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Habitat and biology

The haddock is a demersal species which occurs at depths from 10 to 450 m (33 to 1,500 ft; 5.5 to 250 fathoms), although it is most frequently recorded at 80 to 200 m (300 to 700 ft; 40 to 100 fathoms). It is found over substrates made up of rock, sand, gravel or shells and it prefers temperatures of between 4 and 10 °C (39 and 50 °F). Off Iceland and in the Barents Sea, haddock undergo extensive migrations, but in the north western Atlantic its movements are more restricted, consisting of movements to and from their spawning areas. They reach sexual maturity at 4 years old in males and 5 years old in females, except for the population in the North Sea which matures at ages of 2 years in males and 3 years in females. The overall sex ratio is roughly 1:1, but in shallower areas, females predominate, while the males show a preference for waters further offshore.

The fecundity of the females varies with size: a fish of 25 cm (10 in) length bears 55,000 eggs while a fish at 91 cm (36 in) has 1,841,000 eggs. Spawning takes place from depths of around 50 to 150 m (200 to 500 ft; 30 to 80 fathoms). In the northwestern Atlantic spawning lasts from January to July, although it does not occur simultaneously in all areas, and in the northeastern Atlantic the spawning season runs from February to June, peaking in March and April. The eggs are pelagic with a diameter of 1.2 to 1.7 millimetres (3 ⁄ 64 to 9 ⁄ 128 in), and they take one to three weeks to hatch. 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. Their growth rate shows considerable regional variation and fish at one year old can measure 17 to 19 cm (6 + 1 ⁄ 2 to 7 + 1 ⁄ 2 in), at 2 years old 25 to 36 cm (10 to 14 in), up to 75 to 82 cm (29 + 1 ⁄ 2 to 32 + 1 ⁄ 2 in) at 13 years old. Their lifespan is around 14 years. 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. The fish which spawn in inshore waters are normally smaller and younger fish than those which occur in offshore areas. The younger fish have a spawning season which is less than half of that of the larger and older stock offshore. Once hatched the larvae do not appear to travel far from their spawning grounds, however some larvae spawning off the west coast of Scotland are transported into the North Sea through the Fair Isle - Shetland Gap or to the northeast of Shetland.

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. Once they have reached the settled, demersal, post-larval stage, they gradually switch from pelagic to benthic prey. Adults primarily feed on benthic invertebrates such as sea urchins, brittlestars, bivalves and worms, however, they will feed opportunistically on smaller fish such as capelin, sandeels and Norway pout. Juvenile haddock are an important prey for larger demersal fish, including other gadoids, while seals prey on the larger fish.☃☃

The recorded growth rates of haddock underwent significant change over the 30 to 40 years up to 2011. Growth has been more rapid in recent years, with haddock attaining adult size much earlier than was noted 30–40 years ago. However, the degree to which these larger, younger fish contribute to reproductive success of the population is unknown. The growth rates of haddock, however, have slowed in recent years. There is some evidence which indicates that these slower growth rates may be the result of an exceptionally large year class in 2003. The haddock stock periodically has higher than normal productivity; for example in 1962 and 1967, and to a lesser extent, 1974 and 1999. These result in a more southerly distribution of the fish and have a strong effect on the biomass of the spawning stock, but because of high fishing mortality, these revivals do not have any lasting effect on the population. In general, there was above average recruitment from the 1960s up to the early 1980s, similar to recruitment for Atlantic cod and whiting, this has been called the gadoid outburst. There was strong recruitment in 1999 but since then, the recruitment rate has been very low.