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==Environmental effects== {{Further|Bottom trawling#Environmental damage}} Although trawling today is heavily regulated in some nations, it remains the target of many protests by [[environmentalist]]s. Environmental concerns related to trawling refer to two areas: the lack of selectivity and the physical damage which the trawl does to the seabed.<ref>{{Cite news| last=Urbina|first= Ian| date=July 28, 2015|title=A Renegade Trawler, Hunted for 10,000 Miles by Vigilantes.|work=The New York Times|url=https://www.nytimes.com/2015/07/28/world/a-renegade-trawler-hunted-for-10000-miles-by-vigilantes.html}}</ref> ===Selectivity=== [[File:Trawl catch of myctophids and glass shrimp from a layer associated with the bottom at greater than 200 meters depth.jpg|thumb|Trawl catch of [[myctophid]]s and [[glass shrimp]] from the bottom at greater than 200 meters depth]]Since the practice of trawling started (circa 14th century), there have been concerns over trawling's lack of selectivity.<ref>{{Cite journal |last=Jones |first=Peter |date=May 2018 |title=The long 'lost' history of bottom trawling in England, c.1350β1650 |url=http://journals.sagepub.com/doi/10.1177/0843871418766765 |journal= International Journal of Maritime History |language=en |volume=30 |issue=2 |pages= 201β217 |doi= 10.1177/0843871418766765 |s2cid=134879666 |issn=0843-8714|url-access=subscription }}</ref> 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 (New South Wales)|Clarence River]] found that an estimated 177 tons of by-catch (including 77 different species) were discarded each year.<ref>{{cite journal| last1=Liggins| first1= G.W.| last2= Kennelly| first2= S.J.| year= 1996| title= By-catch from prawn trawling in the Clarence River estuary, New South Wales, Australia| journal= Fish. Res. | volume= 25| pages= 347β367}}</ref> Size selectivity is controlled by the mesh size of the "cod-end" β 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 ([[rhombus]]es) 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 (typically made of stainless steel or plastic) 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 (juvenile fish, shrimp) pass through the grid and enter the sea again. In shrimp trawls, the grid pushes the largest organisms (fish) 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.<ref>{{cite web| url= http://minouw-project.eu/sorting-grids-can-norwegian-success-be-replicated-in-the-mediterranean/ | title= Sorting grids β can Norwegian success be replicated in the Mediterranean? | website= Minouw-Project.eu| publisher=| date=| accessdate=}} </ref> 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.<ref>{{cite news| last1= Alverson | first1= D L| last2= Freeberg| first2= M K| last3= Murawski| first3= S A| last4= Pope| first4= J G| year= 1994| title= A global assessment of fisheries bycatch and discards| publisher= FAO Fisheries | id= Technical Paper No 339 |location=Rome}}</ref> ===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 [[habitat (ecology)|habitats]] and removal of seaweed.{{Citation needed|date=January 2021}} [[File:Entangled porpoise.jpg|thumb|Porpoise Caught in Fishing Net]] 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 [[sessility (zoology)|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.{{Citation needed|date=January 2021}} [[File:Louisiana trawling landsat cropped.jpg|thumb|Aerial Photo of Trawling Turbidity Plume in Louisiana]] 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.{{Citation needed|date=December 2021}} 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.{{efn|These turbidity plumes can be seen on [[Google Earth]] 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.{{Citation needed|date=December 2021}} 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.<ref>{{cite journal| url= | doi=10.1080/00288330.1992.9516500| title= Environmental impact of trawling on the seabed: A review| year=1992| last1=Jones| first1=J. B.| journal=New Zealand Journal of Marine and Freshwater Research| volume= 26| issue=1| pages=59β67| bibcode= 1992NZJMF..26...59J}}</ref> A 2021 study estimated annual carbon emissions from bottom trawling at almost 1.5 billion tonnes (about 3% of the world total) and recommended that more [[marine protected area]]s be established.<ref>{{Cite journal|last1=Sala|first1=Enric|last2=Mayorga|first2= Juan|last3=Bradley|first3=Darcy|last4= Cabral| first4=Reniel B.|last5=Atwood|first5= Trisha B.|last6=Auber|first6=Arnaud|last7= Cheung|first7=William|last8=Costello|first8= Christopher|last9=Ferretti|first9= Francesco|last10= Friedlander|first10=Alan M.|last11= Gaines|first11=Steven D.| display-authors= 3| date= 2021-03-17|title= Protecting the global ocean for biodiversity, food and climate|url=https://www.nature.com/articles/s41586-021-03371-z.epdf?sharing_token=I6r76A0ntYrwdimz3_CxqtRgN0jAjWel9jnR3ZoTv0MwjSp_dqdYRo11ccDn9dqPW5D1xJuK8fpT__q4KFNUwgKdmwi3JyJVwmHRf-bxESQBSr9MbBwkap3XEr49FKSZrw7W6j8yaEyrI67o_vW36vuzGgf5WiXfPupj3TCNKWGPhX2RS00vTHE-BkQvziGb11MFJGGPZ1b_51DKmwtGk8pMFy1ivjK881Fj5VETogYf3fVIstOvZFm7GPLB6_wASYvalgBa2hb_ClZSqyd73EzCcTn8cASv1PLfbmhXewD4sVOkGUUAf_4MCv5oHLi-j547_kNPlxRtvGBZ5qDJn9VBDK_sIGiNYssafmauhDnnlgA_XLjd4OOWVUZQk9Df&tracking_referrer=www.theguardian.com|journal= Nature|language=en|volume=592|issue= 7854| pages=397β402|doi=10.1038/s41586-021-03371-z|pmid=33731930|bibcode=2021Natur.592..397S|s2cid= 232301777| issn=0028-0836}}</ref> 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.<ref>{{cite journal| last= Hiddink| first= J. G. | display-authors= etal| title= Quantifying the carbon benefits of ending bottom trawling| work= Nature | url=| doi= 10.1038/s41586-023-06014-7 | year=2023}}</ref><ref>{{cite journal| last1= Ovando| first1= D.| last2= Liu| first2=O. |last3= Molina| first3= R. | display-authors= etal| title= Global effects of marine protected areas on food security are unknown | work= Nature | number= 621| pages= E34βE36 | year= 2023| doi= 10.1038/s41586-023-06493-8}}</ref> 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.<ref>{{cite journal| first1= S. | last1= Roberts| last2= Hirshfield | first2= M. | url= http://www.oceana.org/fileadmin/oceana/uploads/destructive_trawling/savecorals/News/oceana_coral_report_old.pdf | title= Deep Sea Corals: Out of Sight, But No Longer Out of Mind| archiveurl= https://web.archive.org/web/20090226041832/http://www.oceana.org/fileadmin/oceana/uploads/destructive_trawling/savecorals/News/oceana_coral_report_old.pdf |archivedate=2009-02-26 | work= Oceania. In Frontiers in Ecology and the Environment| date= April 2004| accessdate=}}</ref> Published research has shown that benthic trawling destroys the cold-water coral ''[[Lophelia pertusa]]'', an important habitat for many deep-sea organisms.<ref>{{cite journal| last1= Fossa | first1= J H| last2= Mortensen | first2= P B |last3= Furevik | first3= D M | year= 2002| url= http://www.imr.no/Dokumenter/fossa.pdf | title= The deep water coral Lophelia pertusa in Norwegian waters: distribution and fishery impacts| archiveurl= https://web.archive.org/web/20090226041832/http://www.imr.no/Dokumenter/fossa.pdf |archivedate= 2009-02-26 | work= Hydrobiologia | volume= 471| pages= 1β12| accessdate=}}</ref> [[Midwater trawling|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).<ref>{{cite journal| last1= Ross | first1= A| last2= Isaac | first2=S| year= 2004| title= The net effect? A review of cetacean bycatch in pelagic trawls and other fisheries in the north-east Atlantic| website= uk.whales.org | url= https://uk.whales.org/wp-content/uploads/sites/6/2018/08/net-effect.pdf |place=London| publisher= Greenpeace Environmental Trust| accessdate= August 18, 2024}}</ref>
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