Biology and distribution

The cod can grow quite large; the largest individual measured in Icelandic waters was 186 cm long and 17 years old. Common size in catches is much smaller, or in the range of 45 to 85 cm long in most fishing gear. This corresponds to roughly 4 to 7 year old and 1 to 4 kg fish. Gillnets are the exceptions, as they are used to selectively target much larger fish, or mostly in the size range of 75 to 105 cm. Growth is quite variable, both between years and areas. In general, the cod grows slower in the colder northern Icelandic waters than in the warmer southern. Annual variation in growth can mostly be explained by the stock size of its main prey, the capelin. In general, three year old cod are about 1 kg and in the size range of 30 to 40 cm long, while 10 year old cod are around 10 kg and in the size range of 100 cm. However, variation is large and, especially, cod in the colder waters grow much slower.

Age of maturity is also variable between years and areas. However, it is closely related to growth; slow growing fish mature later than fast growing fish. Hence cod in the colder northern waters does not reach 50% maturity (the age where 50% of the year class is mature) until around the age of seven while its southern counterparts reach 50% maturity at five.

The cod spawns all around Iceland, but the largest and most important fishing grounds are off the south-western coast. Spawning takes place in late winter when mature cod aggregate on the spawning grounds. In previous centuries the main fishing season used to be along the south-west coast at that time, targeting the large spawning cod. After spawning, the cod migrates, spreading out over the continental shelf, but the highest abundance is on the main feeding grounds off the northwest and southeast coast where the warm Atlantic waters meet the cold Polar currents.

After spawning, eggs and larvae drift clockwise around the country with the ocean currents. In autumn they settle down to the bottom and spend most of the time hiding from menacing predators, mostly larger cod. The most important rearing rounds for the juveniles are in the colder waters northwest, north and east of Iceland. In general, they head for deeper waters as they grow and end up on the feeding grounds around maturity.

The food of the cod is very diverse, and this might be one of the reasons for its success. The main food of the juveniles is various zooplankton species; the most important being copepods, krill and capelin larvae. When it grows larger and starts a demersal existence, the main food becomes various benthic invertebrates. At the size of 20 to 30 cm the main food is northern shrimp, although many other prey species are also found in the diet. At larger size an increasing share becomes other fishes, mainly capelin, when the cod is up to about 90 cm in length. At a later stage, however, many larger species are added to the menu, such as redfishes, blue whiting or smaller cod.

The Icelandic cod stock mostly occurs on the Icelandic shelf, although a few tag returns have been from other waters. There is one major exception to this rather localized distribution pattern. The Greenlandic cod stock is largely made up of larvae drift from Icelandic spawning grounds. When this fish matures, it returns to Icelandic waters to spawn. This return has at times had a major impact on cod fisheries in Icelandic waters, contributing to large increase in the landings but also to large problems in assessing the stocks. This Greenland migration has not occurred since 1990 as the Greenlandic stock has been very low, but it might occur in the near future, since the Greenlandic cod stock is currently growing.

Worldwide, the stock is distributed from Cape Cod in the North-western Atlantic, along the coast of Labrador, off southern Greenland, Iceland, Faroe Islands, south to the English Channel, in the North Sea, Baltic Sea and in the Barents Sea.

Hreiðar Þór Valtýsson, University of Akureyri