The Polar cod or Arctic cod (Boreogadus saida) is a fish of the family Gadidae, related to the true cod (genus Gadus). Note that there is another fish with the common name Arctic cod, Arctogadus glacialis.

The Polar cod has a slender body, deeply forked tail, projecting mouth and a small whisker on its chin. It is plainly coloured with brownish spots and a silvery body. It grows to a length of 30 cm. The Polar cod is found further north than any other fish species (beyond 84 degrees N) with a distribution spanning the Arctic seas off northern Russia, Alaska, Canada, and Greenland.

This fish is most commonly found at the water's surface, but is also known to travel at depths greater than 900 m. The Polar cod is known to frequent river mouths. It is a hardy fish that survives best at temperatures of 0-4 degrees Celsius but may tolerate colder temperatures owing to the presence of antifreeze protein compounds in its blood. They group in large schools in ice-free waters.

The Polar cod feeds on plankton in the upper water column (unlike their relative, the Atlantic cod, which feeds on the bottom). It is in turn the primary food source for narwhals, belugas, ringed seals and seabirds. They are fished commercially in Russia and are considered an excellent table fish.

http://www.fishbase.org/Summary/SpeciesSummary.cfm?genusname=Boreogadus&speciesname=saida