— 228 — 



Holland and Germany. But from the Danish coasts fortunately we have so extensive' 

 and trustworthy data, that we are even now able to give a general summary of the con- 

 ditions. We know that on the Danish North Sea coast the ascent of the elvers 

 begins in April or beginning of May. This as well as the scattered notices 

 from other parts of the North Sea show definitely that the ascent begins later in 

 the North Sea than on the Atlantic coasts. And when we come finally to the 

 true Baltic we find the condition, that though the eels live there in quantities and there 

 are important eel fisheries at several places, yet there is no regular ascent of tiny elvers^ 

 as occurs further west in Europe. All this shows with certainty that it is the distance 

 from places lying further to the west in the Atlantic than the coast of France and Great 

 Britain, which specially determines the time of arrival of the elvers at the 

 various coasts of North Europe. For the rest it is also clear that the longer the distances 

 are, which the eel-fry must travel, the more conditions occur on the way to influence the 

 migration (currents, winds, position and form of the coasts, presence of rivers etc.). This 

 is the reason why I have so far only endeavoured to map out the conditions on the 

 coasts of the Atlantic from which further comes the best information. 



We have hitherto only discussed the period at which the eel-fry begin to show 

 themselves on the coasts or to ascend into the fresh water, and the data marked on the 

 Chart, Plate XII, refer thus to the earliest period when the arrival of the eel-fry has been 

 observed at the various places. 



In addition to the distance of the place from the spawning places of the eel and the 

 above-mentioned current and wind conditions etc., there seems also to be another factor 

 which influences the ascent of the fry into the fresh water. According to the available 

 data the eel-fry often collect in quantities in the sea off the mouths of rivers, to which 

 they are specially attracted by the outpouring fresh water. It seems however that they 

 often do not go up the rivers immediately on arrival, but remain for some time in the 

 sea off the mouth, especially, according to the data, when the weather is cold and stormy. 

 Thus for example Couch il. c. p. 312) writes: "this passing upward is indeed in some 

 degree regulated by the state of the season as being mild or cold ; for very young eels 

 have been obtained at the ebb of the tide so early as the 2nd of January, after watching 

 a river with great care they have not been observed passing up the stream until the 

 middle of March, and rarely even then." In addition to this observation, presumably 

 made in Cornwall where Couch investigated the fish-life, several others are to hand 

 which point in the same direction (cf. Kroyer's from Hornbœk in Denmark, p. 210), and 

 we can even find such definite statements (without any very definite foundation however), 

 that the eel-fry do not run up into the fresh water before it has reached a temperature 

 corresponding to the average annual temperature of the place. 



Even if we allow that most of the observers have hardly noticed the phenomenon 

 before this has attained a certain magnitude and that many of the ascending eel-fry 

 especially in the beginning have thus not been remarked, there is scarcely any doubt 

 that the temperature has some influence on the period of the ascent of the eel fry from 



1 So far as known the only small eel-fry hitherto taken on the coasts of the inner Baltic are those of 

 70 — 80 mm. in length mentioned on p. 212 as taken on August 24th in Roslagen (Uppland, not very far from 

 Stockholm). To judge from the size these eel-fry need not be and most probably are, not of the most recent 

 year's group. 



