Science Fair Project Encyclopedia
Structure of a trawl net
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 the more common situation. In this case, the horizonal spread of the net is provided by trawl doors (also known as "otter boards"). Trawl doors are available in a variety of sizes and shapes, and may be specialized to be in contact with the sea bottom, or to remain elevated in the water column. In all cases, doors essentially act as wings, using a hydrodynamic shape to provide horizontal spread. As with all wings, a certain speed must be maintained by the towing vessel for the doors to remain standing and functional. This speed varies, but is generally in the range of 2.5-4.0 knots (nautical miles per hour).
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 morphology. The more uneven the bottom, the more robust the footrope configuration must be to prevent net damage.
Environmental impacts of trawling
Although trawling today is heavily regulated in some nations, it remains the target of many protests by environmentalists. Environmental concerns related to trawling are refer to two areas — a perceived lack of selectivity, and the physical damage which the trawl does to the seabed.
Reports of the lack of selectivity of trawling have been present since its inception (~1600's) and wider implementaion (~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 is not able to be utilised is considered as by-catch.
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.
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 cash. 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 shrimp has specifically been cited as having high levels of bycatch in various parts of the world.
Beacause bottom trawling involves towing heavy fishing gear over the seabed at a speed of several knots, it is destructive to the ocean bottom. The primary source of impact is the doors, which can weigh several tonnes and create furrows when 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 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.
Pelagic trawling is a much "cleaner" method of fishing, in that the catch usually consists of just one species, however it has been attacked in certain instances 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 - food for many bird species. This lead to political pressure for the closure of this fishery, and the bird populations subsequently improved, however, a second population crash in the face of healthy sand eel stocks cast doubt on this link.
Other uses of "trawl"
The noun "trawl" has multiple and 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 "long-lining", it may refer to a series of baited hooks attached to a mainline.
The word "trawling" has come to be used in a number of non-fishing contexts, generally referred to indiscriminate collection with the intent of picking out the interesting bits. For instance, in law enforcement it may refer to the practice of collecting large records of phone calls in the hopes of finding calls made by suspects. The word "trawling" occurs frequently in general literature and is used to mean searching through literature as often as it is used to mean catching fish.
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