236 THE SEA FISHERIES 



the period of depaxture of the silver eel and the arrival of the 

 elvers. 



The elver has long been known to develop from a totally distinct 

 form — ^the Leptocephalus — a practically transparent little fish of 

 the shape of an oleander leaf, and so named because it was originally 

 thought to be a distinct species. The Leptocephali were also 

 thought (by Gunther^) to be abnormally developed forms, but 

 eventually the French zoologist, Delage, succeeded in keeping a 

 Leptocephalus for seven months in an aquarium at Roscoff, and 

 observing its transformation into a young conger. In 1893 the 

 Italian naturalists, Grassi and Calandruccio, traced the develop- 

 ment of young eels from Leptocephali. These naturalists were 

 fortunate in obtaining large numbers of Leptocephali in the Straits 

 of Messina, where strong currents bring them to the surface. In 

 other localities Leptocephali, especially Leptocephalus brevirostris 

 the yoimg of the eel, were found very rarely. It seemed very 

 unlikely that the Baltic eels, for instance, were spawned in the 

 Mediterranean. 



Part of the International Fishery investigations was directed to 

 a systematic search for eel larvae in Northern waters. This search 

 was facilitated by the accidental discovery of a single specimen of 

 Leptocephalus brevirostris near the surface in the sea off the Faroe 

 Islands by the Danish Governments investigation steamer Thar, 

 in May, 1904. The sea was at this point over 500 metres deep. 



Schmidt, the Danish investigator, devised a careful survey off 

 the west coast of the British Isles, and was successful in obtaining 

 large numbers of eel larvae. The reason why these had never been 

 found before is because the eel seeks great depths in the ocean, 

 1000 metres and more, for spawning pin-poses. 



These depths are not found in the Baltic, Skager-Rack or the 

 North Sea. The temperature at depths of 1000 metres is, at least, 

 7" C, that is higher than that encountered at the bottom of the 

 North Sea. The great plateau on which the continent of Europe 

 is built up, falls away somewhat abruptly into great depths, and 

 the 200, 500 and 1000 metres lines run fairly close together. The 

 1000 metre line is nearest the shore off the coast of Spain, where 

 it is only 15 miles distant. The Leptocephali larvae were taken on 

 the Thor by a fine meshed net, which fished through the mid-layers 

 of water, and the greatest number were taken off the south-west of 

 Ireland, between 56° and 43° N. Lat. The temperature here at 

 depths of 1000 metres was over 9° C. all the year round. Larvae 

 were obtained right along the looo-metre line from the Faroes to 



1 An Iwtyoduetim to the Study of Fishes, Edinburgh, A. and C. Black, 1880, 

 p. iSi. 



