IS4 DISCOVERY REPORTS 



MIGRATION 



The migrations of the Sperm whale are not nearly so clearly defined as those of the 

 Whalebone whales. At all times of the year it is found plentifully in the tropics, and 

 males only are found in lesser numbers in higher latitudes at certain seasons. A con- 

 sideration of the season graphs for catches of Sperm whales at South Georgia and Natal 

 will throw some light on the seasonal movements of this species of whale. 



Fig. 64 shows the numbers of male and female Sperm whales caught during each 

 month at Natal for seasons 1918-26 and 1928 and at Cape Province for seasons 1920-5, 

 and Fig. 65 shows the monthly catches of Sperm whales at South Georgia for seasons 

 1926/7-1930/1 and South Shetland for seasons 1923/4 to 1925/6 inclusive. At South 

 Georgia all the Sperm whales are males, and it is seen that there are some indications of 

 bimodality in the graph, with the largest peak in March: at the South Shetlands the 

 bimodality is more marked. The corresponding curve for male Sperm whales at Natal 

 shows a gradual rise from April to July with a high peak in August and September. On 

 comparing the two curves it is seen that the peaks occupy successive positions on the 

 time scale, and that male Sperm whales appear off the coast of Natal after the height of 

 the peak has been passed at South Georgia ; and the peak for Natal is reached at a time 

 when there are no Sperm whales at South Georgia. The inference to be drawn is that 

 the male Sperm whales which appear in some numbers in South Georgian waters in the 

 early months of the year are on their way to lower latitudes, and that the earlier lower 

 peak in the figure for South Georgia represents whales that are migrating southwards 

 towards the end of the year and continue their journey towards the south beyond South 

 Georgia. They go beyond the South Shetlands too, as shown by the curve for that locality. 

 The first peak may represent a migration down the coast of South America. That they 

 go well into the Antarctic and stay for some time is upheld by an examination of the 

 amount of diatom film present on the whales in diflFerent months. Hart (1935) has 

 shown that a stay of at least a month in the Antarctic Zone is necessary for the develop- 

 ment of the diatom film. Table XVI shows the amounts of film observed and shows that 

 infection is absent or slight in the first part of the season and heavy in the second part. 

 This strengthens the view that the whales taken in the early part of the season are on 

 their way farther south, and that those taken in the latter part of the season have been 

 present in the Antarctic Zone for some time and are probably on their way north again. 

 It is of interest to note that the first peak in Fig. 65 occurs in those months in which the 

 curve in Fig. 42 reaches its maximum, and that these southerly migrating males, though 

 in full sexual activity, have left the region frequented by the females at the height of the 

 pairing season to pursue their migration alone. 



It must be remembered, however, that these migrations are performed by a portion 

 only of the Sperm whale population of the oceans, for Sperm whales are plentiful in the 

 tropics at all times of the year. 



Turning now to the curve showing monthly catches of female Sperm whales at 

 Natal, it is seen to be bimodal with a low peak in April and a high one in September. 



