|Distribution of Atlantic cod|
The Atlantic cod (Gadus morhua) is a benthopelagic fish of the family Gadidae, widely consumed by humans. It is also commercially known as cod or codling.  [n 1] Dry cod may be prepared as unsalted stockfish,   [n 2] as cured salt cod or clipfish. [n 3]
In the western Atlantic Ocean, cod has a distribution north of Cape Hatteras, North Carolina, and around both coasts of Greenland and the Labrador Sea; in the eastern Atlantic, it is found from the Bay of Biscay north to the Arctic Ocean, including the Baltic Sea, the North Sea, Sea of the Hebrides,  areas around Iceland and the Barents Sea.
The largest individual on record was 6 feet (1.8 m) long and weighed 211 lb (96 kg), however usually the cod is between 24 inches (61 cm) and 4 feet (1.2 m) long, and weighs up to 88 lb (40 kg). There is generally no difference in weight or size between sexes of Atlantic Cod.  
Atlantic Cod can live for 25 years, and usually attain sexual maturity between ages two and four,  although cod in the northeast Arctic can take as long as eight years to fully mature.  Colouring is brown or green, with spots on the dorsal side, shading to silver ventrally. A stripe along its lateral line (used to detect vibrations)  is clearly visible. Its habitat ranges from the shoreline down to the continental shelf.
Several cod stocks collapsed in the 1990s (declined by >95% of maximum historical biomass) and have failed to fully recover even with the cessation of fishing.  This absence of the apex predator has led to a trophic cascade in many areas.  Many other cod stocks remain at risk. The Atlantic cod is labelled vulnerable on the IUCN Red List of Threatened Species. 
Gravid female in captivity
Adult cod form spawning aggregations from late winter to spring.   Females release their eggs in batches,  and males compete to fertilize them.    Fertilized eggs drift with ocean currents and develop into larvae ("fry"). Age of maturation varies between cod stocks, from ages two to four in the west Atlantic,  but as late as eight years in the northeast Arctic.  Cod can live for 13 years or more. 
The Atlantic cod is one of three cod species in the genus Gadus along with Pacific cod and Greenland cod. A variety of fish species are colloquially known as cod, but they are not all classified within the Gadus, though some are in the Atlantic cod family, Gadidae.
Atlantic cod are a shoaling species and move in large, size-structured aggregations. Larger fish act as scouts and lead the shoal's direction, particularly during post spawning migrations inshore for feeding. Cod actively feed during migration and changes in shoal structure occur when food is encountered. Shoals are generally thought to be relatively leaderless, with all fish having equal status and an equal distribution of resources and benefits.  However, some studies suggest that leading fish gain certain feeding benefits. One study of a migrating Atlantic cod shoal showed significant variability in feeding habits based on size and position in the shoal. Larger scouts consumed a more variable, higher quantity of food, while trailing fish had less variable diets and consumed less food. Fish distribution throughout the shoal seems to be dictated by fish size, and ultimately, the smaller lagging fish likely benefit from shoaling because they are more successful in feeding in the shoal than they would be if migrating individually, due to social facilitation. 
Atlantic cod are apex predators in the Baltic and adults are generally free from the concerns of predation.  Juvenile cod, however, may serve as prey for adult cod, which sometimes practice cannibalism. Juvenile cod make substrate decisions based on risk of predation. Substrates refer to different feeding and swimming environments. Without apparent risk of predation, juvenile cod demonstrated a preference for finer-grained substrates such as sand and gravel-pebble. However, in the presence of a predator, they preferred to seek safety in the space available between stones of a cobble substrate. Selection of cobble significantly reduces the risk of predation. Without access to cobble, the juvenile cod simply tries to escape a predator by fleeing.
Additionally, juvenile Atlantic cod vary their behaviour according to the foraging behaviour of predators. In the vicinity of a passive predator, cod behaviour changes very little. The juveniles prefer finer-grained substrates and otherwise avoid the safer kelp, steering clear of the predator. In contrast, in the presence of an actively foraging predator, juveniles are highly avoidant and hide in cobble or in kelp if cobble is unavailable. 
As apex predators, heavy fishing of cod in the 1990s and the collapse of American and Canadian cod stocks resulted in trophic cascades. Overfishing cod removed a significant predatory pressure on other Atlantic fish and crustacean species. Population-limiting effects on several species including American lobsters, crabs, and shrimp from cod predation have decreased significantly, and the abundance of these species and their increasing range serve as evidence of the Atlantic cod's role as a major predator rather than prey. 
Cod are cannibals and are eaten at various life stages by various other fishes, seals and whales. 
Atlantic cod have been recorded to swim at speeds of a minimum of 2–5 cm/s and a maximum of 21–54 cm/s with a mean swimming speed of 9–17 cm/s. In one hour, cod have been recorded to cover a mean range of 99 to 226 m2. Swimming speed was higher during the day than at night. This is reflected in the fact that cod more actively search for food during the day. Cod likely modify their activity pattern according to the length of daylight, thus activity varies with time of year. 
Swimming and physiological behaviours change in response to fluctuations in water temperature. Respirometry experiments show that heart rates of Atlantic cod change drastically with changes in temperature of only a few degrees. A rise in water temperature causes marked increases in cod swimming activity. Cod typically avoid new temperature conditions, and the temperatures can dictate where they are distributed in water. They prefer to be deeper, in colder water layers during the day, and in shallower, warmer water layers at night. These fine-tuned behavioural changes to water temperature are driven by an effort to maintain homeostasis to preserve energy. This is demonstrated by the fact that a change of only 2.5 °C caused a highly costly increase in metabolic rate of 15 to 30%. 
Stomach sampling studies have discovered that small Atlantic cod feed primarily on crustaceans, while large Atlantic cod feed primarily on fish.  In certain regions, the main food source is decapods with fish as a complementary food item in the diet.  Wild Atlantic cod throughout the North Sea depend, to a large extent, on commercial fish species also used in fisheries, such as Atlantic mackerel, haddock, whiting, Atlantic herring, European plaice, and common sole, making fishery manipulation of cod significantly easier.  Ultimately, food selection by cod is affected by the food item size relative to their own size. However, providing for size, cod do exhibit food preference and are not simply driven by availability. 
Atlantic cod practice some cannibalism. In the southern North Sea, 1–2% (by weight) of stomach contents for cod larger than 10 cm consisted of juvenile cod. In the northern North Sea, cannibalism was higher, at 10%.  Other reports of cannibalism have estimated as high as 56% of the diet consists of juvenile cod. 
Atlantic cod reproduce during a 1– to 2-month spawning season annually. Males and females aggregate in spawning schools and each spawning season yields an average of 8.3 egg batches. Females release gametes in a ventral mount, and males then fertilize the released eggs. Evidence suggests male sound production and other sexually selected characteristics allow female cod to actively choose a spawning partner. Males also exhibit aggressive interactions for access to females.  Based on behavioral observations of cod, that cod mating systems resemble those of lekking species, which is characterized by males aggregating and establishing dominance hierarchies, at which point females may visit and choose a spawning partner based on status and sexual characteristics. 
Cod males experience reproductive hierarchies based on size. Larger cod males are ultimately more successful in mating and produce the largest proportion of offspring in a population. However, cod males do experience high levels of sperm competition. In 75% of examined spawning in one study, sperm from multiple males contributed to offspring. As a result of high competition and unpredictable paternity, males may engage in varied mating strategies and may invest in courtship or may simply ejaculate with other spawning couples. Spawning success also varies according to male size relative to female size. Males that are significantly smaller than females demonstrate significantly lower success rates relative to males that are larger than females. 
Atlantic cod act as intermediate, paratenic, or definitive hosts to a large number of parasite species: 107 taxa listed by Hemmingsen and MacKenzie (2001)  and seven new records by Perdiguero-Alonso et al. (2008).  The predominant groups of cod parasites in the northeast Atlantic were trematodes (19 species) and nematodes (13 species), including larval anisakids, which comprised 58.2% of the total number of individuals.  Parasites of Atlantic cod include copepods, digeneans, monogeneans, acanthocephalans, cestodes, nematodes, myxozoans, and protozoans. 
The Northwest Atlantic cod has been regarded as heavily overfished throughout its range, resulting in a crash in the fishery in the United States and Canada during the early 1990s.
Newfoundland's northern cod fishery can be traced back to the 16th century. On average, about 300,000 metric tons of cod were landed annually until the 1960s, when advances in technology enabled factory trawlers to take larger catches. By 1968, landings for the fish peaked at 800,000 metric tons before a gradual decline set in. With the reopening of the limited cod fisheries in 2006, nearly 2,700 metric tons of cod were hauled in. In 2007, offshore cod stocks were estimated at 1% of what they were in 1977. 
Technologies that contributed to the collapse of Atlantic cod include engine-powered vessels and frozen food compartments aboard ships. Engine-powered vessels had larger nets, greater range, and better navigation. The capacity to catch fish became limitless. In addition, sonar technology gave an edge to detecting and catching fish. Sonar was originally developed during World War II to locate enemy submarines, but was later applied to locating schools of fish. These new technologies, as well as bottom trawlers that destroyed entire ecosystems, contributed to the collapse of Atlantic cod. They were vastly different from old techniques used, such as hand lines and long lines. 
The fishery has only recently begun to recover, and may never fully recover because of a possibly stable change in the food chain. Atlantic cod was a top-tier predator, along with haddock, flounder and hake, feeding upon smaller prey, such as herring, capelin, shrimp, and snow crab.  With the large predatory fish removed, their prey have had population explosions and have become the top predators, affecting the survival rates of cod eggs and fry.
In the winter of 2011–2012, the cod fishery succeeded in convincing NOAA to postpone for one year the planned 82% reduction in catch limits. Instead, the limit was reduced by 22%. The fishery brought in $15.8 million in 2010, coming second behind Georges Bank haddock among the region's 20 regulated bottom-dwelling groundfish. Data released in 2011 indicated that even closing the fishery would not allow populations to rebound by 2014 to levels required under federal law. Restrictions on cod effectively limit fishing on other groundfish species with which the cod swim, such as flounder and haddock. 
Cod populations or stocks can differ significantly both in appearance and biology. For instance, the cod stocks of the Baltic Sea are adapted to low-salinity water. Organisations such as the Northwest Atlantic Fishery Organization (NAFO) and ICES divide the cod into management units or stocks; however, these units are not always biologically distinguishable stocks. Some major stocks/management units on the Canadian/US shelf are the Southern Labrador-Eastern Newfoundland stock (NAFO divisions 2J3KL), the Northern Gulf of St. Lawrence stock (NAFO divisions 3Pn4RS), the Northern Scotian Shelf stock (NAFO divisions 4VsW), which all lie in Canadian waters, and the Georges Bank and Gulf of Maine stocks in United States waters. In the European Atlantic, the numerous separate stocks are on the shelves of Iceland, the coast of Norway, the Barents Sea, the Faroe Islands, off western Scotland, the North Sea, the Irish Sea, the Celtic Sea, and the Baltic Sea.
The Northeast Atlantic has the world's largest population of cod. By far, the largest part of this population is the Northeast Arctic cod, as it is labelled by the ICES, or the Arcto-Norwegian cod stock, also referred to as skrei, a Norwegian name meaning something like "the wanderer", distinguishing it from coastal cod. The Northeast Arctic cod is found in the Barents Sea area. This stock spawns in March and April along the Norwegian coast, about 40% around the Lofoten archipelago. Newly hatched larvae drift northwards with the coastal current while feeding on larval copepods. By summer, the young cod reach the Barents Sea, where they stay for the rest of their lives, until their spawning migration. As the cod grow, they feed on krill and other small crustaceans and fish. Adult cod primarily feed on fish such as capelin and herring. The northeast Arctic cod also show cannibalistic behaviour. Estimated stock size was 2.26 million metric tons in 2008.
The North Sea cod stock is primarily fished by European Union member states and Norway. In 1999, the catch was divided among Denmark (31%), Scotland (25%), the rest of the United Kingdom (12%), the Netherlands (10%), Belgium, Germany and Norway (17%). In the 1970s, the annual catch rose to between 200,000 and 300,000 tons. Due to concerns about overfishing, catch quotas were repeatedly reduced in the 1980s and 1990s. In 2003, ICES stated a high risk existed of stock collapse if then current exploitation levels continued, and recommended a moratorium on catching Atlantic cod in the North Sea during 2004. However, agriculture and fisheries ministers from the Council of the European Union endorsed the EU/Norway Agreement and set the total allowable catch at 27,300 tons.  Seafood sustainability guides, such as the Monterey Bay Aquarium's Seafood Watch, often recommend environmentally conscious customers not purchase Atlantic cod.
The stock of Northeast Arctic cod was more than four million tons following World War II, but declined to a historic minimum of 740,000 tons in 1983. The catch reached a historic maximum of 1,343,000 tons in 1956, and bottomed out at 212,000 tons in 1990. Since 2000, the spawning stock has increased quite quickly, helped by low fishing pressure. The total catch in 2012 was 754,131 tons, the major fishers being Norway and Russia. 
- During the Middle Ages, Middle English used many, many forms of mulvel, milvel, melvel, and milwell to refer to fresh, large cod  and morhwell to refer to smaller ones.  Fresh cod was also known as the common cod,   the Scotch cod,  and as the green fish or greenfish.  "Greenfish", however, now more often refers to other fish. Similarly, "codling" may refer to various morids.
- In South Africa, however, "stockfish" refers to the local hake (Merluccius capensis).
- Former names for salted cod include cured cod,  ling,    and haberdine.   Freshly-salted cod was known as green cod, white cod, corefish,  coursfish,  and green fish or greenfish.  "Green cod" may also refer to the saithe (Pollachius virens), pollack (P. pollachius), or uncommonly to the lingcod (O. elongatus).  "Ling" now more often refers to other fish, particularly the common ling (Molva molva). 
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|Wikispecies has information related to Gadus morhua|
|Wikimedia Commons has media related to Atlantic cod.|
- The Centre for Environment Fisheries and Aquaculture Science
- The history of the northern cod fishery in Canada
- ICES recommendation for the North Sea Cod stock (2007)
- ICES recommendation for the North East Arctic Cod stock (2007)
- Reports on the status of Canadian fishing stocks, including cod
- Governmental Norwegian fact sheet on North-East Arctic Cod
- Atlantic cod-Gadus morhua fishery profiles Status of Atlantic cod fisheries, summarised in FisheriesWiki
- Atlantic cod at the Encyclopedia of Life
- View the Atlantic cod genome in Ensembl
- Government of Canada Seafisheries Landings
- View the gadMor1 genome assembly in the UCSC Genome Browser.