Greenland halibut

Its morphology with the left eye positioned on the dorsal ridge of the forehead gives it an appearance of a cyclops when looking straight at it. The central position of the left eye in the Greenland halibut probably gives it a much wider range of peripheral vision in comparison to other flatfish, where the eye has migrated completely. Its body shape is elongated and compressed dorsoventrally and muscles on both sides are equally developed. Both sides are pigmented, but the left blind side is slightly lighter in color than the right side. The maximum length is about 120 cm (3.9 ft) and the maximum weight is about 45 kg (99 lb), the normal length is 80–100 cm (2.6–3.3 ft) and they usually weigh 11–25 kg (24–55 lb).[3]

Its physical appearance suggests it to be a vigorous swimmer that can swim in a vertical position. Vertical swimming has been observed during tagging experiments. However, video analysis of Greenland halibut behavior in front of a bottom trawl showed no sign of swimming in a vertical position. Though most Greenland halibut are caught in bottom-fishing gear (trawl, longline and gillnet), they have also been caught in surface drift nets, which indicates that they can have a pelagic occurrence. Stomach analysis has also shown that their diet consists mostly of pelagic or bathypelagic organisms. Though the Greenland halibut is a flatfish, it does at times behave more like a roundfish.[6]

The Greenland halibut is a cold-water species found at depths from near the surface to 2,200 m (7,200 ft), but mainly between 500 and 1,000 m (1,600 and 3,300 ft).[2] It is mainly found in waters with temperatures from 1 to 4 °C (34–39 °F), but has also been observed at subzero temperatures down to −2.1 °C (28.2 °F). It has a circumpolar distribution in the Northern Hemisphere and is found in both the North Atlantic and the North Pacific Oceans. In the North Pacific, it ranges from the Sea of Japan near Honshu northwards to the Chukchi Sea, east through the Aleutian Islands, and south as far as northern Baja California in Mexico.[2] In the North Atlantic, it occurs from the British Isles to northern Norway, Faroe Islands, Iceland, and eastern Greenland in the east and from Newfoundland to northwestern Greenland in the west.[3]

Close-up of the head, showing the strong teeth of this predatory species (upperside of fish on left, underside on right)

Although primarily a bottom fish, it is often caught pelagically and it is thought to swim on its ventral edge rather than on the blind side. It is an active, midwater predator that feeds on prawns, fish (such as Atlantic cod, Arctic cod, eelpouts, capelin, and redfish), and squid. It spawns at depths of 700–1,500 m (2,300–4,900 ft) during spring and summer, from April to July at temperatures of 3–5 °C (37–41 °F). Its eggs, larvae, and postlarvae are free-floating in deep water. The larvae complete their metamorphosis on attaining a length of 6–8.5 cm (2.4–3.3 in); the young fish tend to occur in shallower areas than the adults.[3]

The species is not threatened overall and it can be locally abundant, but it is near threatened. Until the 1970s, the species declined due to overfishing, but since the 1980s, an overall slight increase has been seen, although some local populations have further declined.[1] The primary threat is overfishing.[1] The species is a prolific breeder with each female producing tens to hundreds of thousands of eggs,[7][8] but this only happens when reaching maturity around 5 years or older (sometimes even 15 years), making it vulnerable to overexploitation.[1] Secondary threats are related to their deep-water spawning grounds. Some are in areas with oil and gas extraction.[9] Others spawning grounds are in deep near-shore regions where the ecosystem relies on nutrients from meltwater from glaciers, but these are gradually disappearing.[10]

In 2010, Greenpeace International added the Greenland halibut to its seafood red list because some fishing for the Greenland halibut is by bottom trawl, which can cause significant environmental damage, and some populations appear to be overfished.[11] In coastal regions and some offshore regions, fishing for Greenland halibut mainly is done by deep-sea long line fishing (out of reach of seabirds and too cold for sea turtles, issues for this fishing type elsewhere in shallower and warmer waters) and stationary bottom gillnets, which does not cause the same damage as bottom trawling.[12] In 2017, the Marine Stewardship Council certified that the fishery for Greenland halibut was sustainable.[13][14] Because Canada and Greenland share the offshore populations in the Davis Strait–Baffin Bay region in between them, the two have shared the fishing quota and follow the same guidelines in this region.[15] Offshore populations in this region are healthy, stable and well-managed, but in more coastal areas some populations have fallen due to overfishing, although they do get a regular input of young fish from the stable offshore populations.[12][15] Deals on fishing quotas for offshore fishing for Greenland halibut and other species have also been reached between Greenland, Faroe Islands, Norway, and Russia.[16] Among well-monitored populations, the ones in the East Greenland–Iceland region (i.e., Greenland Sea, Denmark Strait and nearby) experienced the greatest decline since the 1970s.[1] In 2019, the parties agreed on a reduction of fishing for Greenland halibut in the Greenland Sea region, following recommendations by biologists.[17][18] In some inshore waters of Greenland, fishing quotas for Greenland halibut have been temporarily raised several times, contrary to recommendations by fisheries biologists, and leading to recommendations of a clearer separation of decisions on quotas and the Greenlandic Government (decisions not fully left to Greenland's Fisheries Commission).[19][20][21] A smaller fishery for the species also exists in the Gulf of Alaska (where relatively uncommon) and the Bering Sea region (where more common), and these populations are not overfished.[7]

Since 1990, the annual total landings reported to the Food and Agriculture Organization have ranged between 87,243 and 146,080 metric tons (85,865–143,773 long tons). In 1999, the largest catches were from Greenland and Norway.[3] These, along with Canada, Iceland and Russia, account for about 75% of the catches of Greenland halibut. Other countries with significant catches of this species are the United States, Faroe Islands, Scotland, Germany, Portugal, and Spain (the last three using EU-quotas).[7][8][22] Fishing is the most important industry in Greenland and Greenland halibut is the second-most important species (after northern prawn), meaning that any changes can have a significant effect on the country's overall economy,[23] as well as the local economy as most inshore fisheries are by small-scale, small-boat fishers.[22] Similarly, the fishery for Greenland halibut is very important to some First Nations and Inuit communities in Canada.[9] In 2001, 1,078 metric tons (1,061 long tons) were landed in Scotland with a value estimated at £1,885,000.[24]

The oil-rich, soft meat is regarded as good, but inferior to that of the Atlantic halibut and European turbot. Traditionally, it was salted, but today it is mostly smoked or frozen and the primary market is in East Asia, where it is regarded as a delicacy.[3][8] However, because of the thick skin, high fat content, and low meat yield, as much as one-third of the fish can be lost in production.[24] In Greenland, the remains are often used as food for the sled dogs (Greenland dogs).[25]