236 PHYSICAL AND BIOLOGICAL CONDITIONS IN THE NORTH SEA. 



state that there are 32 species, of which specimens occur in the 

 Western Baltic occasionally, and are not resident there. These fish 

 are immigrants, and 18 of the species are southern forms — that is, 

 forms whose range extends from the Mediterranean to the British 

 Islands, but not to the Arctic Circle, while of the remaining 14 species, 

 10 are northern forms — that is, species which are abundant within the 

 Arctic Circle, but do not occur in the Mediterranean. The occurrence 

 of the southern forms in the Baltic takes place chiefly in September 

 and October, and, therefore, coincides with the inflow of the southern 

 coast water. The species in question are : — 



Labrax liqms, the Bass ; Sciaena aquila ; Mullus surmulctus, the Red 

 Mullet ; Brama Kayi, Eay's Bream ; ThynniLS vulgaris, the Common 

 Tunny ; Xipliias gladius, the Sword-fish ; Trigla Jiirundo, the Tub, or 

 Latchet ; Mugil chclo, the Grey Mullet ; Lahrus maculahis, the Spotted 

 Wrasse ; Crenilahrus melops ; Gadus minutus, the Poor Cod ; Merluccius 

 vulgaris, the Hake ; Solea vulgaris, the Common Sole ; Orthagoriscus 

 mola, the Sun-fish ; Engraulis encrasicholus, the Anchovy ; Conger 

 vulgaris, the Conger ; Carcharias glaucus ; Trygon pastinaca. 



Now, we cannot consider the herring as a southern fish. It is very 

 improbable that herrings enter the North Sea from the English 

 Channel : on the contrary, the evidence points the other way — namely, 

 to the conclusion that the North Sea herrings come from the north ; 

 and the association of herrings with southern coast water is a fact 

 which requires further examination. Pettersson does not discuss the 

 difiiculty. 



In January, February, and March, the coldest season of the year, 

 there is an influx of water of the same salinity as that previously 

 mentioned — namely 32 to 33 per cent., but of a temperature of only 

 4° to 5° C, which evidently comes from the north along the Norwegian 

 coast. In 1893 this northern coast water was entering the Skagerack 

 in November, and it was found to contain a very characteristic 

 Plankton, or assemblage of minute swimming forms, entirely different 

 from that of the adjacent water of the Baltic current. The latter 

 consisted chiefly of vegetable organisms, such as Diatoms, Cilio- 

 flagellates, etc., intermixed with Copepods, such as Ccntropagcs hamatus, 

 which occur also in the Cattegat and Baltic up to the Aland Islands, at 

 the entrance of the Gulf of Bothnia. 



In the northern coast water, on the contrary, vegetable Plankton was 

 scarce, and the animals were of Arctic or North Atlantic origin, which 

 never appear in the Skagerack during summer, e.g. : — 



Eupliausia incrmis (Krciyer) (Schizopod) ; Hyperoche Kroyeri 

 (Bovallius) {Amphip)od) ; Parathemisio ohlivia (Kroyer) {Amphipod) ; 

 Dii^hyes truncata (M. Sars) {Siphon<yphorc). 



