depth of the fishing ground to ensure that all pipes reach the bottom during the operation (Figure 83).

Electricity can also be used to drive fish. Fish are forced by the electric field to move rapidly toward the planned harvest area. Compared with net driving, electricity requires less investment and labour. However, its effect is not as predictable as oth er methods, particularly in large reservoirs. Therefore, electric driving is operated in combination with netting. This method requires further improvement.

Figure 83. Operation of the air curtain: (1) engine, (2) air compressor, (3) air receiver, (4) steel manifold, and (5) rubber pipe.

Blocking net
This is one of the main fishing devices used in joint fishing. It has several functions. At the start of the harvest, all escape routes are blocked with blocking nets to form an enclosure. Blocking nets are usually set in combination with trammel nets and other fishing gear to force the fish to aggregate in the enclosure. When blocking nets are accompanied by fixed filters, they net can prevent fish from returning and guide the fish toward the fixed filter net where they are caught. If the mesh size and t wine diameter of the net are designed to suite the morphology of Elopichthys bambusa, the net can be used to block fish and to eliminate predators.

Fixed filter net
In the joint fishing method, the fixed filter net is the final chamber used to harvest the fish. It is usually set at a specific location on the fishing ground. The fish are driven by the trammel nets, blocking nets, and other fishing gear and are forced to the filter net for harvesting.

Design and installation of principal fishing gear

Blocking net
This net is ribbon-shaped and made of a single layer of netting (Figure 84).

• Mesh size. The mesh size of the blocking net should not allow harvest-size silver carp and bighead carp to pass through. For example, the minimum harvest size of these two species is set at 2.5 kg in Xinanjiang Reservoir. The mesh size of a gill net neede d to capture such fish is 120 140 mm; therefore, the optimal mesh size of the blocking net would be about 100 120 mm. The entanglement of fish in the meshes of the blocking net should be avoided to improve fishing efficiency.

Figure 84. Structure of a blocking net: (1) float, (2) cork line, (3) head line, (4) netting, (5) sinker, (6) lead line, and (7) foot line.

• Twine materials. The twine materials must enough strength to withstand various external forces, prolong their use, and increase their efficiency. Polyamide twine (210D/6x3, D = denier) and polyethylene twine (0.23/6x3) are both used to make the blocking n ets; however, because of its lower cost, polyethylene twine is most commonly used.

• Hanging coefficient. The acceptable horizontal and vertical hanging coefficients of blocking nets operated in reservoirs are: E1 = 0.60 0.65 and E2 = 0.76-0.80, respectively.

• Height and length. The height of the blocking net depends on the water depth in the fishing ground. Generally, the net should be designed to easily reach from the water surface to the reservoir bottom. The height of the blocking net must be adjust ed to match the water depth. In commercial operations, a main net is used in conjunction with auxiliary nets of different heights to match the average and maximum water depths. For example, the maximum water depth in Dongzhang Reservoir is 32 m (near the dam), the average depth is 15 m, and the depth upstream is only several metres. Based on these conditions, the main and auxiliary blocking nets are designed with different dimensions. The height of the main net is 20 m, which is a little larger than the a verage water depth, and the auxiliary blocking net is used to supplement the main one. In the deeper waters, they are operated jointly; whereas, in coves and other shallow waters, the auxiliary net can be used independently. The heights of the two auxilia ry nets are 10 m and 15 m.

• The length of a blocking net is dictated by the maximum width of the fishing ground. At least three pieces of netting are normally used. Each piece consists of many sections, and each section is usually 50 m long. For very large operations, severa l dozen sections of netting are joined.

• Buoyancy and sinking forces. The blocking net must have a certain buoyancy and sinking force to ensure net stability. These forces are normally calculated with reference to a set gill net. However, higher coefficients of both buoyancy and sinking force are adopted. The buoyancy of a polyamide blocking net should be 0.60 0.65 times the total weight of the net in the air, and the sinking force should be 1 1.2 times the buoyancy of the floats. The buoyancy of a polyethylene blocking net is usually de signed to be 0.55 0.70 times the total weight of the net in the air, and the sinking force is 1.2 1.5 times the buoyancy. The blocking net is supported by PVC foam plastic floats with buoyancy of 300 400 g per float. The drum-shaped ceramic sinkers that a re normally used have a weight of 50 100 g per sinker.

• Lines. The blocking net consists of a head line, foot line, cork line, lead line, and breast line. Polyethylene rope (6 7 mm diameter) or palm rope (8 9 mm diameter) is commonly used. Because most reservoirs have uneven bottoms, the foot line some times does not fit well and some fish escape. To prevent the escape of fish under the foot line, the length of the foot line should be 5 10% longer than the head line.

• Installation. Blocking nets are installed in a similar way to gill nets.

Trammel net
A trammel net is a special type of gill net. It has two wide-meshed outer nets and one fine-meshed inner net. All three nets are fixed to the head and foot lines. The outer nets are made of thick twine, and the inner net is made of thin twine. The hanging coefficient of the outer netting is E12 + E22 = 1 and of the inner webbing E12 + E22