FAO Home>Fisheries & Aquaculture
Food and Agriculture Organization of the United Nationsfor a world without hunger
Bottom otter trawl
Bottom otter trawl
See Tree Map   display tree map

OverviewA bottom otter trawl is a cone-shaped net consisting of a body, normally made from two, four and sometimes more panels, closed by one or two codends and with lateral wings extending forward from the opening. A bottom trawl is kept open horizontally by two otter boards. A boat can be rigged to tow a single or two parallel trawls from the stern or from two outriggers. Bottom trawls usually have an extended top panel (square) to prevent fish from escaping upwards over the top of the net. The mouth of the trawl is framed by an headline with floats to open the trawl vertically and a ground gear, which is designed according to the bottom condition on the fishing ground so as to maximise the capture of targets living close to the bottom and at the same time protect the gear from damage and to facilitate movements across uneven bottom.Accessory EquipmentThe horizontal opening of the net is obtained by two otter boards. The vertical opening is obtained with floats and/or hydrodynamic devices (kites) on the upper edge (floatline) and weights on the groundrope. The groundrope equipped with rubber discs, bobbins, spacers etc. is used at the same time to shield lower leading margin of the trawl from ground damage whilst maintaining ground contact. On very rough bottoms special rock hopper gears are used. The horizontal opening of the trawl is obtained by two otter boards (in cases, for otter twin trawling, two identical trawls are towed side by side, there will be in addition to a pair of otter boards, a sledge between the two nets). There are many models of otter boards: they may be relatively heavy, made of wood, aluminium and steel or a combination of these, rectangular or oval shaped and equipped with a steel sole designed for good contact with the ground. Instruments to monitor gear performance are common in modern bottom otter trawling. Such instruments monitor geometry (door distance, vertical opening, bottom contact, trawl symmetry a.o), water temperature in trawling depth, catch, trawl speed and characteristic with selective grid devices like angle and speed of water flow through the device.Handling EquipmentThe main handling equipment of a Trawlers is a powerful winch with two bobbins (or two or more split winches each one consisting in one bobbin) for storing, shooting and hauling the trawl warps. The trawlers operating otter trawls have gallows, gantries or derricks to handle the heavy otter boards. The net hauling system varies greatly depending on the size of the vessel and the type of the trawl used. A large net drum can be used for shooting, hauling and storing the trawl (including spare ones for additional trawls). Light wing trawls may be hauled in by power blocks. Heavy bobbin trawls may be lifted aboard with gilson winches or quarter ropes. Larger trawlers are arranged with a horseshoe deck layout for handling the trawl.Vessel OverviewOtter bottom trawls can be used by side trawlers (are being phased out gradually), Stern trawlers and Outrigger trawlers. The range of the vessels are from small open boats to large factory trawlers.Fish OperationThe trawl is designed and rigged to have bottom contact during fishing and is, depending on the bottom substrate equipped with different kinds of groundrope with the purpose of shielding lower leading margin of the trawl from ground damage whilst maintaining ground contact and easy move on the bottom. In general the trawler tow a single trawl. However, recent development is towards multitrawl rigging where up to four parallel trawls are towed from the same boat. Twin trawls can be towed from the stern of the vessel or from outriggers. The latter technology is widespread in tropical shrimp fisheries where is called quadtrawling.Target SpeciesBottom and demersal species.Water Area OverviewAll over the world.Gear EnvironmentBottom otter trawls can be operated in a very wide range of depths (from a few meters to around 1 500-2 000 m), mainly at sea, but also, in some cases in inland waters e.g. lakes.ImpactsEnvironmental Bottom otter trawls interact physically with the bottom sediment, which might result in removal or damage of sedentary living organisms (including seaweed or coral) and in the case of uneven bottom surface displacement of stones or other larger objects. On flat sandy/muddy bottom the sediments might be whirled up into the water masses and suspended. The short and long-term impact on the bottom environment is still poorly documented. More research on this impact is urgently needed.Species The major negative impact of bottom otter trawls on the biological environment is related to the capture and frequently discarding of non target sizes and species both of fish and non-fish species. Regulation concerning minimum mesh size in the codend is the most commonly used methods to limit the capture of non-target fish sizes. In recent years such size selectivity has been improved by the introduction of square mesh codends and selection devices like grids. Non-target species are normally reduced by the use of selective devices, like the Turtle Excluder Device (TED) in shrimp tropical trawls and the Nordmoere grid to reduce capture of fish in the northern shrimp fisheries.
Powered by FIGIS