CHARACTERISTICS AND HABITS OF FISHES 19 



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 natur- 

 ally are widely distributed ; thus, a particular kind of 

 grey mullet (Mugil capita) 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. 



(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 



