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at any rate at least ca. 6 — 7° at a depth of ca. 1000 meters. But these conditions 

 are not found anywhere by the eels migrating out from'the fresh and 

 brackish waters of North Europe nearer than in the true Atlantic west of 

 the British Islands and France (and Faeroe— Shetland ridge). 



Further it is at once evident that organisms such as Leptocephalus brevirostris 

 cannot be at home in shallow coastal waters like the North Sea and Baltic. These small, 

 not specially rapidly moving forms, which in their whole structure are so obviously suited 

 to a purely oceanic, pelagic life and which require almost a whole year for their meta- 

 morphosis, cannot normally exist in these shallow waters, where they would be an 

 easy prey to many other fishes and further would be continually exposed to the danger 

 of being driven on the coasts and beaches. It is just in this that we may see the deeper 

 reason, why the metamorphosis of the eel can only take place so far from the coasts out 

 in great depths, as, so long as it is necessary for a species of fish to undergo a meta- 

 morphosis such as the eels, so long is it also necessary that it on spawning should be 

 far from the coasts so that the offspring may find the conditions of life which correspond 

 to its whole organisation. And we may suppose here that the eel in its migrations from 

 the coasts out to the spawning places is guided by an instinct, which enables it to 

 judge a certain pressure and a definite temperature as the goal of its wanderings, a goal 

 which suitability, through countless generations has been made to correspond to the needs 

 of the normal development of the young. 



At the end of this section, it deserves to be specially mentioned that the shortest 

 route for the eels, migrating out from the Baltic, Kattegat, Skager Rak and North Sea 

 to places where there are such great depths in conjunction with such high temperatures 

 as those where the eel larvae were found in far greater numbers than an5rwhere else in 

 the neighbourhood of North Europe, goes through the English Channel to the 

 Atlantic. Yet there is no doubt that a portion also of the eels migrating out from the 

 North Sea find their way to the Atlantic round the north of Scotland. 



7. Biology of the eel in comparison with that of other fishes 



We 'have thus seen, that the eel in order to propagate must undertake migrations 

 often of astonishing extent. We may think for example of the distance from the inner 

 Baltic, where many eels live, to the Atlantic Ocean off the British Isles and France. 

 Another conspicuous feature in the biology of the eel is the enormous difference 

 between the external circumstances, under which it lives during its period 

 of growth —(shallow, fresh or brackish water) and during its spawning period (deep, 

 salt Atlantic Ocean water). 



These conditions are not however peculiar to the eel. On the contrary, we already 

 know many examples amongst fishes, where migrations of large extent are undertaken by 

 the spawners, and the fishery investigations of recent years have added still more. I 

 may for example call to mind Johan Hjort's description of the migration of the cod 

 from and to the spawning places in the Norwegian Sea. At Iceland I have had the 

 opportunity of seeing for myself, how the large cod, which occur in the summer in 



