'Bottom trawling' (known in the scientific community as ''
Benthic trawling'') is a
fishing method which involves towing trawl nets along the sea floor, as opposed to
pelagic trawling, where a net is towed higher in the water column. Bottom trawling can be carried out from one vessel or two vessels fishing cooperatively. It is practiced from a very wide range of fishing vessels, starting with small motor boats powered by engines of several tens of
horsepower and up to large ocean-going trawlers, up to 100 m long, and powered by engines of several thousand horsepower.
History of Bottom trawling
An early reference to fishery conservation measures comes from a complaint about a form of trawling dating from the 14th century, during the reign of
Edward III. A petition was presented to
Parliament in
1376 calling for the prohibition of a ''"subtlety contrived instrument called the wondyrchoum"''. This was an early beam trawl with a wooden beam, and consisted of a net 6m (18 ft.) long and 3m (l0 ft.) wide,
''"of so small a mesh, no manner of fish, however small, entering within it can pass out and is compelled to remain therein and be taken...by means of which instrument the fishermen aforesaid take so great abundance of small fish aforesaid, that they know not what to do with them, but feed and fatten the pigs with them, to the great damage of the whole commons of the kingdom, and the destruction of the fisheries in like places, for which they pray remedy."''.
The response from the Crown was to ''"Let Commission be made by qualified persons to inquire and certify on the truth of this allegation, and thereon let right be done in the Court of Chancery"''. Thus, already back in the Middle Ages, basic arguments about three of the most sensitive current issues surrounding trawling - the effect of trawling on the wider environment, the use of small mesh size, and of industrial fishing for animal feed - were already being raised.
Although trawl nets were used by sailing vessels up to the 19th century, it was only with the development of
steam power and the
diesel engine that bottom trawling became a widely used method of fishing.
Fishing Gear
The design requirements of a bottom trawl are relatively simple, a mechanism for keeping the mouth of the net open in horizontal and vertical dimensions, a "body" of net which guides fish inwards, and a "cod-end" of a suitable mesh size, where the fish are collected. The size and design of net used is determined by the species being targeted, the engine power and design of the fishing vessel and locally enforced regulations.
Beam Trawling
Design of an early beam trawl
The simplest method of bottom trawling, the mouth of the net is held open by a solid metal beam, attached to two "shoes", which are solid metal plates, welded to the ends of the beam, which slide over and disturb the seabed. This method is mainly used on smaller vessels, fishing for
flatfish or
prawns, relatively close inshore.
Otter trawling
Otter trawling derives its name from the "trawl doors" or "otters" which are used to keep the mouth of the net open. As these are towed along the seabed, hydrodynamic pressure pushes them outwards, preventing the mouth of the net closing. They also act like a
plough, digging up to 15cm into the seabed, creating a
turbid cloud, and scaring fish towards the trawlnet mouth.
The net is held open vertically on an otter trawl by
floats and/or kites attached to the "headline" (the rope which runs along the upper mouth of the net), and weighted "bobbins" attached to the "foot rope" (the rope which runs along the lower mouth of the net). These bobbins vary in their design depending on the
roughness of the sea bed which is being fished, varying from small rubber discs for very smooth, sandy ground, to large metal balls, up to 0.5 m in diameter for very rough ground. These bobbins can also be designed to lift the net off the seabed when they hit an obstacle. These are known as "rock-hopper" gears.
Body of the trawlnet
The body of the trawlnet is funnel-like, wide at its "mouth" and narrowing towards the codend, and usually is fitted with wings of netting at the both sides of the mouth. It is long enough to assure adequate flow of water and prevent fish from escaping the net, after having been caught. It is made of diamond-meshed netting, the size of the meshes decreasing from the front of the net towards the codend. Into the body, fish and turtle escape devices can be fitted. These can be simple structures like "square mesh panels", which are easier for smaller fish to pass through, or more complicated devices, such as
bycatch grills.
Cod end
The business end of the net, the cod end is where fish are finally "caught". The size of mesh in the cod end is a determinant of the size of fish which the net catches. Consequently, regulation of mesh size is a common way of managing
mortality of
juvenile fishes in trawl nets.
Process of benthic trawling
The idea that fish are passively "scooped up" is commonly held, and has been since trawling was first developed, but has been revealed to be erroneous. Since the development of
scuba diving equipment and cheap
video cameras it has been possible to directly observe the processes that occur when a trawl is towed along the seabed.
Structure of a benthic otter trawl
The trawl doors disturb the sea bed, create a cloud of muddy water which hides the oncoming trawl net and generates a noise which attracts fish. The fish begin to swim in front of the net mouth, but do not seem to be distressed by it. As the trawl continues along the seabed, fish begin to tire and slip backwards into the net. Finally, the fish become exhausted and drop back, into the "cod end" and are caught.
The speed that the trawl is towed at depends on the swimming speed of the species which is being targeted and the exact gear that is being used, but for most
demersal species, a speed of around 4
knots (7 km/h) is appropriate.
Ecological effect
Bottom fishing has operated for over a century on heavily fished grounds such as the
North Sea and
Grand Banks. Although overfishing has caused huge ecological changes to the fish community on the Grand Banks, concern has been raised recently about the damage which benthic trawling inflicts upon seabed communities. A species of particular concern is the slow growing, deep water
coral ''
Lophelia pertusa''. This species is home to a diverse community of deep sea organisms, but is easily damaged by fishing gear. On
November 18,
2004 the
United Nations General Assembly urged nations to consider temporary bans on high seas bottom trawling.
Satellite image of trawler mud trails off Louisiana coast
The turbidity produced by bottom trawling can have an impact significant distances from the site of the trawl activity. The absolute magnitude of suspended solids introduced into the water column from bottom trawling activity is much greater than any other man-made source of suspended solid pollution in the world's oceans.
[1]
The
UN Secretary General reported that 95 percent of damage to
seamount ecosystems worldwide is caused by
deep sea bottom trawling.
[2]
'Also see:'
Fishery Marine conservation
Current bottom trawling restrictions
Today, most major, responsible fishing countries restrict bottom trawling within their jurisdictions:
[3]
• The
United States National Oceanic and Atmospheric Administration banned bottom trawling off most of its Pacific coast in early 2006 and has restricted the practice severely off its other coasts as well.
[4]
• The
Council of the European Union in 2004 applied “a precautionary approach” and closed the sensitive Darwin Mounds off Scotland to bottom trawling.
• In 2005, the
FAO’s General Fisheries Commission for the Mediterranean (GFCM) banned bottom trawling below 1000 metres and, in January 2006, completely closed ecologically sensitive areas off Italy, Cyprus, and Egypt to all bottom trawling.
•
Norway first recognized in 1999 that trawling had caused significant damage to its cold-water lophelia corals. Norway has since established a program to determine the location of cold-water corals within its EEZ so as to quickly close those areas to bottom trawling.
•
Canada has acted to protect vulnerable coral reef ecosystems from bottom trawling off Nova Scotia. The Northeast Channel was protected by a fisheries closure in 2002, and the Gully area was protected by its designation as a Marine Protected Area (MPA) in 2004.
•
Australia in 1999 established the
Tasmanian Seamounts Marine Reserve to prohibit bottom trawling in the south Tasman Sea. Australia also prohibits bottom trawling in The Great Australian Bight Marine Park near Ceduna off South Australia. In 2004, Australia established the world’s largest marine protected area in the Great Barrier Reef Marine Park where fishing and other extractive activities are prohibited.
•
New Zealand in 2001 closed 19 seamounts within its
EEZ to bottom trawling, including in the Chatham Rise, sub-Antarctic waters, and off the east and west coasts of the North Island.
New Zealand Fisheries Minister
Jim Anderton announced on
14 February 2006 that a draft agreement had been reached with fishing companies to ban bottom trawling in 30 percent of New Zealand's
exclusive economic zone, an area of about 1.2 million km² reaching from sub-Antarctic waters to sub-tropical ones.
[5] But only a small fraction of the area proposed for protection will cover areas actually vulnerable to bottom trawling.
[6]
•
Palau has banned all bottom trawling within its jurisdiction and by any Palauan or Palauan corporation anywhere in the world.
[7]
• The President of
Kiribati, Anote Tong, announced in early 2006 the formation of the world’s first deep sea marine reserve area. This measure—the Phoenix Islands Protected Area—creates the world’s third largest marine protected area and will protect deep sea corals, fish, and seamounts from bottom trawling.
[8] In contrary to these claims a number of deep sea marine reserves existed before this announcement. One of these is the
Tasmanian Seamounts Marine Reserve in Australia that was established in 1999.
[9]
The relative lack of bottom trawling regulation in international waters
Beyond national jurisdictions, most bottom trawling is unregulated either because there is no Regional Fisheries Management Organization (RFMO) with competence to regulate, or else what RFMOs that do exist have not actually regulated. The major exception to this is in the Antarctic region, where the
Convention for the Conservation of Antarctic Marine Living Resources regime has instituted extensive bottom trawling restrictions.
[10] The North East Atlantic Fisheries Commission (NEAFC) also recently closed four seamounts and part of the mid-Atlantic Ridge from all fishing, including bottom trawling, for three years. This still leaves most of international waters completely without bottom trawl regulation.
As of May 2007 the area managed under the 'South Pacific
Regional Fisheries Management Organisation' (SPRFMO)
[11] has gained a new level of protection. All countries fishing in the region (accounting for about 25 percent of the global ocean) agreed to exclude bottom trawling on high seas areas where vulnerable ecosystems are likely or known to occur until an specific impact assessment is undertaken and precautionary measures have been are implemented. Also observers will be required on all high seas bottom trawlers to ensure enforcement of the regulations.
United Nations Proposals for Bottom Trawling Ban
Palau President
Tommy Remengesau has called for a ban on destructive and unregulated bottom trawling beyond national jurisdictions and Palau has led the effort at the
United Nations and in the
Pacific to achieve a
consensus by countries to take this action at an international level.
[12] [13] Palau has been joined by the
Federated States of Micronesia, the
Republic of the Marshall Islands, and
Tuvalu in supporting an interim bottom trawling ban at the United Nations.
[14]
New Zealand Fisheries Minister
Jim Anderton has promised to support a global ban on bottom trawling if there was sufficient support to make that a practical option. Bottom Trawling has been banned in 1/3 of New Zealand's waters
[15]
References
# March, E. J. (1953). Sailing Trawlers: The Story of Deep-Sea Fishing with Long Line and Trawl. Percival Marshal and Company, p. 33. Reprinted by Charles & David, 1970, Newton Abbot, UK. ISBN 071534711X
External links
★
Bottom trawling imagery Annotated satellite images from a number of bottom trawling activities around the world
★
Deep Sea Conservation Coalition Campaign for a ban on deep sea bottom trawling
★
FAO Gear type fact sheets Gear type fact sheet on various types of bottom trawls
★
Greenpeace: bottom trawling facts
★
Oceana: bottom trawling facts
★
UNEP: System-Wide EarthWatch ''Oceans and Coastal Areas'' On the role bottom trawling plays in global fisheries
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