280 THE VERTEBRATE ANIMALS 



food of larger ones. The nitrogen balance was maintained through 

 the excretions of the animals and their death and decay. 



The marine world is a great balanced aquarium. The upper 

 layer of water is crowded with all kinds of little organisms, both 

 plant and animal. Some of these are microscopic in size; others, 

 as the tiny crustaceans, are visible to the eye. On these little 

 organisms some fish feed entirely, others in part. Such are the 

 menhaden l (bony, bunker, mossbunker of our coast), the shad, 

 and others. Other fishes are bottom feeders, as the blackfish and 

 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. 



The Egg-laying Habits of the Bony Fishes. The eggs of most 

 bony fishes are laid in great numbers at the time of spawning. 

 This number varies 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 summer. At the 

 time of spawning the male usually deposits milt, consisting of mil- 

 lions of sperm cells, in the water just over the eggs, thus accomplish- 

 ing fertilization. Some fishes, as sticklebacks, sunfish, toadfish, etc., 

 make nests, but usually the eggs are left to develop by themselves, 

 sometimes attached to some submerged object, but more frequently 

 free in the water. In some eggs a tiny oil drop buoys up the egg 

 to the surface, where the heat of the sun aids development. They 

 are exposed to many dangers, and both eggs and developing fish 

 are eaten, not only by birds, fish of other species, and other water 

 inhabitants, but also by their own relatives and even parents. 

 Consequently a very small percentage of eggs ever reach maturity. 



1 It has been discovered by Professor Mead of Brown University that the in- 

 crease in starfish along certain parts of the New England coast was in part due 

 to over-fishing of menhaden, which at certain times in the year feed almost entirely 

 on the young starfish. 



