Editing Bottom trawling (section)

===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.