Cod is a cold-water species of fish that is distributed on the continental shelves and in the coastal waters of the northern North Atlantic. The range extends from the Bay of Biscay and the Baltic Sea to the Barents Sea, around Iceland, along the southern Greenland and along Newfoundland to North Carolina in North America.
The scientific name of cod is Gadus morhua. The genus name Gadus comes from the Greek word gados, which simply means fish. The origin of the species name morhua can be derived from the Latin word for cod: morua. The cod has a protruding upper jaw, a chin barbel, a light and curved lateral line, three dorsal fins and two ventral fins without spines and a truncated caudal fin. The cod is grouped within the family Gadidae together with haddock, whiting, saithe, and other codlike fishes. More distant relatives are grenadiers, toadfishes and anglerfishes.
Adult cod prefer water temperatures from 2 to 8 °C but is often found in temperatures up to 20 °C. Young cod have special enzymes that act as anti-freeze, which enables them to survive in sub-zero environments. Although cod can be caught at all depths from 1 to 600 m, their preferred depth range is from 10 to 200 m. Generally, cod are demersal, that is, they live on or close to the sea bottom. In the Baltic Sea, cod are often forced to stay in the pelagic zone due to anoxic deeper waters. Within its geographical range cod are generalists: they inhabit all habitats from rocky bottoms with or without macroalgae to muddy and even sandy seabeds.
Adult cod are omnivorous carnivores, which means that they feed on any animal that they can engulf. Their menu includes bristle-worms, mussels, squids, crustaceans and fishes such as sand eel, Norway pout, capelin, sticklebacks, sprat and herring. Adult cod are also cannibalistic and do not hesitate to catch smaller cod. However, cod have preferences: young cod prefer plaice fry before crabs. Their diet appears to affect their skin color; cod that feed on crustaceans acquire a brownish-golden skin while cod that feed on fishes take on a more greenish-blue appearance.
Spawning occurs throughout the geographical range mostly in offshore waters but also in small fjords and bays. The cod along Norway and Iceland both spawn from January through March. In the North Sea spawning continues through April while Baltic cod start spawning late in March but continue through October. Eggs are transparent and measure around 1.5 mm in diameter. Baltic cod produce larger eggs, possibly as an adaptation to float in low salinity waters (10 ‰). The eggs hatch within a month in normal sea temperatures. After another three months, the pelagic larvae settle during the autumn at suitable sea grounds and take up a permanent demersal life. Maturity is attained at around age 3 in the south (Baltic, North Sea, Irish Sea) but not until age 6 in the colder, northern waters (Norwegian, Icelandic waters). A female cod will lay up to 500 000 eggs per kg of her own weight. Consequently, a 3-year-old female of half a kg can produce 250000 eggs; an 8-year-old female of 5 kg can produce 2.5 million eggs per year. A cod can live to over 25 years of age and weigh over 90 kg. Although the fecundity decreases at large weights, a heavy female will still produce an impressive number of eggs. A record 9 million eggs was found in a 34 kg female. Virtually all of these eggs, and subsequent larvae, will die during the first 3 months of their life. In theory, only 2 eggs need to survive to maturity if the number of fishes in the sea should remain constant. Spawning sites are generally located upstream of a stable hydrographic gyre that will transport eggs and larvae to their nursery grounds. Tagging studies have shown that mature cod migrate annually from feeding grounds to these specific spawning sites. The migrations can be extensive. Cod that have been tagged in the northern Baltic have been recaptured more than 500 km south at spawning sites in the southern Baltic. Observations and simulation studies indicate that eggs, larvae and juvenile cod generally disperse northward after spawning. Thus eggs and larvae drift to nursery grounds, juvenile cod migrate to feeding grounds and mature cod migrate to spawning grounds. This lifecycle of migration has been the basis used to define spawning stocks and their preferred spawning locations. At present 14 cod stocks are defined in the Northeast Atlantic and around 10 cod stocks in the Northwest Atlantic. Some of these stocks are large in terms of numbers or biomass, like the Arctic cod stock outside Norway. Others are small, like the Rockall cod stock west of Ireland.
- Norwegian scientists attract young cod to feeding sites by playing recordings of tuba music underwater.
- The Portuguese claim to have 365 ways of preparing dried salt cod (bacalhau) one for each day of the year.
- About 10% of the world fish catch is cod.
- The official state fish of Massachusetts is the Cod fish. It has been a symbol of the Commonwealth for more than 200 years. Cod fish was so important for Massachusetts that on March 17, 1784, according to the Journal of the Massachusetts House of Representatives: "Mr. John Rowe moved the House that leave might be given to hang up the representation of a Cod Fish in the room where the House sit, as a memorial of the importance of the Cod Fishery to the welfare of this Commonwealth." It is still kept in the present House chamber.
- Codfish live to an average of 15 years or so, with a record of 27 years old from the 1960s. You can tell a cod's age from the two white earstones in the skull (otoliths), which have annual growth rings. You really can not tell the age from their weight, as the rate of growth will vary, depending on the water temperature, population size, food, etc.