Distribution. Fishes are found in practically every ocean, lake and river in the world, with a few notable exceptions, such as the Dead Sea, in which the concentration of salt is too high. They appear to exist at all depths of water, and have been found in the sea as deep down as 2,720 fathoms. Fish living at this depth generally possess enormous mouths, long, attenuated, soft bodies, and are equipped with highly developed phosphorescent organs.
The distribution of a particular species appears to depend upon the salinity of the water, the temperature of the water, the kind and quantity of food available and the prevailing intensity of sunlight. It is possible to divide fish into four well-defined groups, according to the salinity of the water in which they are found: (1) Marine fish: those that live always in the sea, for example herring, haddock, shark. (2) Fresh-water fish: those that live always in fresh water, for example carp, trout, pike. (3) Many fish live in brackish water, and appear to be able to accommodate themselves easily to considerable changes in salinity, e.g. sticklebacks, gobies, grey mullets and blennies. Such species naturally are widely distributed; thus, a particular kind of grey mullet (Mugil capito) is found without any appreciable difference in form on nearly every coast of the Atlantic Ocean. (4) The fourth group of fish are migratory. Some species, for example salmon and shad, live and develop in salt water, but ascend rivers to spawn, i.e. to lay their eggs, in fresh water. Others, such as eels and certain pleuronectids, for example the flounder, live and develop in fresh water, and descend rivers to the sea to spawn. Many fresh water fish, e.g. trout, forsake the large streams in the spring and ascend small brooks, where the young can be reared in greater safety.
Of these different groups or species, the marine fishes are industrially by far the most important, for at least two-thirds of all the fish in the world live in the sea, and the capture of these sea-fish in enormous quantities constitutes the fishing industry, with which we are concerned.
The different species of marine fishes can be divided into three well-marked groups, according to their habits and habitats.
Fig. 2
COD (Gadus morrhua)
Length up to 5 ft.; usually caught at about 3 ft.
Food.—Small crustaceans, molluscs, and young fish.
Range.—North of Norway and Iceland to the Bay of Biscay, and from Greenland to New York.