(1) There are the true deep-sea fishes that live at the bottom of the sea, for example cod, haddock, plaice, sole. These are called “demersal” fish. Fish, like birds, inhabit a medium that is continuous throughout the world. A glance at the map of the world will show that the three great oceans—Atlantic, Indian and Pacific—are united in the southern hemisphere. In Tertiary times, it is practically certain that the Pacific and the Atlantic oceans were also united at Darien, and that the Mediterranean was united with the Red Sea. Apart, therefore, from differences in local conditions, for example of temperature and food supply, there is practically no obstacle to the world-wide distribution of any particular species of fish. At the bottom of the sea, the temperature, the food supply, and the general conditions of life are singularly uniform all over the world, consequently there are no barriers at all to the dispersion of demersal fish, and we find various species widely distributed in all seas. Demersal fish, on the whole, are more primitive in type than those that live nearer the surface. They have well-developed senses of touch and smell by means of which they hunt for their food. They differ markedly in structure and shape from surface or shallow-water fish, their bodies being designed to resist the greater pressure of deep water. The body is generally lean and is enclosed by a wall of muscular fibre. Shallow-water fish, if introduced into deep water, would be crushed inward by the pressure. Similarly, the deep-living, demersal fish are unable to accommodate themselves to shallow water and, if placed in it, soon become unhealthy. A cod floats helplessly on its side when placed in shallow water, owing to the dilatation of its swimming bladder. If the bladder is pricked it collapses, and the fish is able to regain an upright position. This is done when cod and other similar demersal fish are kept alive in sea-water tanks on board ship, to be delivered to the markets alive. In Denmark, fish are delivered alive to the shops. When fishes from great depths are brought to the surface, their bodies break into pieces owing to the reduced external pressure, the scales start from their skin and the eyes from their sockets.
There are two distinct types of demersal fish: the “round” and the “flat.” The body of a round fish is more or less circular in cross-section, for example cod, while that of a flat fish is flattened, for example sole, ray.
The most important edible demersal fish can be classified as follows—
(a) The Gadidae—related to the cod.
Cod—inhabits northern waters, notably the North of Britain, Iceland and Newfoundland.
Ling—inhabits northern waters: West of Scotland and Ireland, and North towards Iceland and Newfoundland.
Haddock—inhabits northern waters. Nearly half the total catch is obtained in the North Sea, from the White Sea to the Bay of Biscay.
Whiting—found in great numbers in the North Sea. It is more coastal than the cod or haddock.
Hake—found from Norway to the Mediterranean. The greater part of the catch is obtained off the south-west of Ireland. Hake is also caught off Morocco and in the Bay of Biscay.
(b) The Pleuronectidae—related to the plaice and sole.