FISHES 145 



young, cause a great economic loss to the fisheries 

 of the United States. 



Migrations 



It has been shown that some fish make migrations 

 for the purpose of spawning. The salmons, shad, ale- 

 wives, striped bass, and sturgeons migrate from salt 

 to fresh water for that purpose, and are said to be 

 anadromous. The fresh- water eel, on the other hand, 

 is catadromous, for it migrates from fresh water to 

 the sea to spawn. The general movement of the cod 

 is reported to be shoreward during the spawning 

 season. All species, however, do not make special 

 migrations for the purpose of spawning, but deposit 

 their eggs on suitable grounds in the general vicinity 

 of their habitat. 



Migrations for food are made to a greater or lesser 

 extent by all species. It is readily conceivable, how- 

 ever, that fish which travel in schools have to cover 

 more space to find sufficient food than the solitary 

 dwelling ones. Migrations brought about by the 

 season of the year, or by temperature, are so closely 

 associated with migrations for food that it is fre- 

 quently difficult to determine whether the movement 

 toward warmer water is for the purpose of escaping 

 from a low temperature or to find a more abundant 

 supply of food. 



It is not surprising that some marine fishes wan- 

 der considerable distances in their watery home. It 

 is even possible that occasionally a few of the wan- 

 dering species may cross an ocean. On the other 

 hand, the majority of marine fishes probably do not 



