The flatfish are one of the most extraordinary fish groups in existence. There are something like 540 different kinds worldwide, all of them well adapted to a life on the sea bed.
Halibut go through a series of amazing changes – common to all flatfish – in which they lie on one side while one eye moves over to join the other on the upper side of the head. But no one has ever explained why some species usually have their eyes on the left, while others have them on the right. Flatfish are generally small to moderate sized, but the halibut (a right-eyed flatfish), reaching up to 2.4m (8ft) long, is a giant.
Size says it all
Any really big flatfish caught in British seas can be identified with certainty as a halibut. (The only rival for size it has among flatfish is the related Pacific halibut). In the case of moderate-sized flatfish, halibut recognition can be confirmed by the thickset body with both eyes on the right side of the fish, a medium sized head and a very big mouth, with large, sharp teeth in bothjaws.
The dorsal fm starts level with the front edge of the eye and runs almost the entire length of the body; the tail fin is slightly concave at the edge. A good identifying feature is the strong curve in the lateral line over the pectoral fin.
The halibut is usually a dull greeny brown or olive grey (though is sometimes almost black) on the upper side and pearly white on the blind side.
Down in the deeps
A cold water (but not polar) flatfish, the halibut lives down as deep as 1500m (5000ft) but can be caught in just 100m (330ft), or even from the shore in areas where the land drops off steeply into the sea. It inhabits different depths at different times in its life. Young fish are mostly found in shallow water, but are extremely hard to catch. After about the age of four they move into deeper water.
Large males are usually found very deep, even on the edge of the continental shelf. The really big specimen fish are simply huge – fish of 1.8m (6ft) long are sometimes still caught, and there are true records of fish nearly twice that length.
The halibut lives on a variety of sea beds from sand and gravel to rocky bottoms. The larger fish probably shelter between pinnacles of rock. However, unlike most flatfish, it is not confined all the time to the sea bed, but hunts actively off the bottom. It feeds on a wide range of near-bottom living fish (haddock, redfish and small skate), squid and some crustaceans. Young fish living in shallower water eat small fish such as herring and sandeels, as well as flatfish and crustaceans, including large prawns.
Spawning takes place in late winter and early spring near the sea bed in deep water and close to the edge of the continental shelf. The halibut produces large numbers of eggs (a 200 lb/90kg female is estimated to contain more than two million).
Although spawning takes place close to the sea bed the eggs – once fertilized – float up to about 90m (300ft) below the surface. The young fish, about 7mm (Vsin) long at hatching, rise towards the surface and are swept shorewards at the same time. By the time they are about 4cm (lMdn) long, their eyes have moved over to the right side of the head, and they begin to live on the bottom.
Since the halibut spawns in such deep water and the young fish are difficult to catch, there are many details of their breeding cycle which are not well known. Sadly, the fish is also becoming scarcer.