The principal features of a trawl net.
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|OverviewThe trawl nets are cone-shaped net (made from two, four or more panels) which are towed, by one or two boats, on the bottom or in midwater (pelagic). The cone-shaped body ends in a bag or coded. The horizontal opening of the gear while it is towed is maintained by beams, otter boards or by the distance between the two towing vessels (pair trawling). Floats and weights and/or hydrodynamic devices provide for the vertical opening. Two parallel trawls might be rigged between two otter boards (twin trawls). The mesh size in the codend or special designed devices is used to regulate the size and species to be captured.Accessory EquipmentAccording to the type of trawl net used, netsounder, trawl monitor sensors (depth, distance, height, speed, symmetry, catch a.o), otter boards, beam, and other specific equipment can be employed.Handling EquipmentWinches installed on deck control and store the warps. Auxiliary winches, net drums and lifting tackles are equipment that assist the handling of the trawl gear.Vessel OverviewTrawlers range in size from smaller undecked boats, powered by outboard engines up to large Freezer trawlers and Factory trawlers which fish in the most distant waters.Fish OperationTrawling is the operation of towing a net to catch fish and/or shellfish. The trawls are towed either with bottom contact or in midwater. Different devices providing the forces to keep the trawls open horizontally (otter boards, beams and two vessels and vertically (floats and weights). The catch principle is filtering the water. The towing speed varies, according to the type of trawl and trawling, to the target species, etc., from 1 to 7 knots, the most common being 3 to 5.Target SpeciesBottom, demersal and pelagic species.Water Area OverviewAll over the world.Gear EnvironmentTrawls are used in sea fisheries and to a lesser extent also in freshwater where there is sufficient space for towing and a clean environment (bottom without too many obstacles, open water without too much floating debris).ImpactsEnvironmental See the environmental impact caused by Bottom trawls; Beam trawls; Midwater trawls.The major potential detrimental impact of trawling on species can be the capture and removal from the ecosystem of small sized organisms and non-target species, which frequently are discarded at sea. Such impact can be mitigated by using larger meshes in the codends and/or devices in the trawl that reduce capture of small and unwanted organisms.|