Trawling

Trawling is a method of fishing that involves pulling a fishing net 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 as little as 30 hp (22 kW) engines to large factory trawlers with over 10,000 hp (7.5 MW). 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 instead of trawls are drawn through the water. Trolling is used both for recreational and commercial fishing whereas trawling is used mainly for commercial fishing. Trawling is also commonly used as a scientific sampling, or survey, method.[1]

Fish on Trawler
Trawl net with fish

Bottom vs midwater trawling

Trawl catch of myctophids and glass shrimp from a layer associated with the bottom at greater than 200 meters depth
Trawl catch of myctophids and glass shrimp from the bottom at greater than 200 meters depth

Trawling can be divided into bottom 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 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, halibut and rockfish.

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

Trawling Drawing
Bottom trawling

When two boats are used (pair trawling), 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) 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, shellfish, 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.[1]

Environmental effects

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

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: the 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 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-catch, 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 found that an estimated 177 tons of by-catch (including 77 different species) were discarded each year.[2]

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 allow some legally catchable fish to escape. 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 trawling is responsible for the highest rate of by-catch.[3]

Environmental damage

Setting a trawl in Stephens Passage , Alaska
Setting a trawl

Trawling is controversial because of its environmental impacts. Because bottom 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 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 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 on soft bottoms also stirs up bottom sediments and loading suspended solids 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 combined.[4] These turbidity plumes can be seen on Google Earth in areas where they have high resolution offshore photos (see Bottom trawling). When the turbidity plumes from bottom trawlers are below a thermocline, the surface may not be impacted, but less visible impacts can still occur, such as persistent organic pollutant 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.[5] Published research has shown that benthic trawling destroys the cold-water coral Lophelia pertusa, an important habitat for many deep-sea organisms.[6]

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).[7]

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 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. Troll 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.

See also

Notes

  1. ^ "Not found". Mass.gov. Archived from the original on 2018-04-17.
  2. ^ Liggins, G.W., Kennelly, S.J., 1996. By-catch from prawn trawling in the Clarence River estuary, New South Wales, Australia. Fish. Res. 25, 347-367.
  3. ^ Alverson D L, Freeberg M K, Murawski S A and Pope J G. (1994) A global assessment of fisheries bycatch and discards. FAO Fisheries Technical Paper No 339 Rome, FAO 1994.
  4. ^ Weaver, Dallas E (2007). "'Remote Impacts of Bottom Trawling". Archived from the original on 2009-04-10.
  5. ^ Roberts S, Hirshfield M. (2004) Deep Sea Corals: Out of Sight, But No Longer Out of Mind Archived 2009-02-26 at the Wayback Machine. Oceania. In Frontiers in Ecology and the Environment, April 2004.
  6. ^ Fossa J H, Mortensen P B and Furevik D M. (2002) The deep water coral Lophelia pertusa in Norwegian waters: distribution and fishery impacts Archived 2009-02-26 at the Wayback Machine. Hydrobiologia 471: 1-12, 2002.
  7. ^ Ross A, Isaac S. (2004) The net effect? A review of cetacean bycatch in pelagic trawls and other fisheries in the north-east Atlantic. London, UK: Greenpeace Environmental Trust.
  8. ^ Cambodia volunteers step up battle against illegal fishing

References

  • 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
  • March, E. J. (1953). Sailing Trawlers: The Story of Deep-Sea Fishing with Long Line and Trawl. Percival Marshal and Company. Reprinted by Charles & David, 1970, Newton Abbot, UK. ISBN 0-7153-4711-X
  • FAO (2007) Workshop on standardization of selectivity methods applied to trawling Fisheries Report No. 820. ISBN 978-92-5-005669-2

External links

Aquaculture of catfish

Catfish are easy to farm in warm climates, leading to inexpensive and safe food at local grocers. Catfish raised in inland tanks or channels are considered safe for the environment, since their waste and disease should be contained and not spread to the wild.

Bamboo coral

Bamboo coral, family Isididae, is a family of mostly deep-sea coral of the phylum Cnidaria. It is a commonly recognized inhabitant of the deep sea, due to the clearly articulated skeletons of the species. Deep water coral species such as this are especially affected by the practice of bottom trawling. These organisms may be an important environmental indicator in the study of long term climate change, as some specimens of bamboo coral have been discovered that are 4,000 years old.

Bottom trawling

Bottom trawling is trawling (towing a trawl, which is a fishing net) along the sea floor. It is also referred to as "dragging". The scientific community divides bottom trawling into benthic trawling and demersal trawling. Benthic trawling is towing a net at the very bottom of the ocean and demersal trawling is towing a net just above the benthic zone.

Bottom trawling can be contrasted with midwater trawling (also known as pelagic trawling), where a net is towed higher in the water column. Midwater trawling catches pelagic fish such as anchovies, and mackerel, whereas bottom trawling targets both bottom-living fish (groundfish) and semi-pelagic species such as cod, squid, shrimp, and rockfish.

Trawling is done by a trawler, which can be a small open boat with only 30 hp (22 kW) or a large factory trawler with 10,000 hp (7,500 kW). Bottom trawling can be carried out by one trawler or by two trawlers fishing cooperatively (pair trawling).

Global catch from bottom trawling has been estimated at over 30 million tonnes per year, an amount larger than any other fishing method. Concerns about the environmental impacts of bottom trawling have led to changes in gear design, such as the addition of turtle excluder devices to reduce bycatch, and limitations on locations where bottom trawling is allowed, such as marine protected areas.

Bycatch

Bycatch, in the fishing industry, is a fish or other marine species that is caught unintentionally while catching certain target species and target sizes of fish, crabs etc. Bycatch is either of a different species, the wrong sex, or is undersized or juvenile individuals of the target species. The term "bycatch" is also sometimes used for untargeted catch in other forms of animal harvesting or collecting.

In 1997, the Organisation for Economic Co-operation and Development (OECD) defined bycatch as "total fishing mortality, excluding that accounted directly by the retained catch of target species". Bycatch contributes to fishery decline and is a mechanism of overfishing for unintentional catch.The average annual bycatch rate of pinnipeds and cetaceans in the U.S. from 1990 to 1999 was estimated at 6215 animals with a standard error of 448.The fisherman bycatch issue originated due to the "mortality of dolphins in tuna nets in the 1960s".There are at least four different ways the word "bycatch" is used in fisheries:

Catch which is retained and sold but which is not the target species for the fishery

Species/sizes/sexes of fish which fishermen discard

Non-target fish, whether retained and sold or discarded

Unwanted invertebrate species, such as echinoderms and non-commercial crustaceans, and various vulnerable species groups, including seabirds, sea turtles, marine mammals and elasmobranchs (sharks and their relatives).Additionally, the term "deliberate bycatch" is used to refer to bycatch as a source of illegal wildlife trade (IWT) in several areas throughout world.

Destructive fishing practices

Destructive fishing practices are practices that easily result in irreversible damage to aquatic habitats and ecosystems. Many fishing techniques can be destructive if used inappropriately, but some practices are particularly likely to result in irreversible damage. These practices are mostly, though not always, illegal. Where they are illegal, they are often inadequately enforced.

Fish emulsion

Fish emulsion is a fertilizer emulsion that is produced from the fluid remains of fish processed for fish oil and fish meal industrially.

Fish marketing

Fish marketing is the marketing and sale of fish products.

Fish products

Fish and fish products is consumed as food all over the world. With other seafoods, it provides the world's prime source of high-quality protein;14–16 percent of the animal protein consumed worldwide. Over one billion people rely on fish as their primary source of animal protein.Fish and other aquatic organisms are also processed into various food and non-food products.

Fishing net

A fishing net is a net used for fishing. Nets are devices made from fibers woven in a grid-like structure. Some fishing nets are also called fish traps, for example fyke nets. Fishing nets are usually meshes formed by knotting a relatively thin thread. Early nets were woven from grasses, flaxes and other fibrous plant material. Later cotton was used. 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 are still used.

Fishing techniques

Fishing techniques are methods for catching fish. The term may also be applied to methods for catching other aquatic animals such as molluscs (shellfish, squid, octopus) and edible marine invertebrates.

Fishing techniques include hand-gathering, spearfishing, netting, angling and trapping. Recreational, commercial and artisanal fishers use different techniques, and also, sometimes, the same techniques. Recreational fishers fish for pleasure or sport, while commercial fishers fish for profit. Artisanal fishers use traditional, low-tech methods, for survival in third-world countries, and as a cultural heritage in other countries. Mostly, recreational fishers use angling methods and commercial fishers use netting methods.

There is an intricate link between various fishing techniques and knowledge about the fish and their behaviour including migration, foraging and habitat. The effective use of fishing techniques often depends on this additional knowledge. Which techniques are appropriate is dictated mainly by the target species and by its habitat.Fishing techniques can be contrasted with fishing tackle. Fishing tackle refers to the physical equipment that is used when fishing, whereas fishing techniques refers to the manner in which the tackle is used when fishing.

Fishing trawler

A fishing trawler is a commercial fishing vessel designed to operate fishing trawls. Trawling is a method of fishing that involves actively dragging or pulling a trawl through the water behind one or more trawlers. Trawls are fishing nets that are pulled along the bottom of the sea or in midwater at a specified depth. A trawler may also operate two or more trawl nets simultaneously (double-rig and multi-rig).

There are many variants of trawling gear. They vary according to local traditions, bottom conditions, and how large and powerful the trawling boats are. A trawling boat can be a small open boat with only 30 horsepower or a large factory ship with 10,000 horsepower. Trawl variants include beam trawls, large-opening midwater trawls, and large bottom trawls, such as "rock hoppers" that are rigged with heavy rubber wheels that let the net crawl over rocky bottom.

Fishmonger

A fishmonger (fishwife for female practitioners - "wife" in this case used in its archaic meaning of "woman") is someone who sells raw fish and seafood. Fishmongers can be wholesalers or retailers, and are trained at selecting and purchasing, handling, gutting, boning, filleting, displaying, merchandising and selling their product. In some countries modern supermarkets are replacing fishmongers who operate in shops or fish markets.

Midwater trawling

Midwater trawling is trawling, or net fishing, at a depth that is higher in the water column than the bottom of the ocean. It is contrasted with bottom trawling. Midwater trawling is also known as pelagic trawling and bottom trawling as benthic trawling.

In midwater trawling, a cone-shaped net can be towed behind a single boat and spread by trawl doors, or it can be towed behind two boats (pair trawling) which act as the spreading device. 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.

Whereas bottom trawling can leave serious incidental damage to the sea bottom in its trail, midwater trawling by contrast is relatively benign.

Pair trawling

Pair trawling is a fishing activity carried out by two boats, with one towing each warp (the towing cables). As the mouth of the net is kept open by the lateral pull of the individual vessels, otter boards are not required. By utilising the towing power of two boats, and as no otter boards are needed, a larger net may be worked than would otherwise be possible, or alternatively, the two boats can share increased fuel efficiency[1].

As doors are not necessary, the gear arrangements are simplified, with the warps attaching directly to the wings of the net. Setting and hauling of the nets are carried out by one boat, while the other is only used for towing; usually each will take turns at these operations[2].

Pair trawling is effective on all demersal species. In shallow waters, where the noise from a single vessel may scatter fish, two vessels operating a distance apart tend to herd fish into the path of the net. Catch per vessel often considerably exceeds that attainable through standard bottom trawling. Pair trawlers targeting cod off the coast of New England have reported average per vessel catches of three to six times that from single trawls.Because of the increased efficiency of pair trawling, vessels are able to tow their gears in mid-water at a faster speed to target species which are normally able to escape trawl nets, such as the European seabass. This has caused some controversy, due to the high level of marine mammal bycatch associated, leading to the British government introducing a ban on pair trawling for bass in UK territorial waters.

Raceway pond

A raceway pond is a shallow artificial pond used in the cultivation of algae.

The pond is divided into a rectangular grid, with each rectangle containing one channel in the shape of an oval, like an automotive raceway circuit. From above, many ponds look like a maze. Each rectangle contains a paddle wheel to make the water flow continuously around the circuit.

The Department of Energy's Aquatic Species Program experimented with raceway ponds for the cultivation of algae. Many commercial producers of spirulina still use raceway ponds as their primary method of cultivation. Race way ponds were used for removal of lead from waste water using live Spirulina (Arthospira) sp.

ST Koraaga (1914)

Koraaga was a Castle class steel-hulled trawler built in 1914 by Smiths Dock Company, South Bank, Middlesbrough. She was requisitioned as an auxiliary minesweeper operated by the Royal Australian Navy (RAN) in October 1917 for minesweeping duties during World War I, but she was never commissioned. Koraaga returned to be operated commercially as a fishing trawler until she wrecked when she struck a reef off Bass Point whilst carrying returning to Sydney. She was refloated on the tide after having becoming stranded and drifted till she was finally lost 5 miles east of Black Head, Gerringong New South Wales on 10 September 1931.

Scottish east coast fishery

The Scottish east coast fishery has been in existence for more than a thousand years, spanning the Viking age right up to the present day.

Trolling (fishing)

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 a jetty. Trolling is used to catch pelagic fish such as salmon, mackerel and kingfish.

Trolling can be phonetically confused with trawling, a different method of fishing where a net (trawl) is drawn through the water instead of lines. Trolling is used both for recreational and commercial fishing whereas trawling is used mainly for commercial fishing.

Trolling from a moving boat involves moving quite slowly through the water. This can be accomplished with the use of a special trolling motor. Multiple lines are often used, and outriggers can be used to spread the lines more widely and reduce their chances of tangling. Downriggers can also be used to keep the lures or baits trailing at a desired depth.

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