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Arctic Cod

Life is not easy in the frigid waters of the Arctic. Fish are not as abundant there nor as varied as they are farther to the south. One hardy denizen of the far north is the Arctic cod (Boreogadus saida), a small relative of the Atlantic cod and one of several species of cod-like fish scientifically classified as gadoids. While not harvested commercially by Canadian fishermen, this fish plays a key role in the diet of many Arctic marine mammals, seabirds and fish.

Description

The Arctic cod has an elongate body which tapers markedly toward the tail and may grow to a total length of 38 cm but rarely exceeds 30 cm. While it resembles other members of the cod family such as the Atlantic cod, rock cod, haddock, and tomcod, it can be readily distinguished from them by its more slender body, its deeply forked tail, its projecting lower jaw (they have projecting upper jaws) and the very small size of the "whisker" or barbel under its lower jaw.

Over the back and on the upper sides, it is generally brownish spotted with many black dots. Below it is silvery. The fins are dark, almost black, with a narrow, pale edge. Running along each side of the body from head to tail is a pale lateral line. The scales are very small.

Where Found

Arctic cod are circumpolar in distribution. They have been sighted at a latitude of 84°42'N-farther north than any other fish species. They inhabit Arctic seas off northern Russia, Alaska, Canada and Greenland. In Canadian waters they are found in the Beaufort Sea, the Arctic Archipelago, Hudson Bay, Baffin Bay, along the Labrador coast, eastern Newfoundland coast, and the northern and eastern Grand Banks. Occasionally they have been known to stray into the Gulf of St. Lawrence.

Temperatures of 0 to 4°C are believed to be optimal for the survival of Arctic cod, although Canadian east coast biologists have usually found them in water colder than 0°C and frequently near drifting ice. It has been noted that their size decreases from north to south. Off northern Labrador, the common length range of this fish is 25 to 30 cm. Off southern Labrador, it diminishes to 10 to 25 cm and off eastern Newfoundland to 10 to 18 cm.

These fish are found close to shore among ice floes and also offshore in depths greater than 900 m. Exploratory fishing ventures have revealed that off northern Labrador and Baffin Island, best catches from otter trawls were taken at bottom depths of 100 to 250 m and at bottom temperatures of from -1.4 to 0.6°C. Off southern Labrador and northern Newfoundland, catches were smaller and were mainly taken at depths of 200 to 300 m and at bottom temperatures of -1.2 to 3.6°C. During autumn the fish have been observed to congregate in large numbers and move into coastal waters.

Life History

Both male and female Arctic cod are mature when about 20 cm long and three years of age. In northern Canadian waters, spawning is thought to occur in late autumn and winter. Off northern Russia it is reported to occur during January and February.

The gonads of maturing male Arctic cod are relatively large and comprise about 10 per cent of the total body weight. Fully mature, female Arctic cod produce eggs ranging from 1.5 to 1.9 mm in diameter. At the yearly spawning, they release 9,000 to 21,000 eggs against an average of 2,000,000 to 5,000,000 eggs for the Atlantic cod and 1,800,000 for the haddock. Spawning occurs under the Arctic ice cover and fertilization is external, meaning that the female releases her eggs into the sea where they are fertilized by milt from the male.

Arctic cod are the main consumers of plankton (microscopic forms of animal and plant life) in the Arctic seas. Unlike the Atlantic cod, which are groundfish, they rarely feed on organisms found on the bottom, but rely mainly on those organisms making up the drifting plankton in the upper layers of the water column.

Small Arctic cod, 4 to 6 cm long, feed mainly on the eggs and larvae of copepods and adult amphipods-both very small crustaceans. Intermediate sized fish, 8 to 12 cm long, feed on copepods, amphipods and shrimp-like crustaceans known as euphausiids. Fish more than 12 cm in length feed on copepods, amphipods and arrow worms. Large Arctic cod, in addition to feeding on plankton organisms are to some extent cannibalistic, feeding on smaller members of their own kind.

The weight of these fish can be correlated with their length. One measuring 10 cm would weigh about 10 g, one measuring 20 cm, about 70 g, and one measuring 28 cm, about 180 g.

Age can also be correlated with length, although it has been noted that Arctic cod in the Arctic Ocean usually grow more slowly than those off the Labrador coast. In general, at the age of one year, these fish are about 9 cm long. By the end of the second year they measure about 15 cm, and by the age of three, about 20 cm. Their growth rate appears to decrease slightly after the third year, four-year-olds averaging only about 22 cm and five-year-olds, 25 cm.

Arctic cod have a short life span. Scientific expeditions off the Labrador coast have shown that the oldest individuals there were only six years old. Their age was determined by counting the annual rings of the ear bones, just as the age of a tree is determined by counting the rings on the stump.

Population Estimates

On the basis of current knowledge, it is not possible to estimate the abundance of Arctic cod in the Canadian Arctic. Based on data obtained from exploratory fishing off northern Labrador in September, 1978, it is supposed that there were several hundred thousand tonnes of the fish there at that time.

Estimates of Arctic cod from echo sounder surveys of an area of about 18,000 square miles off southern Labrador and northern Newfoundland during 1978 suggest that there were then about 100,000 tonnes of the fish in the area, mainly the young-of-the-year.

That increase in the population of Arctic cod in the NewfoundlandLabrador area may have been the result of environmental conditions in the Arctic and may have been short lived. Abundance indices since 1979 indicate that the numbers of Arctic cod there have since decreased.

Biological Importance

Like other marine environments, Arctic waters support a complex system of organisms, many of which directly or indirectly influence a number of other life forms. Two major differences exist between the aquatic resources of the Arctic and those of the more temperate zone: a lower species diversification and a lower level of production. The loss of a single species might seriously disrupt the food chain, resulting in radical changes at many levels. Loss of the Arctic cod in northern areas might well be calamitous as it is an important food in the diet of many species of marine mammals, seabirds and fish.

Narwhals, for example, feed predominantly on Arctic cod. The remains of as many as 64 Arctic cod have been found in a single narwhal stomach. White whales (belugas) and ringed seals also feed on this fish to some degree. Seabirds, especially murres, depend heavily on Arctic cod as a source of food. In one study it was estimated that over a 35-day period, at least 1.4 million Arctic cod, and probably 10 times that number, were consumed by a line of seabirds stationed along the edge of 125 km of fast ice.

Atlantic cod have been reported feeding on large numbers of Arctic cod off northeastern Newfoundland during the early spring. Arctic char, Greenland halibut and Atlantic salmon also feed on Arctic cod at various times. It is not known whether Arctic cod compete with capelin for the same food supply. Generally, in most areas they feed in colder waters than the capelin. However, off northeastern Newfoundland and southern Labrador, both Arctic cod and capelin have been observed in the same areas and great numbers of Arctic cod have been taken as a bycatch during the autumn, offshore capelin fishery.

Commercial Use

While Arctic cod are not at present harvested commercially by Canadian fishermen, large numbers have been obtained off Labrador by Soviet trawlers as a by-catch in the offshore capelin fishery. Said to be excellent as a table fish, in Russia where there is a commercial fishery for them, they reach the consumer market in several forms.

The development of a commercial fishery for Arctic cod in Canada may not prove a viable undertaking for east coast fishermen due to the sporadic appearance of the fish in their area. Further, any commercial harvest would have to be carefully controlled in view of the great importance of these fish in the Canadian Arctic food chain.

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Last updated : 2004-08-17

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