Shellfish such as scallops.
How they work
Similar to a beam trawl, a dredge consists of a rugged triangular steel frame and tooth-bearing bar, behind which a mat of linked steel rings is secured. A heavy netting cover joins the sides and back of this mat to form a bag in which the catch is retained. Shellfish are raked out of sand or gravel and swept into the bag. Several dredges are towed together from a tow bar and larger vessels generally tow two bars, one from each side of the vessel. In suction dredges and hydraulic dredges, water is shot into sediments and displaced shellfish are collected in a mesh bag (hydraulic) or sucked to the surface through a pipe (suction).
Bottom trawling and dredging are destructive and wasteful - seabed ecosystems are ploughed up and a wide range of organisms are crushed in the path of the trawl or scooped up in the nets. While the magnitude of these impacts are not the same for all bottom trawl fisheries, and depend on certain factors (e.g. the type of trawl gear used, habitat composition, life history of component species, the natural disturbance regime), even for those bottom trawls operating in the least sensitive benthic environments, these areas are often regularly trawled so have little chance to recover, and there are significant levels of bycatch. Fish that are too small or of the wrong species are thrown overboard, dead or dying. Demersal otter trawls commonly throw away over 30% of their catches (by weight) while beam trawls throw away up to 70% of their catches (by weight).
Considering, firstly, that management bodies have tended to put very few limits on where bottom trawls can operate, and secondly, that there is a serious lack of marine reserves to allow for both recovery and for scientific comparison of unfished and fished areas, Greenpeace does not currently support the use of bottom trawling.