BCO-DMO BlogInstrument: Otter Trawl
Otter trawls have 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, pushes them outwards and prevents the mouth of the net from closing. The speed that the trawl is towed at depends on the swimming speed of the species which is being targeted and the exact gear that is being used, but for most demersal species, a speed of around 4 knots (7 km/h) is appropriate. More: http://en.wikipedia.org/wiki/Bottom_trawling
The trawl net used was a low-rise design constructed by Stephen Lee to target monkfish and flatfish. The net has a one legged bridle and a 137 foot sweep made up of 60 foot wings and a 17 foot bosom. Roller gear included 8 inch rubber disks (cookies) in the bosom, extending to 6 inch cookies on the first 10 feet of the wings, and 5.5 inch cookies on the remainder of the wings. The ground cables were 20 fathoms in length and covered with 4 inch cookies. The body of the net was composed of 6 inch green polyethylene diamond mesh with a fishing circle of 342 meshes. The codend was 6.5 inch knotted green polyethylene material hung on the square with double mesh. The net was towed with Bison #7 doors at an average speed of 2.5 knots.
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