GENERAL CHARACTERISTICS OF THE NORTHERN SEAS 



57 



developed in the post-Pliocene period, while the original Pliocene forms were 

 few. When the salinity of the waters had increased again the main mass of 

 Salmonidae was pushed into the estuaries of the rivers and into river systems 

 and within this new habitat they went through the process of rapid formation 

 of species. If the pre-Quaternary ancestors of the Salmonidae had already 

 possessed the original type of anadromous migration, then the system of 

 migration, as we know it now, developed as a result of the above-mentioned 

 palaegeographic changes. 



It is remarkable that all this relict ice-marine fauna has in its distribution a 



Fig. 13. Distribution of family Salmonidae including the 

 Coregonidae and Osmeridae (Zenkevitch, 1933). 



clearly manifest character of stages, linked with its adaptation to definite 

 salinity. 



A. Svetovidov (1952, 1954) has expressed some very interesting ideas about 

 the distribution of Clupeidae and Salmonidae in the Arctic. It is known that 

 both groups in their origin are connected with the northern half of the Atlantic 

 Ocean. Svetovidov thinks that in the case of both families only a few repre- 

 sentatives of the northern ocean have penetrated into the Pacific Ocean from 

 the Arctic basin and are represented by cold-living forms. The endemic, 

 small herring with a few vertebrates which are representatives of the Arctic 

 herrings are Ciupea harengus pallasi n. maris albi (White Sea) and Cl.h.d.n. 

 suvorovi (Cheshsko-pechora). The origin of the Arctic sub-species of herring 

 is no doubt connected with the Atlantic sub-species CI. harengus harengus. 

 Having migrated to the Pacific Ocean herring has formed there a variety 



