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Trawling

Trawling is a method of fishing that involves actively pulling a fishing net through the water behind one or more boats, called trawlers.

When two boats are used (pair trawling), the horizontal spread of the net is provided by the boats, with one warp attached to each boat. However, single-boat trawling is more common. Here, the horizontal spread of the net is provided by trawl doors (also known as "otter boards"). Trawl doors are available in various sizes and shapes and may be specialized to keep in contact with the sea bottom (bottom trawling) or to remain elevated in the water. In all cases, doors essentially act as wings, using a hydrodynamic shape to provide horizontal spread. As with all wings, the towing vessel must go at a certain speed for the doors to remain standing and functional. This speed varies, but is generally in the range of 2.5-4.0 knots.

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 closed off tail where the fish is collected and is open on the top end as the mouth.

Trawl nets can also be modified, such as changing mesh size, to help with marine research of ocean bottoms.[1]

Double-rigged shrimp trawler hauling in the nets

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.

Nets for trawling in surface waters and for trawling in deep water and over the bottom. Note the "tangles" with ensnared marine life

Reports of the lack of selectivity of trawling have been present since it started (about the 1600s) and it became used more widely (about 1900). Trawl nets may be non-selective, sweeping up both marketable and undesirable fish and fish of both legal and illegal size. Any part of the catch which cannot be used is considered as by-catch some of which is killed accidentally by the trawling process.

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.

Trawl net with fish

Trawling is an environmentally disputed fishing technique. Trawling can cause significant damage to the seafloor, coral shattering, boulders being dragged along the seafloor causing damage to habitats and removal of seaweed can all be unintentional side-effects of trawling. Because bottom trawling involves towing heavy fishing gear over the seabed it can cause large scale destruction on the ocean bottom. 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 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 seabottom 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 opportunstic organisms.

Bottom trawling on soft bottoms also stirs up bottom sediments and puts a hugh 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 hr. into the water column than all the suspended solids pollution from all the sewerage, industrial, river and dredge disposal operations in Southern California, USA combined. These turbidity plumes can be seen on Google Earth™ in areas where they have some high resolution offshore photos. When the turbidity plumes from bottom trawlers are below a thermocline, you may not a surface impact but can still create less visible impacts. 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 led to political pressure for the closure of this fishery; the seabird populations subsequently improved. However, a second population crash of seabirds which occurred with no effect on the stocks of sand eel, cast doubt on this link.

There have been cases of naval ships using a trawl net to catch intruding submerged divers (see here).

The noun "trawl" has many possibly confusing meanings in commercial fisheries. For example, two or more lobster pots that are fished together may be referred to as a trawl. In some older usages "trawling" meant "long-line fishing"; that usage occurs in Rudyard Kipling's book Captains Courageous. (This use is perhaps confused with trolling, where a baited line is trailed behind a boat)

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.

• Clover, Charles. 2004. The End of the Line: How overfishing is changing the world and what we eat. Ebury Press, London. ISBN 0-09-189780-7

• Information on the destructive side effects of trawling