Cod is scientifically known as Gadus belongs to the family Gadidae. Cod is a common name used for all the members of this family. Cod is a much admired food fish characterized by dense, blistering white meat, stumpy fat content and easygoing savor. Cod livers are used for manufacturing cod liver oil which is an imperative source of vitamin A, vitamin D, vitamin E, omega-3 fatty acids, EPA and DHA. The larger cod caught at the time of spawning are known as skrei. The young Atlantic cod or haddock prepared in stripes for cooking is known as scrod. The Atlantic cod has the property of changing colour at different depths of water. It has two colour phases, the grey-green and reddish-brown. It weighs about 5-12 kg but specimens weighing about 100 kg have been recorded. Cod feeds on molluscs, worms, squid, crabs and starfish. Some fishes migrate to warm waters in winter in order to spawn. A large female is known to lay up to 5 million eggs in the mid-ocean out of which only few are known to survive. The saltwater cod are also famous along the British Columbia, Canada and Northwestern US coastal areas. These fishes are darker in colour and are about three times larger than their fellows of the eastern coasts.
Pollock and coalfish are also found on the identical grounds where cods dwell. Pollock have shovel-like tails with pale lateral lines and measure about 1m in length with 15 kg body weight. Some grows to about 2m. The meat of cod is clammy, flaky and becomes white when cooked. Cod are popular ingredients of fish and chips in United States and Atlantic along with haddock. They are also very popular in Spain and Portugal. At present cod are at risk in the United States and Atlantic due to habitat destruction and overfishing. Cods enclose three rounded dorsal and two anal fins. The pelvic fins are somewhat small and set in front of the pectoral fins. The first ray of the pelvic fin is extended and is set just behind the gill cover. The upper jaw extends over the lower jaw and is demarcated by the presence of a chin barbel. They have a distinct white coloured lateral line running from the gill slit just above the pectoral fins. The eyes are medium-sized and are of the same length as that of the chin barbel. The back tends to be greenish or sandy brown while the belly is white in colour. The individuals inhabiting the deeper areas like the rocks are fairly dark in colour so darker shades are not uncommon.
Spawning occurs in the months of January- April at the depth of 200 meters and at the temperature range of 4-6°C in specific spawning grounds. Pre-spawning courtship involves display of fins and grunting by male which results in pair formation. While spawning, the pair swims in circle. Eggs are planktonic and hatch between 8-23 days. The hatched larvae are 4 mm in length. The planktonic phase lasts for about 10 weeks and the larvae amplify in size about 40 times supplementary and become 2 cm. The young ones now revolutionize their feeding habitat from planktons to small crustaceans like the diminutive isopods and minute crabs. They accomplish to a length of about 8 cm in the first six months and 18 cm by the end of first year. By the end of second year they become 25-35 cm. at higher altitudes the growth rate is less. They arrive at maturity when they are 3-4 years old and are 50 cm in length.
They prefer varied habitats like the grounds especially the inshore. They are gregarious fishes and live in schools. The shoals are frequently observed at the time of spawning. They are active hunters feeding on small crabs, haddock, whiting, sand eels, lobsters, squid, mussels, worms, mackerel, and molluscs. The younger fishes avoid larger prey. They are attacked by a number of parasites. The infestation of cod by the cod worm is very common. This worm is a crustacean which starts its life as a free swimming larva. The first host is the lumpsucker. The larva attaches to the lumpsucker with the hooks and sucks blood. They mature and the fertilized female carries her eggs to the cod. The life cycle is repeated. Cod dwell from warm sea waters to the deeper region of Northern Atlantic. Cod liver is tinned and eaten.