676 



BIOLOGY OF THE SEAS OF THE U.S.S.R. 



side of the Ocean off the shores of Japan, the 0° and 16° isotherms are 

 separated in winter by only 10°, whereas off the American shores the zone of 

 eparation is more than 30°. In summer there is a 15° belt between the iso- 

 therms 10° and 26° on the western side of the Ocean and 40° on the eastern. 

 This influences not only the climate of the coastal regions of the mainland 

 but also the whole biological environment, and most of all the marked pheno- 

 mena of oceanic convergence on the western side of the Ocean. 



These peculiarities create conditions in the northwestern part of the Pacific 

 Ocean for the existence of quantitatively very rich flora and fauna, and zones 



Fig. 319. Diagram of the Arctic (/), boreal (2), tropical (5) and 

 mixed (4) zones on both sides of the Pacific Ocean. 



of heterogeneity where Arctic boreal and subtropical meet. There are some 

 mixed tropical and subtropical zones. This is most apparent in the zone 

 where the waters of the Oyashio and Kuroshio meet in the pelagic region ; 

 we are therefore led to the conclusion that a mixed zone exists here rather 

 than a subtropical region, since the boreal and tropical fauna and flora 

 resemble each other very closely and are partly intermixed. 



The qualitative variety of the population is increased also as a result of the 

 great vertical range (down to 1,100 km) and of the much greater biotopic 

 variety (a large number of archipelagos and the presence of coastal features). 

 The fauna of the northwestern part of the Pacific and its adjacent seas is at 

 least twice as rich as that of the seas of northwestern Europe. The deep-water 

 fauna of the Sea of Okhotsk, the Bering Sea and the adjacent part of the 

 Pacific (with the Aleutian, Kuril-Kamchatka and Japanese trenches) is ex- 

 tremely rich; its variety is probably considerably greater than that of any 

 other part of the world ocean. 



