The boats that are used for trawling are called trawlers. Trawlers vary in size; from small open boats with only 30 hp to large factory trawlers with over 10,000 hp. Trawling can be carried out by one trawler or by two trawlers fishing cooperatively (pair trawling).
Trawling can be contrasted with trolling, where baited fishing lines are drawn through the water instead of trawls. Trolling is used both for recreational and commercial fishing whereas trawling is used mainly for commercial fishing.
The scientific community divides bottom trawling into benthic trawling and demersal trawling. Benthic trawling is dragging the trawl along the very bottom of the ocean and demersal trawling is towing the trawl just above the benthic zone.
Midwater trawling is also known as pelagic trawling. Midwater trawling catches pelagic fish such as anchovies, shrimp, tuna and mackerel, whereas bottom trawling targets both bottom living fish (groundfish) and semi-pelagic fish such as cod, squid, halibut and rockfish.
Bottom trawling can leave serious incidental damage to the sea bottom and deep water coral reefs, in its trail; by contrast midwater trawling is relatively benign.
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. This is used to catch shrimp, shell fish, cod, scallops and many others. Trawls are tunnel shaped nets that have a closed off tail where the fish are collected and is open on the top end as the mouth.
Although trawling today is heavily regulated in some nations, it remains the target of many protests by environmentalists. Environmental concerns related to trawling refer to two areas: a perceived lack of selectivity and the physical damage which the trawl does to the seabed.
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 marketed fish from their catch. A number of methods to minimize this have been developed for use in trawling. Bycatch reduction grids 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 sources of impact are the doors, which can weigh several tonnes and create furrows if dragged along the bottom, and the footrope configuration, which usually remains in contact with the bottom across the entire lower edge of the net. Depending on the configuration, the footrope may turn over large rocks or boulders, disturb or damage sessile organisms or rework and re-suspend bottom sediments. Published research has shown that benthic trawling destroys the cold-water coral Lophelia pertusa, an important habitat for many deep-sea organisms.
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. However, it should be noted that most areas with significant natural sea bottom disturbance events are in relatively shallow water. In mid to deep waters, bottoms trawlers are the only significant area wide events and the destruction has been viewed as clear-cutting the bottom. These impacts result in decreases in species diversity and ecological changes towards more opportunistic organisms.
Bottom trawling on soft bottoms also stirs up bottom sediments and puts a huge load of suspended solids into the water column. The magnitude of the problem can be visualized by noting that one bottom trawler put more than 10 times the amount of suspended solids pollution per hour into the water column than all the suspended solids pollution from all the sewerage, industrial, river and dredge disposal operations in Southern California combined. These turbidity plumes can be seen on Google Earth in areas where they have some high resolution offshore photos (see Bottom trawling for an example). When the turbidity plumes from bottom trawlers are below a thermocline, you may not have a surface impact but can still create less visible impacts such as persistent organic pollutant transfer into the pelagic food chain.
Midwater (pelagic) trawling is a much "cleaner" method of fishing, in that the catch usually consists of just one species and does not physically damage the sea bottom.
Bycatch of non-target species, including dolphins, sea turtles, sharks, and others is a particular problem with most trawling.
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.