Bottom trawling
Bottom trawling is trawling along the seafloor. 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, 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 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 or a large factory trawler with. Bottom trawling can be carried out by one trawler or by two trawlers fishing cooperatively.
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. A 2021 paper estimated that bottom trawling contributed between 600 and 1500 million tons of carbon dioxide a year by disturbing carbon dioxide in the sea floor – emissions approximately equivalent to those of Germany, or the aviation industry. However, these values are highly uncertain and have been criticized as overestimates. International attempts to limit bottom trawling have been ineffective.
History
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 6 m long and 3 m 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.
Another source describes the wondyrchoum as:
three fathom long and ten men's feet wide, and that it had a beam ten feet long, at the end of which were two frames formed like a colerake, that a leaded rope weighted with a great many stones was fixed on the lower part of the net between the two frames, and that another rope was fixed with nails on the upper part of the beam, so that the fish entering the space between the beam and the lower net were caught. The net had maskes of the length and breadth of two men's thumbs
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.
Until the late 18th century sailing vessels were only capable of towing small trawls. However, in the closing years of that century a type of vessel emerged that was capable of towing a large trawl, in deeper waters. The development of this type of craft, the sailing trawler, is credited to the fishermen of Brixham in Devon. The new method proved to be far more efficient than traditional long-lining. At first its use was confined to the western half of the English Channel, but as the Brixham men extended their range to the North Sea and Irish Sea it became the norm there too.
By the end of the 19th century there were more than 3,000 sailing trawlers in commission in UK waters and the practice had spread to neighbouring European countries. Despite the availability of steam, trawling under sail continued to be economically efficient, and sailing trawlers continued to be built until the middle of the 1920s. Some were still operating in UK waters until the outbreak of World War II, and in Scandinavia and the Faroe Islands until the 1950s.
English commissions in the 19th century determined that there should be no limitation on trawling. They believed that bottom trawling, like tilling of land, actually increased production. As evidence, they noted that a second trawler would often follow a first trawler, and that the second trawler would often harvest even more fish than the first. The reason for this peculiarity is that the destruction caused by the first trawl resulted in many dead and dying organisms, which temporarily attracted a large number of additional species to feed on this moribund mass.
Bottom trawling does not only have a long tradition in European waters. It was also recognized in 1704 during the Edo era in Japan as a common fishing method. A slightly different approach was developed where the "Utase Ami" or "trawled bottom net" was deployed from a sideways sailing boat.
Bottom trawling has been widely implicated in the population collapse of a variety of fish species, locally and worldwide, including orange roughy, barndoor skate, shark, and many others.
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
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 large rectangular otter boards which are used to keep the mouth of the trawl net open. Otter boards are made of timber or steel and are positioned in such a way that the hydrodynamic forces, acting on them when the net is towed along the seabed, push them outwards and prevent the mouth of the net from closing. They also act like a plough, digging up to into the seabed, creating a turbid cloud, and scaring fish towards the net mouth.The net is held open vertically on an otter trawl by floats and/or kites attached to the "headline", and weighted "bobbins" attached to the "foot rope". These bobbins vary in their design, depending on the roughness of the sea bed that is being fished, varying from small rubber discs for very smooth, sandy ground, to large metal balls, up to 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" gear.
Body of the trawl
The body of the trawl is funnel-like, wide at its "mouth" and narrowing towards the cod end, and usually is fitted with wings of netting on 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 cod end is the trailing end of the net 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.Environmental damage
Trawling gear produces acute impacts on biota and the physical substratum of the seafloor by disrupting the sediment column structure, overturning boulders, re-suspending sediments and imprinting deep scars on muddy bottoms. Also, the repetitive passage of trawling gear over the same areas creates long-lasting, cumulative impacts that modify the cohesiveness and texture of sediments. It can be asserted nowadays that due to its recurrence, mobility and wide geographical extent, industrial trawling has become a major force driving seafloor change and affecting not only its physical integrity on short spatial scales but also imprinting measurable modifications to the geomorphology of entire continental margins.Bottom fishing has operated for over a century on heavily fished grounds such as the North Sea and Grand Banks. While overfishing has long been recognised as causing major ecological changes to the fish community on the Grand Banks, concern has been raised more 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 17 November 2004, the United Nations General Assembly urged nations to consider temporary bans on high seas bottom trawling. A global analysis of the impacts of bottom trawling found that the impact on seabed biota was strongly dependent on the type of gear used, with otter trawls estimated as having the smallest impact and removing 6% of biota per pass while hydraulic dredges had the largest impact and removed 41% of biota per pass. Other research found trawled canyon sediments contained 52 percent less organic matter than the undisturbed seafloor. There were 80 percent fewer sea worms in the trawled region and only half as much diversity of species in the trawled seafloor.
Image:Louisiana trawling landsat cropped.jpg|right|thumb|Satellite image of trawler mud trails off the Louisiana coast