11 



It is not by chance tbat the Biologicai Station happens to be engaged just 

 at the present time with investigations into tlie biology of the eel, and that at the 

 request of the Director I uow pubhsh the iollowiug contribution towards the elu- 

 cidation of its natural history, based on the cruise of the »Sallingsund« in 1906, 

 since, so long as every one was confused and uucertain as to the reproduction 

 of the eel, not even knowing where its spawning piaces were to be found, and 

 the possibility indeed could not be excluded that it spawned in freshwater, thougli 

 all probability spoke against this — so long as these primary questions remained 

 unanswered, it was difticult to think of making detailed investigations on the more 

 secondary questions such as the growth and age of the eel. 



The biological investigations of recent years had, however, removed the 

 obscurity which had previously covered the <juestiou of the eel, so that the time 

 had come for making the above-mentioned more detailed investigations. 



It will perhaps be of interest to give here quite briefly a summary of the 

 investigations of the last ten years. 



The question of the reproduction of the eel awakeaed considerable interest 

 even in olden times. Thus, Aristotle occupied him.self with it and concluded that 

 it was a generatio cequivoca — that the eel arose from the mud on the bottom of 

 the sea. But in spite of this early notice, two thousand years elapsed before any- 

 thing essential appeared to throw light upon the obscure subject; not that many 

 diverse things were not written during this long period concerning such au ecouo- 

 mically important fish, but they were of no interest. Thus, Blanchard in his book: 

 »Les poissons des eaux douces de la France«, says, »The views formulated and 

 the treatises written concerning the reproduction of the eel are quite innumerable 

 and quite devoid of interest as actual observations are wanting.« It is only within 

 the most recent times that clearness has come, thanks to Itahan and Danish 

 investigators. 



In the year 1893 Grassi and Calandruccio succeeded in showing that 

 Leplocephalus hreviroslris, which had long been known in the Mediterranean but 

 had always offered some difficulty relative to its position in classification, was a 

 young stage in the development of the common eel; they observed how it grad- 

 ually assumed the well known form of the eel. It was thus proved that the eel 

 living in South Europe, which is quite the same species as our o\vn, spawns out 

 in the deep water of the Mediterranean. It was improbable, however, that the 

 eel, wliich lives in the fresh and salt waters of Northern Europe, should migrate 

 to the Mediterranean in order to spawn. It had certainly become gradually known 

 that the northern eels after a space of time, the length of which was not known, 

 left these waters and migrated away — where was also not known. And it was 

 clearly shown by C. G. Joh. Petersen in 1895 that before their departure they 

 assumed a special breeding-dress, chauging from yellow to silver, so that this 

 together with the relatively greater development of the sexual organs indicated a 

 spawning migration. 



It was also well-known that in the spring countless quantities of young 

 eels came to our waters, so that the eel must have spawned at one place or 

 another — but where? It was only when the international investigations had 

 extended their sphere of operations to the waters of the Atlantic bordering on 



