The haddock (Melanogrammus aeglefinus) is a saltwater ray-finned fish from the family Gadidae, the true cods.

It has a distinctive black lateral line contrasting with the whitish background colour and which curves slightly over the pectoral fins.

It also has a distinctive oval black blotch or ‘thumbprint’, sometimes called the "Devil's thumbprint",[8] which sits between the lateral line and the pectoral fin,[9] a feature which leads to the name of the genus Melanogrammus which derives from Greek "melanos" meaning "black" and "gramma" meaning letter or signal.

Occasionally there are differently coloured variants recorded which may be barred, golden on the back or lack the dark shoulder blotch.

[6] The largest stocks are in the North Sea, off the Faroe Islands, off Iceland and the coast of Norway but these are discrete populations with little interchange between them.

The overall sex ratio is roughly 1:1, but in shallower areas, females predominate, while the males show a preference for waters further offshore.

Following metamorphosis, the past larval fish remain pelagic until they attain a length of around 7 cm (3 in), when they settle to a demersal habit.

[6] The most important spawning grounds are in the waters off the central coast of Norway, off the southwest of Iceland, and over the Georges Bank.

[12] In their larval stages, haddock mainly feed on the immature stages of copepods, ostracods and limacina with their diet changing as they grow, moving on to larger pelagic prey such as amphipods, euphausiids, eggs of invertebrates, zoea larvae of decapods and increasing numbers of copepods.

For example, the cod worm, Lernaeocera branchialis, starts life as a copepod, a small, free-swimming crustacean larva.

The first host used by cod worm is a flatfish or lumpsucker, which they capture with grasping hooks at the front of their bodies.

[18][19] The female worm, with her now fertilized eggs, then finds a cod, or a cod-like fish such as a haddock or whiting.

There, the worm clings to the gills while it metamorphoses into a plump, sinusoidal, wormlike body, with a coiled mass of egg strings at the rear.

There, firmly rooted in the cod's circulatory system, the front part of the parasite develops like the branches of a tree, reaching into the main artery.

[10][18][19] The haddock was first formally described as Gadus aeglefinus in 1758 by Carolus Linnaeus in the 10th edition of volume one of his Systema naturae with a type locality given as "European seas".

[21] The 5th edition of Fishes of the World classifies the haddock within the subfamily Gadinae, the typical cods, of the family Gadidae, which is within the superfamily Gadoidea of the order Gadiformes.

All the stocks in eastern Atlantic are assessed by ICES, which publish a recommendations on an annual basis for Total Allowable Catch.

[11] Haddock currently resides on the Greenpeace seafood red list due to concerns regarding the impact of bottom trawls on the marine environment.

The haddock populations in the western Atlantic (offshore grounds of Georges Bank off New England and Nova Scotia) are also considered to be harvested sustainably.

[41] Unlike cod, haddock is not an appropriate fish for salting and preservation is more commonly effected by drying and smoking.

[43] Grimsby fish market sources its haddock from the North East Atlantic, principally Iceland, Norway and the Faroe Islands.