4. Bottom Pair Trawl
As mentioned above, bottom pair trawls are gradually replacing the otter board trawls for exploitation of fish. Now, bottom pair trawls are very popular in the Northern and Southern regions. Most pair trawlers have engines of 200-450 Hp per boat. There are two types of trawl used. The first one is an ordinary trawl and the second is the Chinese trawl with a very big mesh size (see Table 7 ).
The nets have head ropes of 18-84 m in length. The length of trawl net fully stretched ranges from 40 m to 120 m.
The main parameters of the structure of the ordinary pair trawls are given in Table 9 .
To operate the pair trawl, the two boats must have the same horsepower and size. This trawling method is applied throughout the whole country. However, only some provinces of the Southern region where the fisheries have developed use a big boat called a “Female boat” in cooperation with a small boat called the “male boat”. During operations, the small boat is responsible for keeping only the end of a towing warp, the big boat not only keeps the end of other towing warp but also hauls the towing warps and the trawl and also stores the catch.
Net Setting:
The main boat (the setting boat) reduces its speed to the lowest possible level and the cod-end is deployed into the sea, the body of net, wings and headline leg and tow leg follow in turn.
Table 9. The parameters of ordinary bottom pair trawls
Provinces |
Horse power of boat (Hp) |
Length of trawl net (m) |
Length of head rope (m) |
Mesh size in cod end (mm) |
Kien Giang |
90-150 |
50-58 |
22.2-23.8 |
25.0-30.0 |
62.0-100.0 |
23.8-28.6 |
25.0-30.0 |
250-400 |
Vung Tau |
90-150 |
44.0-55.0 |
35.0-48.0 |
28.0-34.0 |
44.0-48.0 |
50.0-52.0 |
30.0-34-0 |
155-225 |
47.0-70.0 |
39.0-40.0 |
30.0-40.0 |
250-400 |
Khanh Hoa |
23-45 |
48.0-48.5 |
19.2-21.0 |
30.0 |
49.5-52.0 |
21.5-23.0 |
30.0 |
46-84 |
Da Nang |
23-45 |
40.0-65.0 |
18.0-30.0 |
13.0-17.0 |
40.0-68.0 |
20.0-41.0 |
16.0-18.0 |
46-84 |
Nghe An |
<23 |
40.0-45.0 |
28.0-30.0 |
18.0 |
Hai Phong |
250-400 |
60.0-82.0 |
32.0-42.0 |
40.0 |
The other boat reaches to the left side of the main one, a messenger line is thrown to the main vessel in order to haul one wing of the net.
Net Hauling:
Both boats reduce their speed to the minimum and turn their bows to the right until they are moving in the opposite direction to towing. The two boats then haul the towing warps synchronously, then the sweep line, the headline leg and the tow leg.
The auxiliary boat hauls the sweep line until the headline leg and the tow leg is very to close to the gallows then the headline leg and tow leg are linked together with the messenger line to transfer them to the main boat. The auxiliary then moves away from the main boat.
After receiving the wings from the auxiliary boat, the main boat continues using a crane to haul each part of the net from the wing to the cod-end. This operation is carried out on the right side (starboard) of the boat, at that time the boat is controlled carefully to keep the net from being to close to the vessel's side or drifting toward the propeller. Depending on the catch, one or two cranes may be used to haul the net.
Apart from the above fishing method, some provinces use other methods to haul nets.
The duration of trawling in a haul is from 2-4 hours. The fishing trip lasts from 1 to 3 weeks. The main catches are bottom fishes, semi-pelagic fishes and trash fish. The average ratio of trash fish is usually 40-55% of the total catch.
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