Atlantic halibut

The Atlantic halibut, Hippoglossus hippoglossus, is a flatfish of the family Pleuronectidae. They are demersal fish living on or near sand, gravel or clay bottoms at depths of between 50 and 2000 m. The halibut is among the largest teleost (bony) fish in the world. Halibut are strong swimmers and are able to migrate long distances. Halibut size is not age-specific, but rather tends to follow a cycle related to halibut (and therefore food) abundance.

The native habitat of the Atlantic halibut is the temperate and arctic waters of the northern Atlantic, from Labrador and Greenland to Iceland, the Barents Sea and as far south as the Bay of Biscay and Virginia. It is the largest flatfish in the world, reaching lengths of up to 4.7 m and weights of 320 kg. Its lifespan can reach 50 years.

Description
The Atlantic halibut is a right-eyed flounder. It is flattened sideways and habitually lies on the left side of its body with both eyes migrating to the right side of its head during development. Its upper surface is a uniformly dark chocolate, olive or slate colour, and can be almost black; the underside is pale. The end of the caudal fin is concave. Young fish are paler with more mottled colouration.

Biology
The Atlantic halibut has a relatively slow growth rate and late onset of sexual maturity, with males attaining maturity at seven to eight years old, females at 10 to 11 years, and individuals are thought to live up to 50 years. Little is known about their breeding except their spawning is seasonal, although its timing varies somewhat with location. In the eastern Atlantic, spawning occurs chiefly in March, April and May, although may span from January to June. Off the American coast, however, the spawning season appears to continue through the summer as late as September. After spawning, both sexes migrate northwards in search of food. Young Atlantic halibut individuals feed on crustaceans such as crabs and prawns. These halibut lie motionless and invisible on the sea bed, capturing any fish that pass within reach, although they may also hunt for fish in open water.

Habitat
This marine fish usually lives on the ocean floor at depths between 50 and 2000 m, but it occasionally comes closer to the surface. The larvae are pelagic, drifting relatively helplessly, but at around 4 cm, they migrate to the bottom. Young between the ages of two and four years live close to the shore, moving into deeper waters as they grow older.

Role in ecosystem
The Atlantic halibut occupies a relatively high trophic level in the food chain.

Diet
The diet of the Atlantic halibut consists mainly of other fish, e.g. cod, haddock, herring, pogge, sand eels and capelin, but it will also eat cephalopods, large crustaceans and other benthos organisms.

Predators
Atlantic halibut are eaten by seals, and are a staple food of the Greenland shark.

Commercial fishing
The Atlantic halibut was formerly a very important food fish, but due to its slow rate of population growth, it is unable to recover quickly from overfishing, and the fishery has largely collapsed. Consequently, fish labelled as "halibut" is usually one of the other large flatfishes, often Pacific halibut, Hippoglossus stenolepis.

Farming
Due to its popularity as a food fish, Atlantic halibut has attracted investment in fish farming. As of 2006, five countries - Canada, Norway, the UK, Iceland and Chile - were engaged in some form of Atlantic halibut aquaculture production.

Conservation status
Following overfishing, the Atlantic halibut now faces a high risk of extinction in the wild, and in 1996 the IUCN rated it as Endangered and placed it on its Red List.

The Atlantic halibut is a US National Marine Fisheries Service Species of Concern. The American Fisheries Society has classified the species as "Vulnerable". In 2010, Greenpeace International added the Atlantic halibut to its seafood red list of "fish that are commonly sold in supermarkets around the world, and which have a very high risk of being sourced from unsustainable fisheries."