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These waters are the Baltic, Skagcr Rak with the "Norwegian Channel" and the 

 Norwegian Sea west of Norway. It is common to these that the eel lives in all though 

 not in equally great quantities. There remain the Kattegat and the more southerly 

 portion of the North Sea, but the depths there are so small that to judge from Grassi 

 and Cal.\ndruccio's investigations in the Mediterranean the possibility of the eel being 

 able to breed in these waters may already be excluded. 



The greatest depths in the Baltic are 4—500 meters, in the Skager Rak ca. 800 

 meters, whilst in the Norwegian Sea between Norway and the submarine ridge which 

 joins Shetlands— Faeroes— Iceland— Greenland the depth sinks to at least 4000 meters. 



As mentioned in Chap. 4 the larvae of the eel were found under such conditions in 

 the Atlantic, that they must undoubtedly have been born at places where the depth is 

 at least ca. 1000 meters. This depth corresponds to a pressure of at least 100 atmos- 

 pheres. The eel cannot find such a great depth or so great a pressure 

 anywhere in the Baltic, Skager Rak or Norwegian Channel. There is 

 however the Norwegian Sea which has considerable depths, towards which therefore one 

 might think the eels, for example from Norway, the Skager Rak, Kattegat and Baltic, 

 might migrate in order to spawn. But here it is obviously the hydrographical condition 

 which places hindrances in the way. This will be evident from the summary given above 

 of the hydrographical conditions in the deepest parts of (i)the Baltic, (2) the Skager 

 Rak, (3) the western part of the Norwegian Sea (Faeroe- Shetland Channel) and 

 lastly, (4) and (5), in the Atlantic at two of the places where the eel larvae were founds 



It can be seen from the Table, that the temperature at the place stated in the Nor- 

 wegian Sea was negative (under 0') at 1000 meters depth, whilst in the same depth at 

 the station in the Atlantic, where the eel larvae were found in greatest numbers, the 

 temperature was between S'/a and 9°. And the temperatures in 1000 meters 

 depth are negative not only at the place mentioned but everywhere in the Norwegian 

 Sea. (See also the Chart, Plate X, where the isotherms for 1000 meters depth are 

 given.) 



It appears therefore from the Table, that the temperatures at the deepest places in 

 the Baltic and Skager Rak are considerably lower than at the corresponding depths in 

 the Atlantic where the eel larvae were found. Further, the salinity in the Baltic is totally 

 different from that in the Atlantic (respectively lo-l9 7oo and 35-44 %o)- 



We thus see that the physical conditions in the deepest part of the Baltic 

 and Skager Rak and in the Norwegian Sea are very different from those 

 in the Atlantic, where the eel larvae occur in great number, and this furnishes 

 the only and at the same time the very natural explanation of why the eel does not 

 breed in the first-named waters, so that is has not been possible to find there the earliest 

 known Leptocephalous stages, namely, the larvae. 



To judge from the conditions under which I have found the larvae, it seems certain 

 therefore that the eel to be able to breed must find itself at places, which have a depth 

 of at least ca. 1000 meters in conjunction with a relatively high temperature, namely 



I 5 is the place (S. W. of Ireland) where the greatest number of Leptocephali were taken. 4 is the most 

 northerly station (W. of the Faeroes) where I found them. The temperatures and salinities o£ the upper water- 

 layers are not included, as the first and partly also the second vary considerably according to the period of 

 the year, further because they have no interest for the question considered. 



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