Demersal Trawl

Demersal trawling

Demersal Trawl (image © Seafish)

Demersal trawls are large nets that are pulled through the water with the bottom edge of the net touching the seabed to target bottom-dwelling species living on and near the seabed. At each edge the net is held open by metal ‘trawl doors’ which act as hydroplanes pulling the net mouth open. The foot rope at the front of the net runs along the seabed, disturbing fish and sending them up into the belly of the net. Mesh sizes are set large to reduce unwanted bycatch of juvenile fish. The majority of Cornish demersal trawlers use 110mm, which is larger mesh than the required size of 100mm. When fishing in the Celtic sea (off Cornwall’s north coast) demersal trawlers have to use square mesh panels in the upper surface of the net. These are designed to reduce the catch of undersized and unwanted fish, such as juvenile pollack and haddock. Many skippers voluntarily use these panels in their nets when fishing outside this area too.  Lower head ropes are also used to make the gear more selective, and many Cornish fishermen are trialling gear that has been improved or modified in other ways.
A small number of vessels now employ twin demersal trawls. By towing two trawls they increase their efficiency in comparison to using a single large demersal trawl. 
The gear used in a demersal trawl has an impact on seabed habitats. The trawls can reduce biodiversity and productivity of the habitat, particularly when used for the first time in a new area. However, the gear can only be used on relatively soft seabeds, and years of fishing effort has meant the majority of suitable areas have already been fished. These soft sandy or gravelly seabeds are thought to recover relatively quickly from the effect of demersal trawls. Tidal currents and wave action will also affect the area, and research published in 2013 showed that in some parts of Cornwall this natural impact is likely to exceed any damage caused by fishing effort. 
There are no problems with bycatch of cetaceans and seals with demersal trawling, but the method does have a well-documented physical impact on seabed marine life. It also poses a threat to rare species of sharks, skates and rays such as Angel sharks and Long-nosed skate.
Want to find out more? The following scientific papers give more detail on the impact of demersal trawling:
Sewell, J. & Hiscock, K., 2005. Effects of fishing within UK European Marine Sites: guidance for nature conservation agencies. Report to the Countryside Council for Wales, English Nature and Scottish Natural Heritage from the Marine Biological Association. Plymouth: Marine Biological Association. CCW Contract FC 73-03-214A. 195 pp.
Diesing, M., Stephens, D., and Aldridge, J. 2013. A proposed method for assessing the extent of the seabed significantly affected by demersal fishing in the Greater North Sea. – ICES Journal of Marine Science, DOI:10.1093/icesjms/fst066.