26 



When these immigratiiig eels, elvers or > Aalefaring« as A. Feddersen ver}' 

 happily calls them, come to the differeiit coastal tracts of Denmark, canuot at 

 present be determined; the time of immigration may in general be placed in the 

 months of March to June, and is certainly earlier on the west coast of Jutland 

 than in eastern Denmark; but in May 1906 I found tliem in great quantities in 

 Ribe River and in the riveroutlets into the Great Belt and Isse Fjord, and in the 

 first days of June on the coasts of the Smaaland sea. 



When the elvers arrive at our coasts, they have a very characteristic 

 appearance, which differs considerably from that of the older eels. The form 

 of the body is quite the same, but the coioiu- is quite different. In these post- 

 larval eels the pigmentation is restricted to the top of the head, the end of the 

 tail and as a rule to two dark streaks below on the sides of the fish, whilst the 

 body is otherwise glasselear and transparent, so that the vertebral column, the 

 heart and the alimentary eanal are seen distinctly. On the belly the yellowish 

 brown colour is quite lacking which is so characteristic of the older eels and is 

 due to the deposit of guanin crystals in the peritonenm; this therefore has not 

 yet taken place. This is in faet the most distinctive mark of the glass-eels, as 

 they are cailed, as it remains long after the pigment on the back and the sides 

 has so far developed that these take on the wellknown colour of the eel. 



If we wish therefore to define the term »glass-eel«, the definition would 

 be: eels with colourless, glassclear belly through which the alimentary canal is 

 distinctly seen because guanin crystals do not occur as yet in the peritonenm. 



Towards the summer however this mark of recognition also disappears; it 

 will then be as a rule only by means of their small size and especially their small 

 thickness that specimens of the the Ogroup can be distinguished from those of the 

 following year's group. 



All the glass-eels which stream to our coasts in spring do not succeed 

 in galning the fresh water. TJiey seem indeed to have the ability to detect the 

 fresli water, which streams out into the sea and mixes with it near the outlets of 

 the rivers; at these one can find quantities of them hidden under stones, vegeta- 

 tion and washed up zostera, and it seems as if they stop here bel'ore going further 

 up into the rivers. Great quantities however come into the coasts at piaces where 

 no fresh water runs out; they remain then in the salt water, the stock of eels in 

 which is mainly recruited in this manner and not only by the older yellow eels 

 migrating out from fresh water. 



The glass-eels taken during their migration out at sea have always had a 

 quite emjity digestive truet; they have not yet begun to feed, and corresponding 

 to this the pigmentation is extremely slight. When they have found a resting 

 place, they begin to feed and the pigmentation advances. Tlieir food is chieHy 

 Daphniæ and larvæ of insects. 



If a glass-eel in which the pigmentation is as described above is starved, 

 we find that this does not advance. Thus I have kept some glass-eels in summer 

 from the middle of May to the end of August; they got nothing to eat, only the 

 water (salt water) was renewed several times. At the end of this time they had 

 still quite the same appearance as when captured, whilst all the specimens of the 



