298 DISCOVERY REPORTS 



7 days later 525 miles to the north (27' S., 153° 26' E.) having maintained an average speed of 

 3-6 knots in its northward migration. If the migrating near-term fin whales travel at 4-6 knots they 

 will take 21-31 days to cover the distance from the Antarctic to sub-tropical breeding areas. We may 

 therefore assume that about a month will elapse between the time of leaving the Antarctic whaling 

 grounds and giving birth to the calf. 



10 



so 



60 



20 30 40 



LENGTH IN METRES 



Text-fig. 10. Monthly foetal length frequencies of southern hemisphere fin whales, Balaenoptera physalus, 



smoothed and converted into percentages. 



D J F 

 MONTHS 

 Text-fig. II. Diagram to show expected changes in the shapes of the monthly foetal length frequencies 



for the fin whale. See text for explanation. 



Referring now to the average foetal growth curve for fin whales, this means that foetuses above 

 about 475 m. in length should be under-represented in the Antarctic samples. The probable eflFect 

 of this on the shape of the monthly length frequency curves is indicated in Text-fig. 1 1 bv the broken 

 lines. The February sample should be only slightly affected, but from March onwards the eflFect on 

 the frequency distribution should be considerable. Thus the model frequency curve for March 

 becomes skewed in the reverse direction to those for December and January. This is precisely what 

 the actual length frequencies of the monthly samples show (Text-fig. 10). 



