SEASONAL CHANGES IN THE COMPOSITION OF THE STOCK 269 



and tropical centres. More significant is the local variation in the sex ratio of Hump- 

 backs. In each season there is a clear excess of females in the Antarctic pelagic catches, 

 whereas in the tropics there is an excess of males which is specially marked off the west 

 coast of Australia and, apparently, at Madagascar. There seems to be a definite segre- 

 gation of the sexes here which is of some importance in relation to the effect of whaling 

 on the stock of whales (see Table 30, p. 290). The explanation presumably lies in some 

 difference in the migrations of the two sexes. Perhaps on the southward migration the 

 males tend not to travel so far south as the females, or perhaps some of them remain 

 in tropical waters during the summer. In the temperate waters of Natal the sexes are 

 about equal, and at Saldanha Bay and South Georgia the figures are rather too small to 

 be conclusive. 



SEASONAL CHANGES IN THE LOCAL 

 COMPOSITION OF THE STOCK 



The term 'seasonal changes' refers here to regular changes which take place from 

 month to month every year. The statistics of catches clearly show that in the course of 

 the summer changes do take place in the make up of the whale population on the 

 Antarctic whaling grounds, but it is not easy to give any precise account of them, for 

 the available data are in many respects inadequate and unsatisfactory. The subject can- 

 not be neglected, however, for it is closely concerned with the migrations and distribution 

 of the different species and classes of whales, and with the effect of whaling on the stock. 



It would be desirable to compare in each month the ratio of species and sexes, the 

 average length, the percentage of immature, pregnant, resting and lactating whales, and 

 even the average age. The International Statistics give abundant data on the species 

 ratio in each month, but little else. Hjort, Bergersen, Lie and Ruud analyse the catch 

 statistics in respect of the monthly species ratio and output of oil per Blue whale unit, 

 and for the season 1934-5 §* ve tne monthly percentage of sexually mature and immature 

 whales (1935, p. 21). For some information on fluctuations in average length and the 

 incidence of different classes of whales I have the Discovery Committee's records of 

 whales examined at shore stations and in factory ships, and some records of the catches 

 of one or two factory ships made by whaling inspectors. 



From such data inferences must be drawn with caution for, as already pointed out 

 (p. 228), the catches of whales are not necessarily representative of the composition of 

 the stock, and any changes in the catches of factory ships may result not only from 

 changes in the composition of the stock but also in part from changes in the locality in 

 which the ship is working. In this respect data from the land stations at South Georgia 

 may be a little more reliable, though even there the catchers do not necessarily work 

 quite the same area throughout the season (see Kemp and Bennett, 1932). 



