DISTRIBUTION AND MIGRATION 



341 



Figure 186. Northern limits of Humpbacks, Sei Whales, and Sperm Whales in the N. Pacific. 



{Omura, 1955.) 



ticularly marked in, for instance, animals caught off Kamchatka, but 

 absent in those caught N.E. of the Aleutian Islands. This may indicate 

 that, although some interchanges between the N.E. and N.W. groups in 

 the Bering Sea have been established by marks, such interchanges are 

 rare, and that, in the autumn, each group generally returns south along 

 a specific route. The same is probably true of Blue Whales and Hump- 

 backs as well, although Japanese observers found that Humpbacks 

 occasionally cross over from the American to the Asian side, and vice 

 versa. 



Like the three Rorquals we have discussed, the Sei Whale is a true 

 cosmopolitan and frequents all seas, except the very cold ones. While it 

 moves towards the poles in the spring, and towards the equator in the 

 autumn, the Sei Whale keeps well clear of the ice, and is rarely found 

 north of the Aleutian Islands (Fig. 186) or south of South Georgia. As a 

 rule, it reaches the Antarctic fairly late, and leaves it fairly early, so that 

 no Sei Whale has ever been found completely covered with a film of 

 diatoms (which takes at least six weeks to form). In some seasons (e.g. 

 1957-8 and 1958-9) an exceptional number of Sei Whales were caught 



