PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 2-i7 



Biological Station on Heligoland. The fish of the Baltic have been con- 

 sidered by Mobius and Heincke in a paper* which has already been cited, 

 while Fr. Heincke alone has treated the fishes of the Heligoland Bight 

 in a similar manner, f Papers on the other animals of these districts are 

 also contained in the publications of the same institutions. For the 

 rest of the region I have to rely on various ichthyological works, and on 

 my own experience. 



The peculiarity in the general distribution of fishes in the North Sea 

 is that southern species are found along its eastern side, and at its 

 northern as well as its southern entrance, but are absent on its western 

 shores, and in its central and western portion. The reason of this, 

 as Heincke points out, is that there are two routes by which southern 

 species can enter the North Sea, namely, that of the Gulf Stream — the 

 drift of Atlantic surface water which bathes all the western coasts 

 of the British Isles, flows past the Shetlands to the coast of Norway, 

 and sends a twig into the Baltic ; and secondly, the route which leads 

 from the English Channel, a slow stream of southern warmer water 

 passing along the Dutch and other continental coasts. 



Southern Species entering the North Sea from the South. 



In discussing distribution it is necessary to distinguish between 

 littoral species, which inhabit the zone of sea-weeds, surface species, 

 which feed and swim in the open water, and bottom species, which feed 

 on the bottom ; Heincke uses for the latter two divisions the terms 

 aperticolous and fundicolous. He also distinguishes three divisions 

 according to the frequency of their occurrence, namely, common 

 resident species, rarer resident species, and occasional immigrants. We 

 do not know with sufficient completeness the seasonal movements 

 of the majority of fishes in a particular locality, but there is evidence 

 to show that the aperticolous species are most migratory, the fun- 

 dicolous less so, and the littoral species least of all. It is fairly 

 certain that aperticolous southern species enter the North Sea from 

 the Straits of Dover only in summer, and that they retire in winter. 

 The aperticolous southern forms found in the Heligoland Bight are 

 Scomber scomber, the Mackerel ; Caranx trachurus, the Scad ; Bclone 

 vulgaris, the Gar-fish ; Merluccms vulgaris, the Hake ; Tliynnus vulgaris, 

 the Tunny ; Engraulis cncrasicholus, the Anchovy. 



* Fische der Ostsee. Vicrter Bcricht dcr Commission zur Untersuchung der deutschen 

 Mcere, 1883. 



t W issenschaflliche Meeresuntersuchungen heratisgegeben von der Kommission zur Unter- 

 suchung dcr deutschen Mcere, und der Biologischen Anstalt auf Helgoland, Ncue Folge, 

 Erste Band, Heft 1, 1894. 



