— 158 - 



young-fish trawl) that the young (at any rate the pelagic young) of almost all the food- 

 flshes which spawn within the region investigated were taken in large numbers. It was 

 necessary therefore to find out the reason for not obtaining the Leptocephali and if pos- 

 sible to fill up this gap in the "Thor"s investigations. For it must be called a gap that 

 we had not been able to find the larval stages of so common and so important a fish 

 as the eel in greater numbers, with a ship which had devoted so many months' work 

 in so wide an area to the investigation of the food-fishes, more especially as these larval 

 stages had been taken in quantities in the Mediterranean which had not been specially 

 subjected to systematic fisheries investigations. 



Several reasons for the absence of Leptocephalus brevirostris in the collections of 

 the "Thor" might be thought of, and I may briefly mention them here. Firstly, the invest- 

 igations were carried on in the months of April to September. If the young eels (elvers), 

 which wander early in spring into brackish and fresh water on most of the North 

 European coasts, come from the silver eels which left the coasts in the preceding autumn 

 — and this so far as I know has been the general opinion — it might easily be under- 

 stood why we had not been able to take the Leptocephali in the period from April to 

 September. If this view was correct, the young elvers which wander in during the 

 early spring must be the earliest existing stages of the eel at that period, and it would 

 therefore be useless to seek for the still earlier developmental stages, the Leptocephali, 

 in the months from April to September. A second reason for not finding the Leptoce- 

 phali in quantities might be sought for in the view, that they do not lead, at least 

 normally, a pelagic mode of life, but live buried in the bottom at great depths where it 

 is difficult to catch them. This view in fact had already been put forward to explain 

 the apparent absence of the Leptocephali on the coasts of North Europe (cf. p. 154). 



Lastly it was also possible, that in spite of the wide area over which our investiga- 

 tions extended we had not investigated the places where the eel spawns. 



With regard to the first two possibilities it must be admitted that the probability of 

 their being correct was weakened by the discovery of the Leptocephalus mentioned, on 

 the surface to the west of the Faeroes as late as at the end of May. It was also 

 generally known that the elvers appear on the coasts of West and North Europe 

 already in winter and early spring (March, April); further, I have taken the migrating 

 elvers off the Icelandic coasts so early as in April. Lastly there were also the state- 

 ments of Grassi and Calandruccio that Leptocephalus brevirostris occurs in the 

 Mediterranean from February to September. 



There only remained the third possibility, that our investigations in 1903 and 1904, 

 in spite of the large area investigated, had not been made at the places where the hydro- 

 graphical and depth conditions permit the eel to spawn. With respect to the depth, 

 there could hardly be any doubt that, with the researches in the Mediterranean in mind, 

 we had investigated in sufficiently deep water, as many of our stations were at depths of 

 ICXX)— 2000 meters and more. On the other hand the hydrograp hical conditions at 

 our deep stations were very different from those known from the Mediterranean, where 

 the Leptocephali were found in great numbers and where the temperature even in the 

 greatest depth (about 4400 meters) does not fall under ca. 13"^. Thus, water, the tempera- 

 ture of which even at a depth of ca. 600 meters is negative (under 0°), covers the bottom 

 of the whole of the deep basin between Norway and the submarine ridge between Green- 



