Otter Trawl

              The most popular form of trawl fishing in Thailand is by otter trawl, in which otter boards are used for horizontal spreading of the net mouth. Most otter trawl nets consist of two panels; this is called a “two-seam net”.  The mouth is oval-shaped when viewed from front.  Two wings stretch out to increase the swept area and to guide fish in the net’s path down to the cod-end.  There are two typed of otter trawl: one for shrimps and the other for fish.

              The otter trawl for shrimp is usually operated from small fishing boat, 8-16 m in length and with the low to medium power of the main engine (30-120 h.p.).  The netting is 30-60 mm mesh-size, polyethylene 250 d/6 or 380 d/6-12 for the wings, upper panel and the belly, and 20-25 mm mesh-size polyethylene 380 d/9-15 for the cod-end.  In most cases, the triangular piece of netting at the top wings of both panels is omitted.  The size of net depends on the power of the fishing boat; the head-rope varies from 11-23 m and the ground-rope from 13-24 meters.  The difference between them is 1-2 meters.  Both are made of polyethylene or polypropylene.  The ground rope is weighted with a chain, or with lead sinkers.  Otter-boards are rectangular and flat, made of wood and iron, 50-100 cm wide and 100-200 cm long, with a bridle chain and a back strap.  The sweep lines or hand-ropes are10-36 m long, 14-26 mm in diameter, made of polyethylene, polypropylene or a combination rope.

              The warps are 14-28 mm in diameter, also made of polyethylene or polypropylene.  A capstan winch is used for hauling warps and lifting the catch in the cod-end onto the foredeck of the fishing boat.  The net is pulled by hand at stern.  Four to eight fishermen take part in a fishing operation.  The shrimp otter trawls are mostly operated from Nakhon Si Thammarat to Songkhla province, and the catches consist of shrimps and trash-fish.

              The fish otter trawls are the largest single fishery in Thailand.  Most vessels used in this case are comparatively big, from 15 to over 30 m in length, with the main engines ranging from 100 to 500 h.p.  The fishing expeditions take one or two weeks, sometimes even longer.  The two-seam type of net is used, 120-180 mm mesh-size, polyethylene 700 d/12-21 netting for the wings, square, upper panel and belly, and 20-30 mm mesh-size polyethylene 380 d/9-15 netting for the cod-end.  This net differs from the shrimp otter trawl net in that it has a triangular piece of netting at the top wings of both panels.  The head rope is 28-40 m, and the ground rope 30-46 m long.  The difference in their lengths is 2-6 metres.  Both ropes are made of wire and combination rope.  Wooden and rubber rollers, sometimes covered with spherical plastic capsul are attached on the ground rope for weighting and nothing.  Otter-boards are rectangular and flat, 1-2 m wide, 1.2 x 2.4 m long, made of wood and iron.  They have a fixed bracket and a bridle chain or fixed iron holders, and sometimes 1-5 plastic floats are attached at the front top part of the boards, so as to prevent the sinking of the boards into the muddy sea bed.  Gallows, which are necessary for this type of trawl, are fixed at the stern of the boat.  The necessary for this type of trawl,  are fixed at the stern of the boat.  The sweep lines or hand-ropes are 35-80 m long, 22-32 mm thick combination ropes.  The warps are 14-18 mm thick wires, coiled on the warp drum winches on both sides of the boat, or in the middle of a stern trawler.  Warps are hauled by a warp drum winch, the net is pulled by a capstan winch, and passes through a pulley on the crane boom at the fore-deck (or stern-deck of a stern trawler).  The cod-end is hauled in the same way.  Ten to twenty men are needed for a fishing operation.  The main catch are bottom fishes and trash-fish.  The major fishing ports are Samut Prakan, Samut Sakhon, Songkhla and Phuket.

              Setting: When the gear is to be prepared, the boat streams along the desired bourse, into wind if desirable.  The codend is thrown out and the working ropes retied in their correct positions on head line and wings and the net streamed out astern, the sweep lines run out.  The otter boards are connected to the warps and unhooked from the gallows, all is ready for setting.

              This may be accomplished by releasing the winch brakes simultaneously so allowing the otter boards to drop into the water and spread while “on the run” or alternatively the warps may first be eased out until the otter boards are just below the surface and seem to be spreading satisfactorily before the warps are released.

              All is now ready to set the otter boards and run out the warps to their required extent.