336 WHALES 



Tristan da Cunha in the southern winter, and it is there that the Southern 

 Right Whale cow seems to give birth to her young. 



Figures of the annual catch, as well as blubber measurements, indicate 

 that the migrations of Blue, Fin and Humpback Whales do not coincide, 

 and that the Fin Whale returns to the Antarctic a little later in the spring 

 than the Blue Whale. Thus the percentage of Fin Whales caught increases 

 perceptibly as the Antarctic season advances. Air temperature, too, seems 

 to be a factor influencing the time of migration. For instance, if the 

 average September air temperature over South Georgia is lower than 

 32 -4° F., Blue Whales will pass the island earlier than Fin Whales, but 

 if it is above 32 -9° F., Fin Whales will pass by first. During the first half 

 of January, i.e. just before they begin to migrate north, the number of 

 Humpbacks in the Antarctic seems to be at its maximum. There are 

 indications that cows accompanied by calves, and thus unable to keep 

 up with their congeners, arrive in the Antarctic somewhat later and that 

 they possibly stay there some^vhat longer. In any case, Chittleborough 

 states that pregnant Humpback cows, at least, are late in passing the 

 Australian coast in the course of their migration to the north. 



The percentage of the catch represented by pregnant Blue and Fin 

 Whale cows decreases as the season advances, a phenomenon that is the 

 more striking since at the start of the season, when the embryos are small, 

 cases of pregnancy might be more easily overlooked by the inspectors. 

 While the explanation might be that pregnant cows arrive or leave earlier, 

 it seems more likely that the drop in the percentage is due to the fact that, 

 as the season advances, cows wean their calves to an increasing extent, 

 at which stage they may be caught. In this way, the percentage catch of 

 'resting' cows is increased, and that of pregnant cows drops automatically. 

 There are indications that pregnant Fin Whale cows stay longer in the 

 Antarctic than pregnant Blue Whale cows, possibly because the former 

 give birth to their calves about one month later than the other. 



The increase in the percentage of immature animals caught as the 

 Antarctic season advances is probably due to their late arrival or late 

 departure, but may also be caused by the fact that they have just grown big 

 enough to exceed the size limit. 



It has already been pointed out that not the entire Antarctic population 

 migrates into warm waters during the winter season. A small part, at least, 

 appears to stay behind at about 50° S. On the other hand, recent observa- 

 tions seem to suggest that during the summer season not all Rorquals 

 migrate into the cold waters of the Antarctic. A certain number of them 

 obviously stay behind in the temperate waters of about 40° S., or even 

 closer to the equator. 



So far we have restricted our remarks to the migration of Humpback, 



