196 THE OCEAN RIVER 



until it becomes three inches long, though still in appearance 

 a typical leptocephalus. When it arrives at its destination 

 in the river mouth it undergoes a great change. The leaflike 

 shape disappears and it becomes narrow and round and begins 

 to look like a miniature of its parents. In the rivers and streams 

 where it takes up its home for most of its remaining life it 

 grows to be a foot or more in length. Then, as much as twenty 

 years later, it begins the journey back to the spawning grounds 

 of the Sargasso Sea, where a new generation begins the long 

 ocean circuit. 



Eels return to their original deep-sea home when they are 

 ready to breed, but salmon follow an opposite course. From 

 the age of eighteen months or so they live on the floor of the 

 ocean, where they feed on abundant red shrimps. At maturity 

 they are impelled to seek the river in which they were hatched 

 and to return to the headwaters for spawning. Still other 

 fishes, like the tuna, confine their long migrations to salt 

 water, but we are no better able to explain their uncanny 

 ability to follow the same path, generation after generation. 



The European tunny spend their winters in the subtropical 

 Atlantic in deep water and grow there to maturity. During 

 the spring they move northward by way of the coasts of 

 Spain and Portugal, where they spawn. Large numbers of 

 them enter the Mediterranean, but others follow the herring 

 and mackerel schools and so reach the North Sea. We know 

 little of the return of older fishes and of the first southward 

 migration of newly hatched tunny to the subtropical deep 

 waters. 



The same is true of the bluefin tuna of the western Atlantic, 

 which is believed to be a separate offshoot of the same species 

 as the European tunny. It is rarely that any but scattered indi- 

 viduals of these giant mackerels are seen south of the Florida 

 Straits. Somewhere about the end of May, however, the first 

 schools begin to appear off Cat Cay in the Florida Current, 

 directly opposite the Straits from Miami. They average 500 



