THE VERTEBRATES 423 



annual catch is in some years near 100,000,000 pounds and 

 worth to the fishermen about $2,000,000. The cods live in 

 deep water (20 to 100 fathoms), and are captured only with 

 baited hooks and lines. A cod over six feet long and weigh- 

 ing over 200 pounds was once taken; but from 12 to 40 

 pounds are the sizes usually caught. They spawn near the 

 shores of New England between December and April. The 

 United States Fish Commission hatcheries liberate more 

 than 75,000,000 young fry annually. It is easy to collect the 

 cod eggs for hatching, for in a 20-pound female, the ovaries 

 (popularly called " roes ") contain more than 2,500,000 eggs, 

 which are so small that a quart bottle will hold about 335,000 

 eggs. Think of how abundant codfishes would be if all the 

 eggs of a thousand females were to hatch and grow to 

 maturity, and one-half of these were to be equally prolific 

 females. However, since cods do not appear to be either 

 increasing or decreasing rapidly, we are justified in concluding 

 that, on the average, two eggs from each female produce 

 mature individuals (the two sexes about equal in number). 

 The others are destroyed by enemies or die from diseases. 

 This is a good illustration of the intensity of the struggle for 

 existence, which, to a great extent, affects all animals and 

 plants ( 499). 



Next to the Columbia salmon and the cod, the shad is the 

 most important fish caught in waters of North America. It is 

 captured in the spring when it ascends rivers to spawn. The 

 annual catch is about 14,000,000 fish, weight 50,000,000 

 pounds, and worth more than $1,600,000. The fact that the 

 shad is taken only at the spawning season would long ago 

 have made the fisheries unprofitable if the United States 

 Fish Commission had not engaged in artificial propagation. 

 More than 200,000,000 young shad are annually " planted " 

 in the rivers of the Atlantic Coast. They have been intro- 

 duced since 1871 on the Pacific Coast, where they do not 

 naturally occur; and have now become abundant in the 



