2. Bottom Otter Board Trawl

     According to the classification of the species caught, the bottom otter board trawls may be divided into two groups: Shrimp otter board trawl and Fish otter board trawl.

2.1. Fish Otter Board Trawl

    The fish otter board trawl has developed mainly in large-scale fisheries. They are used on fishing vessels of 135-500 Hp. Most of the nets are two-seam types with a mesh size ranging from 80 to 240 mm for the wings and from 30 to 40 mm for the cod-end with polyethylene twine of 380D/5x3-380D/25x3 or polyethylene 700D/5x3-700D/15x3. The ground rope ranges from 21.8 to 40 m.

    The otter boards are made of wood of a rectangular or oval shape. Depending upon boat size, the trawlers can operate on the fishing grounds continuously from 7-20 days.

    Before 1985, the fish otter board trawls developed and expanded. Up to the present, because the numbers of living marine resources have declined and because the towing speed of otter board trawlers are slow, the catch obtained cannot cover the fishing cost. Therefore, the numbers of otter board trawlers have decreased year by year. Instead of fish otter board trawlers, nowadays, fishermen use pair trawlers with engines of 135-500 Hp per boat. Using pair trawls, the towing speed is increased and the production increases. Fishing operations can be carried out in off-shore areas.

Net Setting:

    The net is neatly prepared on the deck at the stern. When it is operated, the cod-end is deployed first, then the body, wings, ground rope, head rope, headline leg and towing leg and the sweep line follow. At that time, the otter board is tied to the sweep rope and unhooked from the gallows for gradual deployment into the waters and opened when the vessel goes ahead. When the net is stable, it is continuously set to the end of the warps.

Net Hauling:

     The towing warp is recovered and the otter boards are hung on the gallows. The winch recovers the towing rope as far as the ground rope. The crane will then in turn lift each part of the wings, body and cod-end. The cod-end is then pulled to the foredeck for discharging the fish.

2.2. Shrimp Otter Trawl

     The fishing grounds of' the shrimp otter board trawls are the coastal areas, with a water depth of less than 30 m. In the North of Vietnam, small boats with engines of less than 60 Hp are always used. But in the South, fishermen use big fishing boats for catching shrimp. The boats have engines from 33 - 450 Hp. Though the big boats are used for exploitation of shrimp, they still operate in coastal areas. Therefore, the operation of these boats has a negative impact on the small-scale fisheries. Now, most of the otter board trawlers are used to catch shrimp, the fish otter board trawlers are very few in number.

     Fishing operation of the shrimp otter board trawl is same as those of the fish otter board trawl.

     The mesh size in the wings is 26-50 mm and made of polyethylene 380D/2x3 -380D/12x3. The mesh size at the cod-end is 12-24 mm and made of polyethylene 380D/3x3-380D/9x3. The size of net depends on the power of' the fishing boat. Fishermen use a “tickler” in front of' the ground rope and the head rope. The two ends of the tickler are kept in the same place with the ends of' ground rope. The length of the chain is 1.5 - 3 .5 m shorter than the length of ground rope and moving in front of the ground rope. The tickler is used to round up the shrimp in the sand that jump up when contacted by the iron-chain, thus, the shrimp are easily caught by the net.

     The otter boards are rectangular and flat, and are made of wood and iron, they are 0.40 - 0.75 m in width and 0.96 -1.80 m in length. The warps are 12-20 mm in diameter and are made of polyethylene or steel wire, their length depends upon the depth at the fishing ground. The parameters of' single trawls are given in Table 8 .

Table 8. Theparameters of single trawls in some provinces

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