Editing Bottom trawling (section)

==Fishing gear==
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The design requirements of a bottom trawl are relatively simple, a mechanism for keeping the mouth of the net open in horizontal and vertical dimensions, a "body" of net which guides fish inwards, and a "cod-end" of a suitable mesh size, where the fish are collected. The size and design of net used is determined by the species being targeted, the engine power and design of the fishing vessel and locally enforced regulations.

===Beam trawling===
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File:FMIB 36649 Beam Trawl.jpeg|A beam trawl
File:FMIB 34023 Grampus.jpeg|A detail
File:Scheepsmodel smack LT 1203 - Danny Pylyser - Marcel Vermoote - NAVIGO Nationaal Visserijmuseum - 0481.jpg|Ship model of the smack for beam trawling, NAVIGO National Fisheries Museum, [[Belgium]]
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The simplest method of bottom trawling, the mouth of the net is held open by a solid metal beam, attached to two "shoes", which are solid metal plates, welded to the ends of the beam, which slide over and disturb the seabed. This method is mainly used on smaller vessels, fishing for [[flatfish]] or [[prawn]]s, relatively close inshore.

===Otter trawling===
[[File:FMIB 36645 Otter trawl.jpeg|thumb|right|Otter trawl]]
Otter trawling derives its name from the large rectangular '''otter boards''' which are used to keep the mouth of the trawl net open. Otter boards are made of timber or steel and are positioned in such a way that the hydrodynamic forces, acting on them when the net is towed along the seabed, push them outwards and prevent the mouth of the net from closing. They also act like a [[plough]], digging up to {{convert|15|cm|in|0}} into the seabed, creating a [[turbid]] cloud, and scaring fish towards the net mouth.

The net is held open vertically on an otter trawl by [[Float (nautical)|float]]s and/or kites attached to the "headline" (the rope which runs along the upper mouth of the net), and weighted "bobbins" attached to the "foot rope" (the rope which runs along the lower mouth of the net). These bobbins vary in their design depending on the [[surface roughness|roughness]] of the sea bed which is being fished, varying from small rubber discs for very smooth, sandy ground, to large metal balls, up to {{convert|0.5|m|ft|1}} in diameter, for very rough ground. These bobbins can also be designed to lift the net off the seabed when they hit an obstacle. These are known as "rock-hopper" gear.

===Body of the trawl===

[[File:Pelagisches Schleppnetz.jpg|thumb|left|Pelagic (midwater) otter trawl. 1: trawl warp, 2: otter boards, 3: longline chains, 4 hunter, 5: weights 6: headline with floats, 7: pre-net, 8: tunnel and belly, 9: codend]]
The body of the trawl is funnel-like, wide at its "mouth" and narrowing towards the cod end, and usually is fitted with wings of netting on both sides of the mouth. It is long enough to assure adequate flow of water and prevent fish from escaping the net, after having been caught. It is made of diamond-meshed netting, the size of the meshes decreasing from the front of the net towards the codend. Into the body, fish and turtle escape devices can be fitted. These can be simple structures like "square mesh panels", which are easier for smaller fish to pass through, or more complicated devices, such as [[bycatch]] grills.

===Cod end===
The cod end is the trailing end of the net where fish are finally "caught". The size of mesh in the cod end is a determinant of the size of fish which the net catches. Consequently, regulation of mesh size is a common way of managing [[death|mortality]] of [[juvenile (animal)|juvenile]] fishes in trawl nets.