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The Atlantic cod (Gadus morhua) is a well-known benthopelagic fish of the family Gadidae, widely consumed by humans. It is also commercially known as cod or codling. Dry cod may be prepared as unsalted stockfish or as cured salt cod or clipfish.
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.
It can grow to 2 meters in length and weigh up to 96 kilograms (212 lb). It can live for 25 years and usually attains sexual maturity between ages two and four, but cod in the northeast Arctic can take as long as eight years to fully mature. Colouring is brown to green, with spots on the dorsal side, shading to silver ventrally. A stripe along its lateral line 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 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 VU (vulnerable) on the IUCN Red List of Threatened Species.
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. 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.
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 it has been hypothesized that ultimately, the smaller lagging fish benefit from shoaling because they are more successful in feeding in the shoal than they would be if migrating individually because of social facilitation.
Atlantic cod are apex predators and adults are generally free from the concerns of predation. Juvenile cod, however, may serve as prey for adult cod, which sometimes practice cannibalism. Research has revealed that 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 like 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 behavior according to the foraging behavior of predators. In the vicinity of a passive predator, cod behavior 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.