— 256 — 

 SECTION IV 



Resume 



The eels of North and West Europe spawn in the Atlantic west of Europe (west of 

 the British Isles and France). The conditions necessary for the spawning of the eel and 

 for the development of its first stages have been found to be: (l) depths of at least about 

 lOOO meters (corresponding to a pressure of ca. too atmospheres), (2) a temperature of 

 the water in 1000 meters of over ca. 7° C. (accompanied by a salinity of over ca. 35'20 %o)- 



It results from this that the silver eels migrating from the Baltic and North Sea 

 cannot find the conditions required for the spawning and development of the larvae (Lep- 

 tocephaii) anywhere nearer than the west of the British Isles and France. Thus, in the 

 Baltic, Kattegat, Skager Rak and North Sea the depths are not suitable, and in the Nor- 

 wegian Sea west of Norway where the depths are sufficient the tejnperature js too low. 



The larvae of the eel {Leptocephaht-s brevirostris Kaup), see PL X, have been found 

 by the "Thor" in the Atlantic along the whole distance from west of the Faeroes (ca. 6l° 

 N. L.) to off Brittany (ca. 48° N. L.), see PI. X'. The most northerly station where they 

 were found by the "Thor", namely off the Faeroes, must be considered as indicating ap- 

 proximately the northern boundary for the eel larvae in the Atlantic, determined by the 

 temperature in deep water which further to the north becomes too low. On the other 

 hand, keeping in mind the observations in the Mediterranean, there can be no doubt that 

 later investigations will find them much further to the south than the "Thor" has been, 

 that is, along the coasts of France, Spain, Portugal and West Africa as far as the 

 temperatures will permits 



The region of the distribution of the eel larvae or of the spawning places of the eel 

 may thus be defined as the belt along the continental slope towards the eastern part of 

 the Atlantic depths, where the depths are ca. 1000 meters or more and where the temp- 

 erature in 1000 meters depth is at least ca. 7° C. This belt, which runs from north to 

 south somewhat parallel to the continental coasts, is rather narrow in comparison with 

 the whole extent of the great Atlantic basin, and it is this which constitutes a barrier 

 between the eels of North America and Europe {Angmlia chrysypa and vulgaris). Both 

 of these spawn indeed in deep water in the Atlantic, but not so far from the coasts that 

 intermingling is possible under normal conditions. In connection with this it has also been 

 shown that the vertebrae of ca. 300 Ç eels have been counted (ca. 200 from Europe and 

 ca. 100 from North America) without any number being found common to the European 

 Angm'lla vulgaris {ill — 119 vertebrae) and the American Angiiilla chrysypa (103 — 110 

 vertebrae) (cf p. 240). 



The youngest developmental stage of the common eel {Anguilla viUgaris) as yet 

 known is Leptocephalus brevirostris previously found only in the Mediterranean. This 

 was found in the Atlantic for the first time by the "Thor" on May 22nd, 1904 west of the 

 Faeroes, but only a single specimen and a single specimen was likewise taken later on 



I Our investigations in 1906 have shown that the larvae of the eel occur in quantities in the Bay of 

 Biscay wheie we found them a.s far to the south as to otT the North co.ast of Spain. (Note added during pre.ss.) 



