Trawling
Trawling is an industrial method of fishing that involves pulling a fishing net through the water behind one or more boats. The net used for trawling is called a trawl. This principle requires netting bags which are towed through water to catch different species of fishes or sometimes targeted species. Trawls are often called towed gear or dragged gear.
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 engines to large factory trawlers with over 10,000 hp. Trawling can be carried out by one trawler or by two trawlers fishing cooperatively.
Trawling can be contrasted with trolling. While trawling involves a net and is typically done for commercial usage, trolling instead involves a reel, rod and a bait or a lure and is typically done for recreational purposes. Trawling is also commonly used as a scientific sampling, or survey, method.
Bottom vs. midwater trawling
Trawling can be divided into bottom trawling and midwater trawling, depending on how high the trawl is in the water column. Bottom trawling is towing the trawl along or close to the sea floor. Bottom trawling is an industrial fishing method in which a large net with heavy weights is dragged across the seafloor, scooping up everything in its path. Bottom trawling can be disadvantageous because it can stir up significant amounts of sediments that lie on the sea bed and can harm some marine species. It also causes water pollutants to mix with some plankton, which in turn will move into the food chain, which will then create harmful algae blooms leading to insufficient oxygen. A 2021 study estimated that greenhouse gas emissions from bottom trawling were as much as aviation. However, the estimation methods in the original article published in the journal Nature, have been criticized by other scientists, claiming that the green house gas emission estimates are uncertain. Pr 2024 there is an intense scientific debate going on about this and no final conclusion can yet be drawn. Newer, trial methods employing bottom trawling gear that do not touch the seabed could potentially have lower environmental impact than livestock or fed aquaculture if employed.Midway trawling or pelagic trawling target fishes that are living in the upper water column of the ocean. The funnel shaped trawl nets are hauled by one or two boats. This method is generally used to catch fishes of a single species. Unlike bottom trawling, this type of trawl does not come into contact with the sea bed and hence is not involved in damage of marine habitat. Some species caught with this trawling method are mackerel, herring, and hoki. However there may be some disadvantages in using this method as in the process of catching the targeted species of fish, one may end up capturing non targeted fish accidentally and thus discarding of juvenile commercial species of fish may impact on the population. Still, bycatch level is typically lower.
Midwater trawling is also known as pelagic trawling. Midwater trawling catches pelagic fish, whereas bottom trawling targets both bottom-living fish and semi-pelagic fish.
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
When two boats are used, 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. Trawl doors are available in various sizes and shapes and may be specialized to keep in contact with the sea bottom 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 and weight on the lower edge 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.
Environmental effects
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, 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 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 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 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. By-catch reduction grids or square mesh panels of net can be fitted to parts of the trawl, allowing certain species to escape while retaining others. In fish trawls, the grid is mounted so the smallest organisms pass through the grid and enter the sea again. In shrimp trawls, the grid pushes the largest organisms through a hole in the roof of the net, reducing by-catch of fish. The latter type of grid is mandatory in Norway and has been in use for 20 years. The grids are typically equipped with sensors that measure the angle of the grid, so the fishermen can tell whether the grid is working correctly.
Studies have suggested that shrimp trawling is responsible for the highest rate of by-catch.
Physical damage
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 stirs up bottom sediments, loading suspended solids into the water column. It is estimated that 21.87 gigatons of sediment from the sea floor is resuspended annually due solely to the activity of trawlers. For scale, the amount of sediment deposited into the ocean by all rivers in the world is estimated to be 17.8 to 20 gigatons annually. 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. Rototilling the sea floor and resuspending bottom sediment affects the nutrient levels and changes the entire chemistry of the ambient water, greatly reducing the photosynthesizing ability of plants and kelps while also impacting any animal living on the ocean floor. An article published in New Zealand Journal of Marine and Freshwater Research determined that the resuspended sediment creates anaerobic turbid conditions capable of killing scallop larvae that use the ocean floor as a habitat as they mature. The study also revealed that for filter feeders, despite there being more particulate matter in the water after a trawl, the protein per unit weight of sediment decreased, meaning they have to filter much more water for the same nutritional value. A 2021 study estimated annual carbon emissions from bottom trawling at almost 1.5 billion tonnes and recommended that more marine protected areas be established. Both the findings and the conclusions in the study have been scrutinized in more recent scientific works that do not come to the same conclusions as the mentioned study does.
Despite these scientific disputes that to a large extent are oriented around scientific modelling, other effects of trawling are not disputed. A vast array of species are threatened by trawling 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, an important habitat for many deep-sea organisms.
Midwater 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.