THE YOUNG FISH 



become more and more pigmented and are called 'elvers', and these migrate 

 up river, even across damp grass etc., to ponds where they live until adult. 



At the onset of maturity, they migrate downstream, becoming silvery 

 in colour and with enlarging eyes, and so out to sea. The ovaries of the 

 females at this stage arc enlarging but are far from properly mature, and the 

 males show little sign of development. They thus disappear into the sea, 

 never to return, and the next we know is the young new generation from 

 the Sargasso on its drifting passage back across the Atlantic. This raises 

 interesting problems and theories. As there is a certain amount of overlap 

 between the spawning grounds of both the European and the American 

 eel, how do the larvae become so completely segregated ; Never a European 

 larva seems to go to America and never an American to Europe ! An attempt 

 has been made to explain this on the length of life of the leptoccphalus 

 stage which in the American eel is about one year but in the European three, 

 corresponding to the time necessary to make their respective journeys. Thus, 

 should an American type of leptoccphalus be carried towards Europe it 

 would never have time to make the journey. Should any European type be 

 drifted to America it would arrive too soon in its development and would 

 not react properly to the presence of the river water. Thus it would not 

 migrate up the rivers but go on being carried about in the ocean currents 

 and so become lost. 



As recently as 1959 Dr. Tucker, lately of the British Museum, suggested a 

 new hypothesis to account for this strange life history and migration. Briefly it 

 is this. He considers that the European adult eel cannot reach the spawning 

 grounds 3,500 miles away as the gut is rapidly disintegrating with the onset 

 of maturity; they camiot feed and they could not cover such a distance and 

 retain sufficient vitality to spawn even if they reached there — laying 10 

 million eggs is no small achievement. Their instinct is to attempt to do so, 

 but they soon die. Only the American eels, which have so much less 

 distance to travel and are larger and in better condition, reach the spawning 

 ground. Eggs laid to the cast of the Sargasso Sea, as they rise through the 

 water layers, are thought to reach a lower temperature than those in the 

 west and thus grow to be European eels with 110-119 vertebrae and are 

 carried by the current system eastwards across the Atlantic. Eggs laid to 

 the west may rise through a layer with a sudden temperature increase and 

 grow to be American eels with 103-111 vertebrae and are carried to the 

 American coast. We have already seen (p. loi) that this is not an improbable 

 effect of temperature at just this critical period. The length of life of the 

 leptoccphalus is considered to be dependent upon the conditions of its 

 environment and not a factor dependent upon heredity. Its metabolic rate 

 will be higher in the warmer waters towards America and it will be ready 



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