Trawling is an important method of commercial fishing in the seas. It involves dragging a huge net through the waters attached to one or two boats called trawlers. Trawling is said to have begun as a method of fishing on a large-scale during 17th century in Europe. Modern trawling is said to have actively begun in 19th Century Europe with the use of steam propulsion for fishing boats. This involves using a large conical net that is flattened and dragged over the floor of the sea bed and its middle waters to catch fish. The current trawlers use the same mechanism, though they have become more sophisticated and the processes more refined.
 Ocean Floor
The ocean’s foundation rests on the sea floor which forms the physical support for the marine environment. The greater discoveries of the deep sea that began in the 1960s revealed that the ocean’s floor is similar to landscape with plains, mountain ridges, deep canyons, mountains, coral habitats and more. While many marine mammals and fish roam the ocean’s many areas, there are some that are limited to surviving around their familiar habitats within the deep sea region. The deep ocean floor has its own mountains called seamounts. They rise 1,000 metres upwards above the surrounding sea-floor. Estimates reveal that there are tens of thousands of seamounts across the world's oceans: there are more than 800 in the Atlantic Ocean, and 30,000 and more are believed to be in the Pacific Ocean. Seamounts are rich in biodiversity and their surfaces are often colourful forests of cold water corals, soft seapens, sponges and seawhips, mobile animals such as sea spiders and lobster-like crustaceans that all live in sheltered crevices of the mounts. Thriving communities of small worms and crustaceans occupy softer areas of the mounts. In the waters around the seamounts live large populations of fish, swimming in the constant currents. Many seamount-dwelling species are confined to only one or two individual seamounts and cannot be found elsewhere.
 Destructive Trawling
Fishing by trawling is a disputed method due to the impact it has on the deep sea environment. A destructive trawler can plough and raze the ocean floor and kill its many marine species. It is calculated that a single net can snare 1-1/2 tonnes of coral every hour as the trawler passes by on the ocean’s surface. Environmentalists, conservationists and marine biologists are debating the destruction afflicted by deep sea trawling done for commercial fishing. They warn that if the seafloor continues to be ravaged by trawlers the ocean bed environment will be affected leaving healthy fish populations, marine habitats threatened. Bottom trawls use large, heavy nets and drag them across the floor bed so that can snare several tonnes of fish that live down close to the ocean floor. Species including the cod, haddock, shrimp, rockfish, pollack, flounder and scallops are caught using trawlers and dredges. Among the many habitats and species that are damaged in this fashion by deep sea trawling include seaweeds, deep sea coral and sponge life. Footropes and furrows from trawlers are said to cause furrows on the ocean floor and destroy sessile organisms and disturb bottom sediments.
In recent times fishermen are using equipment like roller gear, rock hopper with large wheels that looks like earth-moving equipment to increase their catch and these can crumble and ravage coral and rock habitats that are the homes to marine wildlife. Benthic trawling or bottom trawling, that drags nets across the sea floor, is said to destroy cold-water coral (lophelia pertusa) that serves as an important habitat for many organisms.
 Trawl Structure
Trawling is conducted with one or two boats. Pair trawling involves using two boats that provide the horizontal spread of the net with one warp attached to each boat. Single-boat trawling is more common where trawl ‘doors’ keep the net spread horizontally. Trawl doors come in varying shapes and sizes and are used to keep in contact with the ocean floor or above it. These doors act as wings to tow the vessel in the seas. Trawl nets can be modified. For example their mesh size can be adjusted keeping in mind the size of the fish the trawlers want to catch in a particular area.
 Trawling By-catch
Lack of selectivity in trawling is a prime concern as trawl nets are often non-selective, and end up catching not just commercially viable fish but also those that are not. The part of the catch that cannot be used is called by-catch that is also killed accidentally in the trawling process. While all fishing methods do encounter some capture of undesirable species that are not viable for fishermen, conservationists urge the use of many methods devised to minimize by-catch. By-catch reduction grilles and square mesh panels of net can be fitted to the trawl so that unwanted species are allowed to escape. Shrimp by-catch in various parts of the world is high and is a cause for concern.
Trawling by-catch is a serious concern too, as often while fishing for a single species, the bottom trawler snares and kills other species that are not required for commercial purposes. It is said that pelagic trawling or trawling on the upper levels of the high seas, is a ‘better’ method of trawling as it involves netting a single species. But this is again disputed as there are conservationists who argue that by-catch can be used for food for certain sea birds. The argument is inconclusive as both types of trawling continue.
 Environmental Hazards
The millions of pounds or sea life and organisms that are destroyed by a single trawler can take centuries to replace as many are slow growing ecosystems. It is estimated that two-thirds of all corals live in the deep sea. Deep sea corals range widely in size, shape and color, and coral gardens and reefs serve as habitat for diverse types of fish and marine wildlife. Corals also serve an important role in deep sea habitat and provide protection from currents and predators, and as nurseries for young fish and feeding, breeding and spawning areas for numerous fish and other ocean wildlife. Species such as orange roughy, blue ling and roundnose grenadier cling to underwater seamounts and these locations provide rich habitats for cold-water corals and the other animals and plants. Zooplanktons that top the food pyramid in the deep sea are often found on sea mounts.
The North Pacific coral and sponge areas support many economically viable species like rockfish, atka mackerel, walleye pollock, Pacific cod, sablefish, flatfish, crabs. In the Oculina Banks of the Atlantic many kinds of fish including groupers, bass, jacks, snappers, porgies and sharks thrive. Fishermen's observations and scientific studies have pointed out that the disappearance of corals in these regions causes significant changes in the distribution of fish and other ocean wildlife.
While opponents of trawling argue that the harm it causes to the ocean habitat and organisms is large and threatening in magnitude, its defenders dispute it saying the damage is minimal compared to the levels of destruction caused by natural calamities.
 Did You Know?
- Increasing use of new deep-water trawling equipment means that increasing number of fishers are trawling beyond the continental shelf.
- Harvesters are trawling at depths up to 400 m and, in some places, more than 1,500 m.
- Because deep-living species tend to grow more slowly than shallow-water species, the long-term impact of trawling is magnified as trawl depths increase.
- Some of the cold water coral reefs being destroyed by high seas bottom trawling are 8000 years old.
- In 2001, up to50% of Norwegian coral habitat had been damaged by deep sea trawling.
 World Debates
There has been widespread call for the ban of bottom-trawling by many environmentalists. Countries with deep-sea bottom-trawling fleets include Russia, New Zealand and Spain. Fleets are also known to operate out of Norway, Estonia, Denmark, Portugal, Japan, Lithunia, Lativa and Iceland. A coalition known as Deep Sea Conservation Coalition has called for a ban on bottom trawling due to the damage it causes to deep-sea fragile ecosystems. They argue that these countries are responsible for 95% of bottom trawl catch in the high seas in 2001. The coalition urged the UN to declare a global moratorium on bottom-trawling. The UN declined. The debate continues.
- Fisf Net USA
- Deep-sea trawling's 'great harm'
- Bottom Trawling | Greenpeace International
- The Impact of Globl Trawling: Mapping Our Footprint On The Seafloor
- What Is Destructive Trawling?
- History And Animals
- Deep Sea Bottom Trawling and the Eastern Tropical Pacific Seascape: A Test Case for Global Action
- Putting Humanity Before Technology
- The Impact of Demersal Trawling on NE Atlantic Coral Ecosystems with particular reference to the northern Rockall Trough