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Midwater Otter Trawls
Midwater Otter Trawls
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Characteristics
OverviewA midwater otter trawl is a cone-shaped net which is towed in mid-water. It consists of a cone-shaped body, normally made of four panels, ending in a codend and the net has lateral wings extending forward from the opening. The horizontal opening is maintained by otter boards. Floats and/or sailkites on the headline and weights on the groundline provide for the vertical opening. Large modern midwater trawls are rigged in such a way that the weights in front of and along the groundline provide for the vertical opening of the trawl. The cable transmitting acoustic signal form the net sonde might also provide a lifting force that maximize the vertical trawl opening. To reduce the resistance of the gear and achieve a large opening, the front part of the trawls are usually made from very large rhombic or hexagonal meshes. The use of nearly parallel ropes instead of meshes in the front part is also a common design. The largest mesh sizes used so far are 128 m and, on modern large midwater trawls, approximately three quarters of the length of the trawl is made with mesh sizes above 400 mm.Accessory EquipmentHydrodynamic designed otter boards are used for midwater trawling. The fishing depth is usually controlled by means of a netsounder (netsonde) or depth recorders. As for other types of Trawl nets, catch sensors can be installed in the codend to give information about the amount of caught fish. Tension sensors can also be used in combination with balanced winches to guarantee a stable, optimal opening. Horizontal sonars and fish finding equipment is a prerequisite for successful operations.Handling EquipmentMost of the vessels engaged in midwater trawling are equipped with net drums. Trawl winches installed on deck control the warps and store them when not in use. The larger midwater trawlers use pumps for emptying the codend.Vessel OverviewMidwater otter trawls are used especially by Stern trawlers.Fish OperationA midwater otter trawl is towed by a single boat. Midwater trawling is a well-aimed fishing activity. The fish shoals are located by sonar (but some midwater trawling can also be carried out on slightly scattered fish staying in a given layer of water) and the vessel is towing in the direction of the shoal. The fishing depth level, which is controlled by the net sounder, is regulated by the length of the warps and/or the towing speed.Target SpeciesPelagic species, mainly fish and cephalopods, northern shrimp (also small crustaceans, Antarctic krill )Water Area OverviewAll over the world.Gear EnvironmentMidwater, or pelagic, fishing is carried out mainly at sea, on the continental shelf, sometimes in deeper waters, also in some lakes.ImpactsEnvironmental No impact on bottom habitat and bottom structure.Species In most cases it is a single species fishery, bycatch rates of other species are low. Incidental catch of dolphins and marine mammals may occur in some areas.
 
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