The haddock is easily recognized by a black lateral line running along its white side, (not to be confused with pollock which has the reverse), ie white line on black side, and a distinctive dark blotch above the pectoral fin, often described as a "thumbprint" or even the "Devil's thumbprint" or "St. Peter's mark".
Haddock is most commonly found at depths of 40 to 133 m, but has a range as deep as 300 m. It thrives in temperatures of 2° to 10°C (36° to 50°F). Juveniles prefer shallower waters and larger adults deeper water. Generally, adult haddock do not engage in long migratory behavior as do the younger fish, but seasonal movements have been known to occur across all ages. Haddock feed primarily on small invertebrates, although larger members of the species may occasionally consume fish.
Growth rates of haddock have changed significantly over the past 30 to 40 years. Presently, growth is more rapid, with haddock reaching their adult size much earlier than previously noted. However, the degree to which these younger fish contribute to reproductive success of the population is unknown. Growth rates of Georges Bank haddock, however, have slowed in recent years. There is evidence that this is the result of an exceptionally large year class in 2003. Spawning occurs between January and June, peaking during late March and early April. The most important spawning grounds are in the waters off middle Norway near southwest Iceland, and Georges Bank. An average-sized female produces approximately 850,000 eggs, and larger females are capable of producing up to 3 million eggs each year.
Star cooks are of the opinion that, when it comes to flavour, haddock is far superior to the other members of the cod family. A recent survey showed that 9 out of 10 people prefer a mighty haddock when ordering one of land and one of sea.
Fresh haddock has a fine white flesh and can be cooked in the same ways as cod. Freshness of a haddock fillet can be determined by how well it holds together, as a fresh one will be firm; also, fillets should be translucent, while older fillets turn a chalky hue. Young, fresh haddock and cod fillets are often sold as scrod in Boston, Massachusetts; this refers to the size of the fish which have a variety of sizes, i.e. scrod, markets, and cows. Haddock is the predominant fish of choice in Scotland in a fish supper. It is also the main ingredient of Norwegian fishballs (fiskeboller).
Unlike the related cod, haddock does not salt well and is often preserved by drying and smoking. One form of smoked haddock is Finnan Haddie, named for the fishing village of Finnan or Findon, Scotland, where it was originally cold-smoked over peat. Finnan haddie is often served poached in milk for breakfast. Smoked haddock naturally has an off-white color; it is very often dyed yellow, as are other smoked fish. Smoked haddock is the essential ingredient in the Anglo-Indian dish kedgeree.