THE HUMPBACK WHALE 77 



creasing size, as does the length of the flipper, the width of which, however, has a de- 

 creasing rate so that as it gets longer it gets relatively narrower. 



Measurements nos. 9, width of flukes at insertion, and 21, greatest width of skull, 

 both have a value of k below unity. These measurements are transverse to the long axis 

 of the whale and their values show that the relative rates of transverse growth at the 

 extremities decrease with increasing length. 



The value of k for measurement no. 13, the genito-anal distance, is below unity for 

 males, as would be expected in view of the fact that this measurement is situated in the 

 posterior region of the body. In the female, however, not only is the value of k for this 

 measurement less than unity, but it is also negative in sign, showing that the measure- 

 ment not only becomes relatively, but absolutely, smaller with increasing total length 

 of the whale, and that the rate of decrease diminishes with increasing length. It is sug- 

 gested that the decrease in length of this measurement is correlated with increasing size 

 of the vulva as breeding seasons succeed each other, so that the vulva encroaches on the 

 genito-anal distance. The last point in Fig. 71 may indicate that this increase in vulvar 

 size stops with the attainment of physical maturity. 



MIGRATION 



The migrations of the Humpback whale have been more observed than those of any 

 other species, because of its habit of haunting inshore waters when on the move from 

 north to south, and vice versa, along the coasts of the southern continents. The sugges- 

 tion of Risting (191 2) that the Humpback whales are not distributed throughout the 

 oceans, but are bound intimately to definite highways formed by certain of the great 

 ocean currents, appears to be correct. 



The figures of the total catch of Humpbacks at Natal for seasons 1918-28 inclusive 

 are plotted by months in Fig. 79, which shows a very distinct bimodal graph. The first 

 peak in July represents the migration towards the north, and the second peak in Sep- 

 tember represents the return migration to the south. According to Harmer (1931) a 

 similar bimodal graph (July and October) is obtained when the catches made at Angola, 

 West Africa, are plotted by months, and the two peaks represent the migration to the 

 north and back again. Olsen (1914-15) states that the main body of Humpbacks does 

 not strike the southern extremity of the coast of South- West Africa but approaches 

 the shore to the north of Saldanha Bay, and then follows the coast northwards, a 

 statement which is borne out by the figures of the whaling returns from the whaling 

 stations of South Africa. 



On the coasts of Australia similar migrations take place. Dakin (1934) gives inter- 

 esting information on the migration of Humpbacks there. In Western Australia whaling 

 started in 19 12 between latitudes 25° and 26° S, and the whales taken were mostly 

 Humpbacks. In July and the first half of August all the whales were travelling north- 

 wards and the return migration started about August 24. In 191 3, at Norwegian Bay, 

 the northward migration started on June 24 and continued throughout July. By 

 August 17 the return migration had reached its peak. In 19 14 the north-bound whales 



