Sentinel Fisheries - Greenland Halibut
The Greenland halibut is a deep-water flatfish which is known to many people under many names. To Americans it’s the Greenland halibut, to eastern Canadians the “Greenland turbot” or “Newfoundland turbot”.
This marine fish is similar to the common or Atlantic halibut, except that it is much smaller (reaching a maximum size of 120 centimetres and a weight of 25 kilograms). The upper side is also darker in colour. Hence, it’s other names: the black or blue halibut and the lesser or mock halibut.
In the past, Canadian fishermen were forced to compete with many foreign fleets seeking the Greenland halibut in the northwestern Atlantic. Since the 1977 establishment of the 200-mile fishing zone, however, the foreign effort has been phased out in many areas, and the Canadian harvest of this lucrative species has vastly improved.
Greenland halibut thrive in the cold, northern waters of the Pacific and Atlantic oceans, and is most plentiful wherever there are rich stocks of sea prawn. In the northwestern Atlantic, they are especially abundant in the deep coastal bays or fjords of West Greenland, off the continental shelf of Baffin Island and in the Ungava Bay area of Hudson Strait. They are also found at greater depths along the continental slope of Labrador, and in the deepwater bays of northeastern Newfoundland. Upon approaching maturity there appears to be a general migration to Davis Strait.
Though the abundance of these fish diminishes south of the northern slopes of Newfoundland’s Grand Banks, Greenland halibut have also become relatively plentiful in the Gulf of St. Lawrence in the last few years. A prespawning winter concentration occurs in the Laurentian Channel to the southwest of St. Georges Bay, and summer feeding concentrations are found at the mouth of the St. Lawrence River to the north of Anticosti. The southeastern tip of the Scotian Shelf appears to mark the southern edge of this species’ distribution.
Although Greenland halibut can be found in small numbers at depths of less than 100 metres, most of them are caught near the sea bottom at depths of between 200 to 600 metres. In the southern part of the range, however, they go as deep as 1,500 metres. Optimum water temperature for the species ranges from 1° to 3°C. Ordinarily, Greenland halibut can tolerate a wide range of temperatures, but this is less true during the reproductive phase.
The Reinhardtius hippoglossoides belongs to an order of flat, bilaterally symmetrical fish, the Pleuronectiformes. The members of this order undergo an amazing transformation during the larval stage.
They begin life swimming with the dorsal fin upwards, like any salmon or trout. Gradually, however, one eye migrates across the top of the larva’s skull to position itself close to the eye on the other side of the head. There are corresponding modifications to the skull bones, nerves and muscles. The eyeless side, for example, becomes flat while the other side grows slightly rounded. Then, the developing fish turns over and swims on its flat, eyeless side.
A few features distinguish the Greenland halibut from other flatfish. Normally, the eyes of flatfish are located on the top, coloured side of the body, and the blind side is white. Most such fish in the north Atlantic are rightsided. That means that individuals of the species lie on the left side as the eye migrates from the left to the right during larval development. In the Greenland halibut, however, the left eye has not completely migrated to the right side, but is located on the upper edge of the forehead. Moreover, the blind side is not white, but dark grey, while the other side is nearly black. Furthermore, the fish is not perfectly symmetrical so that some members of the species, those smaller fish that tend to swim in the middle levels of the ocean rather than along the seabed, have been known to swim with the dorsal fin upwards.
These special characteristics make the Greenland halibut unusually mobile, and the position of the left eye allows it a greater field of vision than is possessed by most flatfish.
The eggs, as many as 160,000 from a single fish, drift in the middle depths for some weeks, later rising in the form of larvae into the surface waters where they are carried by currents.
In the Gulf of St. Lawrence, mature Greenland halibut concentrate and spawn in winter in the Laurentian Channel off the southwest tip of Newfoundland. The young fish then move to nurseries on the north side of Anticosti Island where small shellfish provide plentiful food.
The Greenland halibut is a voracious feeder. Whole, yet only slightly smaller fish of species which share the same niche in the ocean have been found in the stomachs of halibut.
Small fish (less than 20 centimetres in length) feed on plankton and shrimplike crustaceans, while larger fish (up to 80 centimetres) in the southern Labrador and Newfoundland areas, eat mainly capelin. Those that swim in the deep channels of northern Labrador and West Greenland live mainly on shrimp. Very large halibut feed heavily on larger fish such as squid, cod, redfish and even other Greenland halibut.
The numbers of male and female Greenland halibut are roughly equal, as is their growth rate, until they reach a length of about 45 centimetres at the age of six to seven years. After that, the abundance of males decreases, and those remaining grow much more slowly than the females. Fish larger than 90 centimetres are all female. The reason is that much of the energy previously used for body growth by the early maturing males is subsequently diverted to the formation of products needed for egg fertilization. Females also tend to live longer, with specimens as old as 20 years being recorded, while males seldom live longer than 12 years.
Casualty rates are high among Greenland halibut. The most significant predator of adults is the Greenland shark, found in great numbers in the same waters and at the same depths as halibut. Fishermen frequently find, on retrieving hooked lines, that their catch has been mutilated by the sharks. Other important predators of adult fish are seals and two species of Arctic whales – the white whale and the narwhal.
The larvae of Greenland halibut are eaten by cod and salmon, whereas the young, bottom-dwelling fish and medium-sized adolescents are eaten by cod and by larger Greenland halibut.
Except during spawning, they are able to range from ocean surface to seabed.
For management purposes, Greenland halibut are treated as three stocks in the Northwest Atlantic: Baffin Island/West Greenland, Labrador/East Newfoundland Stock, Gulf of St. Lawrence.
Smaller fish (less than 20 centimetres) are frequently taken as a by-catch by the shrimp trawlers working bottom depths of 200 to 400 m or in the salmon drift nets near the surface. Larger fish tend to be taken from deeper areas by longliners and deepsea trawlers in depths of over 1,000 m.
The fishery for Greenland halibut is rather new to the Gulf of St. Lawrence with landings up to 1976 varying from 600 to 1,000 t each year. In 1977 and 1978, landings increased to 4,000 t, and the next year, it more than doubled to almost 10,000 t. The 1980 catches however, declined to just over 7,000 t.
Until 1979, most of the landings represented by-catches taken during summertime on the north side of Anticosti Island by Quebec fishermen fishing for shrimp. Some of the harvest was also taken by Newfoundland trawlers to the southwest of St. Georges Bay in the Laurentian Channel where the Greenland halibut form a pre-spawning concentration during the winter.
Since 1979, however, the fishery has been carried out almost exclusively by the Quebec gillnet fishermen fishing for Greenland halibut on the southwest side of Anticosti Island, and near the mouth of the St. Lawrence River.
Adaptation of document : Turbot (Greenland Halibut), by Bowering, W. R., Coll. Underwater World, published by Communications Directorate, Fisheries and Oceans Canada, 5 p. 1984.