Report on the Distribution of the Larva- of the Eel. 5 



The Distribution of the Young Glass Eels. 



Our obsei-vations in the North Sea extend over a period of nine 

 years, from April, 1904, to March, 1913. 



From the table supplied, it is seen that all the records of glass-eels 

 occur within the period of the year extending from November to 

 May, and although more numerous observations have been made in 

 the other half of the year, from April to November, no records have 

 been made of these transparent glass-eels in the northern Noi-th Sea. 

 Thus the Scottish records confirm the statements made by Schmidt 

 that the migration of the glass-eels is limited to the winter and 

 spring months, and that these glass-eels are derived from the meta- 

 morphosing stages of the previous summer found out in the Atlantic. 

 The records of the Leptocephali on the west, and the time of 

 occuiTence of the larvae, in the North Sea show us that an annual 

 stock is passing inwards round the North of Scotland in the months 

 of September to January or February. Our records of the Lepto- 

 cephali indicate that the migration in the North of Scotland is aided 

 greatly by the strong north-easterly current, and the occun-ence of 

 glass-eels in the Noiih Sea as early as November points to the fact 

 that the young eels reach the North Sea via the North of Scotland at 

 least as readily as by the apparently much shorter route through 

 the English Channel. 



Schmidt has shown that this strong easterly current which aids 

 the eels in their migration also carries enormous quantities of 

 Atlantic pelagic organisms into the North Sea. Thus he shows 

 that such passive organisms as Salps (Salpa fusiformisj, which were 

 limited in the North Atlantic in May to the west of the Hebrides, 

 and were absent from the Norwegian and North Seas, were found 

 in these latter areas towards the end of July and August, having 

 pushed themselves to the north, and also into the North Sea, in large 

 numbers. 



Now the larval eels were also found at these localities to the West 

 of Scotland from. May to September, yet from our records the glass- 

 eels do not arrive in the North Sea much before November, that is to 

 say, some months after the advent of the Salps ; although the larval 

 eels are by no means such passive organisms as vSalps. The records 

 of capture help us considerably, for it may be said almost as an 

 invariable rule that the glass-eels are only caught at the surface at 

 night. There are no surface records for mid-day hauls, and, indeed, 

 the greatest number obtained in one haul of quarter-hour duration 

 was 18, got at St. 42 (56° 28' N. 0° 53' W.) on February 11, 

 1911, about 3 a.m. This peculiarity has been well brought 

 out by Johansen for the North Sea, Skagerak, and Cattegat; and 

 Schmidt has also shown that the Leptocephali behave in a similar 

 manner, approaching the surface waters during the darkness. 

 These diurnal vertical movements must play no inconsiderable part 

 in modifying the influences of the surface currents on the horizontal 

 distribution of the eel-larvae, and it is therefore more easy to under- 

 stand why the glass-eels should be later in arriving in the Nor'th Sea 

 through the influence of the currents than some other pelagic but 

 more passive Atlantic organisms. 



The first records of the arrival of the new stock of eels in Scottish 

 waters were made in November. Two of the specimens were pro- 



