Trawling
Encyclopedia
Fishing
Fishing is the activity of trying to catch wild fish. Fish are normally caught in the wild. Techniques for catching fish include hand gathering, spearing, netting, angling and trapping....
that involves pulling a fishing net
Fishing net
A fishing net or fishnet is a net that is used for fishing. Fishing nets are meshes usually formed by knotting a relatively thin thread. Modern nets are usually made of artificial polyamides like nylon, although nets of organic polyamides such as wool or silk thread were common until recently and...
through the water behind one or more boats. The net that is used for trawling is called a trawl.
The boats that are used for trawling are called trawlers or draggers. Trawlers vary in size; from small open boats with only 30 hp
Horsepower
Horsepower is the name of several units of measurement of power. The most common definitions equal between 735.5 and 750 watts.Horsepower was originally defined to compare the output of steam engines with the power of draft horses in continuous operation. The unit was widely adopted to measure the...
engines 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
Pair trawling
Pair trawling is a fishing activity carried out by two boats, with one towing each warp . As the mouth of the net is kept open by the lateral pull of the individual vessels, otter boards are not required...
).
Trawling can be contrasted with trolling
Troll (angling)
Trolling is a method of fishing where one or more fishing lines, baited with lures or bait fish, are drawn through the water. This may be behind a moving boat, or by slowly winding the line in when fishing from a static position, or even sweeping the line from side-to-side, e.g. when fishing from...
, where baited fishing line
Fishing line
A fishing line is a cord used or made for angling. Important parameters of a fishing line are its length, material, and weight...
s instead of trawls are drawn through the water. Trolling is used both for recreational
Recreational fishing
Recreational fishing, also called sport fishing, is fishing for pleasure or competition. It can be contrasted with commercial fishing, which is fishing for profit, or subsistence fishing, which is fishing for survival....
and commercial fishing
Commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...
whereas trawling is used mainly for commercial fishing. Trawling is also commonly used as a scientific sampling, or survey, method http://www.mass.gov/dfwele/dmf/publications/tr_38.pdf.
Bottom versus midwater trawling
Trawling can be divided into bottom trawlingBottom trawling
Bottom trawling is trawling along the sea floor. It is also often referred to as "dragging".The scientific community divides bottom trawling into benthic trawling and demersal trawling...
and midwater trawling, depending on how high the trawl (net) is in the water column. Bottom trawling is towing the trawl along (benthic trawling) or close to (demersal trawling) the sea floor. Midwater trawling is towing the trawl through free water above the bottom of the ocean or benthic zone.
Midwater trawling is also known as pelagic
Pelagic zone
Any water in a sea or lake that is not close to the bottom or near to the shore can be said to be in the pelagic zone. The word pelagic comes from the Greek πέλαγος or pélagos, which means "open sea". The pelagic zone can be thought of in terms of an imaginary cylinder or water column that goes...
trawling. Midwater trawling catches pelagic fish
Pelagic fish
Pelagic fish live near the surface or in the water column of coastal, ocean and lake waters, but not on the bottom of the sea or the lake. They can be contrasted with demersal fish, which do live on or near the bottom, and reef fish which are associated with coral reefs.The marine pelagic...
such as anchovies, shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
, tuna
Tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...
and mackerel
Mackerel
Mackerel is a common name applied to a number of different species of fish, mostly, but not exclusively, from the family Scombridae. They may be found in all tropical and temperate seas. Most live offshore in the oceanic environment but a few, like the Spanish mackerel , enter bays and can be...
, whereas bottom trawling targets both bottom living fish (groundfish
Groundfish
Groundfish are fish that live on, in, or near the bottom of the body of water they inhabit. Some typical saltwater groundfish species are sole, flounder, and halibut....
) and semi-pelagic fish such as cod
Cod
Cod is the common name for genus Gadus, belonging to the family Gadidae, and is also used in the common name for various other fishes. Cod is a popular food with a mild flavor, low fat content and a dense, flaky white flesh. Cod livers are processed to make cod liver oil, an important source of...
, squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
, halibut
Halibut
Halibut is a flatfish, genus Hippoglossus, from the family of the right-eye flounders . Other flatfish are also called halibut. The name is derived from haly and butt , for its popularity on Catholic holy days...
and rockfish
Rockfish
Rockfish may refer to one of the following fishes:* One of several species in the Sebastes genus of the Sebastidae family, including the Shortraker rockfish, Rougheye rockfish, Blue rockfish, Yellow tail rockfish and many others....
.
The gear itself can vary a great deal. Pelagic trawls are typically much larger than bottom trawls, with very large mesh openings in the net, little or no ground gear, and little or no chaffing gear. Additionally, pelagic trawl doors have different shapes than bottom trawl doors, although doors that can be used with both nets do exist.
Net structure
Pair trawling
Pair trawling is a fishing activity carried out by two boats, with one towing each warp . As the mouth of the net is kept open by the lateral pull of the individual vessels, otter boards are not required...
), the horizontal spread of the net is provided by the boats, with one or in the case of pelagic trawling two warps 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
Bottom trawling
Bottom trawling is trawling along the sea floor. It is also often referred to as "dragging".The scientific community divides bottom trawling into benthic trawling and demersal 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 funnel shaped nets that have a closed off tail where the fish are 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.http://www.whoi.edu/oceanus/viewArticle.do?id=14448
Environmental effects
Environmentalist
An environmentalist broadly supports the goals of the environmental movement, "a political and ethical movement that seeks to improve and protect the quality of the natural environment through changes to environmentally harmful human activities"...
s. 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.
Selectivity
Since the practice of trawling started (around the 15th century), there have been concerns over trawling's lack of selectivity. Trawls 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 by-catchBy-catch
The term “bycatch” is usually used for fish caught unintentionally in a fishery while intending to catch other fish. It may however also indicate untargeted catch in other forms of animal harvesting or collecting...
, some of which is killed accidentally by the trawling process. By-catch commonly includes valued species such as dolphins, sea turtles, and sharks, and may also include sublegal or immature individuals of the targeted species.
Many studies have documented large volumes of by-catch that are discarded. For example, researchers conducting a three-year study in the Clarence River
Clarence River (New South Wales)
The Clarence River is situated in northeastern New South Wales, Australia. The river originates on the watershed that marks the Queensland border. After flowing south and northeast for 394 km it then empties into the Pacific Ocean at Iluka/Yamba. On its journey it passes through the towns of...
found that an estimated 177 tons of by-catch (including 77 different species) were discarded each year.
Size selectivity is controlled by the mesh size of the "cod-end"—the part of the trawl where fish are retained. Fishermen complain that mesh sizes which allow undersized fish to escape also allows some legally–catchable 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 marketable 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.
Studies have suggested that shrimp
Shrimp
Shrimp are swimming, decapod crustaceans classified in the infraorder Caridea, found widely around the world in both fresh and salt water. Adult shrimp are filter feeding benthic animals living close to the bottom. They can live in schools and can swim rapidly backwards. Shrimp are an important...
trawling is responsible for the highest rate of by-catch.
Environmental damage
Trawling is controversial because of its environmental impacts. Because bottom trawlingBottom trawling
Bottom trawling is trawling along the sea floor. It is also often referred to as "dragging".The scientific community divides bottom trawling into benthic trawling and demersal trawling...
involves towing heavy fishing gear over the seabed, it can cause large scale destruction on the ocean bottom, including coral shattering, damage to habitats
Habitat (ecology)
A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism...
and removal of seaweed.
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, possibly dragging them along with the net, disturb or damage sessile
Sessility (limnology)
In limnology, sessility is that quality of an organism which rests unsupported directly on a base, either attached or unattached to a substrate. It is a characteristic of vegetation which is anchored to the benthic environment. There are two families of sessile rotifers: Flosculariidae and...
organisms or rework and re-suspend bottom sediments. These impacts result in decreases in species diversity and ecological changes towards more opportunistic organisms. The destruction has been likened to clear-cutting in forests.
The primary dispute over trawling concerns the magnitude and duration of these impacts. Opponents argue that they are widespread, intense and long-lasting. Defenders maintain that impact is mostly limited and of low intensity compared to natural events. However, 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.
Bottom trawling
Bottom trawling
Bottom trawling is trawling along the sea floor. It is also often referred to as "dragging".The scientific community divides bottom trawling into benthic trawling and demersal trawling...
on soft bottoms also stirs up bottom sediments and loading suspended solids
Suspended solids
Suspended solids refers to small solid particles which remain in suspension in water as a colloid or due to the motion of the water. It is used as one indicator of water quality....
into the water column. One bottom trawler can 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
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
combined. These turbidity plumes can be seen on Google Earth
Google Earth
Google Earth is a virtual globe, map and geographical information program that was originally called EarthViewer 3D, and was created by Keyhole, Inc, a Central Intelligence Agency funded company acquired by Google in 2004 . It maps the Earth by the superimposition of images obtained from satellite...
in areas where they have high resolution offshore photos (see Bottom trawling
Bottom trawling
Bottom trawling is trawling along the sea floor. It is also often referred to as "dragging".The scientific community divides bottom trawling into benthic trawling and demersal trawling...
). When the turbidity plumes from bottom trawlers are below a thermocline
Thermocline
A thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
, the surface may not be impacted, but less visible impacts can still occur, such as persistent organic pollutant
Persistent organic pollutant
thumb|right|275px|State parties to the Stockholm Convention on Persistent Organic PollutantsPersistent organic pollutants are organic compounds that are resistant to environmental degradation through chemical, biological, and photolytic processes...
transfer into the pelagic food chain.
As a result of these processes, a vast array of species are threatened around the world. In particular, trawling can directly kill coral reefs by breaking them up and burying them in sediments. In addition, trawling can kill corals indirectly by wounding coral tissue, leaving the reefs vulnerable to infection. The net effect of fishing practices on global coral reef populations is suggested by many scientists to be alarmingly high. Published research has shown that benthic trawling destroys the cold-water coral Lophelia pertusa
Lophelia pertusa
Lophelia pertusa is a species of cold-water coral which grows in the deep waters throughout the North Atlantic ocean, as well as parts of the Caribbean Sea and Alboran Sea. L...
, an important habitat for many deep-sea organisms.
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. However, environmental groups have raised concerns that this fishing practice may be responsible for significant volumes of by-catch, particularly cetaceans (dolphins, porpoises, and whales).
Regulation
In light of the environmental concerns surrounding trawling, many governments have debated policies that would regulate the practice.Other uses of the word "trawl"
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 fishingLong-line fishing
Longline fishing is a commercial fishing technique. It uses a long line, called the main line, with baited hooks attached at intervals by means of branch lines called "snoods". A snood is a short length of line, attached to the main line using a clip or swivel, with the hook at the other end....
"; that usage occurs in Rudyard Kipling
Rudyard Kipling
Joseph Rudyard Kipling was an English poet, short-story writer, and novelist chiefly remembered for his celebration of British imperialism, tales and poems of British soldiers in India, and his tales for children. Kipling received the 1907 Nobel Prize for Literature...
's book Captains Courageous
Captains Courageous
Captains Courageous is an 1897 novel, by Rudyard Kipling, that follows the adventures of fifteen-year-old Harvey Cheyne Jr., the arrogant and spoiled son of a railroad tycoon...
. (This use is perhaps confused with trolling, where a baited line is trailed behind a boat. Troll
Troll (disambiguation)
A troll is a mythological creature, related to elves, dwarves, or monsters.Troll may also refer to:-Places:* Troll , a Norwegian Antarctic research station on Queen Maud Land** Troll Airfield, an airfield nearby...
also has several meanings.)
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 volumes of telephone call records hoping to find calls made by suspects. It also occurs frequently in reference to research methods, where it means searching through written sources for relevant information.
External links
- Anthoni, J Floor (2003) FAQs about marine reserves and marine conservation
- Known Trawling Grounds of the World (2000) World Resources Institute.
- Information on the destructive side effects of trawling
- Natural Resources Defense Council: Protecting Ocean Habitat from Bottom Trawling