270 DISCOVERY REPORTS 



this belief is not necessarily entirely correct because the whalers refrain from taking 

 any Sei whales which are available while the larger species are numerous. In addition, 

 in seasons when whaling slackens off in the early months of the year owing to the small 

 numbers of Blue and Fin whales present, there is frequently a marked return of the 

 larger species and improvement in the catch towards the end of the season. It is of 

 interest to note that CoUett (1886) states that the whalers at Varangerfjord said that 

 when the Sei whale came in the Blue whale left the coast and went out to sea. 



A further point which confirms the likelihood that the season graph gives a correct 

 indication of the occurrence of the Sei whale at South Georgia is the general absence 

 of a skin film of diatom in this species. Hart (1935) found that in fifty-two Sei whales 

 examined for the presence of diatom film only ten were infected, and of these only one 

 sufficiently heavily for a patch of film to be visible to the naked eye. He has also shown 

 that at least a month is required to develop the film in Antarctic waters, so that the 

 conclusion to be drawn from his observations is that only one of these whales could 

 be definitely said to have spent more than one month in high latitudes. 



The graph does not represent simply the end point of a southward migration from 

 which the whales return to the north after a stay of a month or two. Reference to 

 Fig. 74 shows that the composition of the Sei whale population during these 3 months 

 is constantly changing. In February the majority of the females are pregnant ; in April 

 the majority are resting ; in March the numbers of the two classes are approximately 

 equal. The proportion of immature whales is fairly constant throughout the period. 

 This means either that the pregnant females arrive and depart first, or else that after 

 the arrival of the first schools, in which the females are mostly pregnant, there is a further 

 arrival of schools in which the majority of the females are not pregnant, so that the 

 earlier population is greatly diluted by the non-pregnant females. In either case the 

 pregnant females arrive first, though data are insufficient to show which of the two 

 alternatives relating to the subsequent movements of the schools is correct. It may 

 be suggested that the resting whales are later in arriving on the South Georgian grounds 

 because they have been delayed in lower latitudes by lactation, and that the pregnant 

 ones, having been unhampered by sucking calves, have been able to leave the north 

 sooner. 



The southward migration, as in the case of the Blue and Fin whales, is undoubtedly 

 a feeding migration. The appearance of Sei whales in South Georgian waters coincides 

 with the departure of either species and the deduction would appear likely that the 

 food of Sei whales then becomes abundant and that of the other species scarce. This 

 theory is further supported by the known habits of Sei whales in the northern hemi- 

 sphere, where they feed on copepods and for which diet they appear to be specially 

 suited by their fine baleen. But examination of the stomach contents merely reveals 

 that the Sei whales of South Georgia are feeding on the same Eiiphausia superba as are 

 the Fin and Blue whales. The March influx of Sei whales cannot, therefore, be due to 

 the seasonal abundance of any special food. Can it be that the Sei whale, being smaller 

 than the Fin and Blue whales, merely takes a longer time to travel the same distance 



