238 



A STUDY OF VERTEBRATES 



the sea bass, living almost entirely upon mollusks and crusta- 

 ceans. Still others are hunters, feeding upon smaller species of 

 fish, or even upon their weaker brothers. Such are the bluefish. 

 squeteague or weakfish, and others'. 



What is true of salt-water fish is equally true of those inhabiting 

 our fresh-water streams and lakes. It is one of the greatest prob- 

 lems of our Bureau of Fisheries to discover this relation of various 

 fishes to their food supplies so as to aid in the conservation and 

 balance of life in our lakes, rivers, and seas. 



Migration of Fishes. Some fishes change their habitat at dif- 

 ferent times during the year, moving in vast schools northward 

 in summer and southward in the winter. In a general way such 

 migrations follow the coast lines. Examples of such migratory 

 fish are the cod, menhaden, herring, and bluefish. The migra- 

 tions are due to temperature changes, to the seeking after food, 

 and to the spawning instinct. Some fish migrate to shallower 

 water in the summer and to deeper water in the winter ; here the 



reason for the migra- 

 tion is doubtless the 

 change in temperature. 

 The Egg-laying 

 Habits of the Bony 

 Fishes. The eggs of 

 most bony fishes are 

 laid in great numbers, 

 varying from a few 

 thousand in the trout 

 to many hundreds of 

 thousands in the shad 

 and several millions in 

 the cod. The time of 

 egg-laying is usually 

 spring or early sum- 

 mer. At the time of 

 spawning the male 



usually deposits milt, consisting of millions of sperm cells, in the 

 water just over the eggs, thus accomplishing fertilization. Some 



Development of a trout. 1, the embryo within the 

 egg ; 2, the young fish just hatched with the yoke 

 sac still attached ; 3, the young fish. 



