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Trawling is a method of Fishing that involves actively pulling a Fishing net through the water behind one or more boats.
The vertical opening of a trawl net is created using flotation on the upper edge ("floatline") and weight on the lower edge ("footrope") of the net mouth. The configuration of the footrope varies based on the expected bottom shape. The more uneven the bottom, the more robust the footrope configuration must be to prevent net damage.
Size selectivity is controlled by the mesh size of the "cod-end" - the part of the trawl where fish are retained. Fishermen complain that a mesh size which allows undersized fish to escape also allows a proportion of legal-landing sized fish to escape as well. There are a number of "fixes", such as tying a rope around the "cod-end" to prevent the mesh from opening fully, which have been developed to work around technical regulation of size selectivity. One problem is when the mesh gets pulled into narrow diamond shapes (rhombuses) instead of squares.
The capture of undesirable species is a recognized problem with all fishing methods, and unites environmentalists, who do not want to see fish killed needlessly, and fishermen, who do not want to waste their time sorting unsellable fish from their catch. A number of methods to minimize this have been developed for use in trawling. Bycatch reduction grills or square mesh panels of net can be fitted to parts of the trawl, allowing certain species to escape while retaining others. Trawling for shrimps has specifically been cited as having high levels of bycatch in various parts of the world.
The primary focus of dispute over the impact of trawl gear is on the magnitude and duration of these impacts. Opponents of trawl gear argue that the impact of trawl nets is widespread, intense, and long-lasting. Defenders of trawl gear maintain that impact is mostly limited, and of low intensity compared to natural events.
Pelagic trawling is a much "cleaner" method of fishing, in that the catch usually consists of just one species. However, some cases of it have been attacked for depleting resources which are important sources of food for certain sea birds. An instance of this was the RSPB linking a population crash of sea birds in the North Sea to pelagic fishing for sand eels, which are food for many seabird species. This lead to political pressure for the closure of this fishery, and the seabird populations subsequently improved; but a second population crash of seabirds while sand eel stocks are healthy, cast doubt on this link.
There have been cases of naval ships using a trawl net to catch intruding submerged divers: see Frogman.
The word "trawling" has come to be used in a number of non-fishing contexts, usually meaning indiscriminate collection with the intent of picking out the useful bits. For instance, in law enforcement it may refer to collecting large records of Telephone calls hoping to find calls made by suspects. The word "trawling" occurs frequently in general Literature and is used to mean searching through literature for information more often than it means catching fish.