148 THE WEALTH OF THE SEA 



pollutions, or other causes is not present, the young 

 fish will perish. The periodical fluctuations in the 

 abundance of various species which frequently are 

 noticed are probably in a large measure due to fa- 

 vorable or unfavorable conditions prevailing on the 

 spawning grounds. An unfavorable spawning season 

 of any species is certain to be reflected in the com- 

 mercial catch at a time when the fish of such a season 

 should reach a marketable size. 



In most fishes the eggs are fertilized at the time 

 they are laid. In some forms, as in some of the skates 

 and rays, the eggs are fertilized internally and some- 

 time before they are laid. In some of the top min- 

 nows, as for example in Gamhusia, the important 

 mosquito-eating fish, the eggs are fertilized and also 

 hatched internally. The young are then brought 

 forth alive as well formed fish. Many of the sharks 

 also produce live young, and it is among these ani- 

 mals that the nearest approach to the process of 

 reproduction in mammals is found in fishes, for the 

 embryo shark probably obtains a part of its nourish- 

 ment from the blood of the mother shark. The baby 

 shark, however, is not suckled and receives no atten- 

 tion whatever from the parent after birth. 



The care of the eggs and the young for some time 

 after hatching is not unusual among fishes. The 

 members of the large sunfish family and the stickle- 

 backs build nests and guard their eggs and young, 

 and many of the marine catfishes have the interest- 

 ing habit of transferring the eggs to the mouth of 

 the male, where they are hatched and where the 

 young are guarded until they are ready to go out 



