262 



DISCOVERY REPORTS 



It would appear from these figures that whales with anything up to four corpora lutea 

 are definitely smaller than their fellows and might therefore be less liable to attack, 

 although it is questionable whether a gunner can readily discriminate between a whale 

 of 80 ft. and one of, say, 85 ft. The former, being newly mature, have not had time to 

 calve, so that none of them will be absent from the catch through being protected while 

 in lactation. It is therefore probable that the youngest adults are over-represented on 

 the whaling grounds. This will go far to offset any tendency that whalers might have to 

 discriminate against them. 



1-5 



. 1-0- 



J D-5 



.0-0 



T-5- 



AGE OF WHALES 

 2 4 6 8 10 12 14 16 18 20 22 24 2E 2B 30 2 4 6 6 10 12 14 16 IB 20 22 24 



— -. 1 i i 1 1 1 1 1 1 1 1 1 1 i I 1 1 1 I 1 I ' 



1934-35 



1935-36 







Q 10 20 30 10 20 



Fig. 13. Logarithmic age distribution of adults. 



It is probable therefore that all ages of mature whales are taken in proportion to their 

 presence on the whaling grounds and that the age distributions which we are considering 

 may be taken to be a random sample of the Antarctic population. 



In a constant animal population under natural conditions the rate of recruitment by 

 births must be just sufficient to balance the mortality at various stages in the life history. 

 If we assumed that a constant stock was subject to constant birth and death rates, the 

 number of living in year groups would take the form of a decreasing geometrical progres- 

 sion. The age distribution of such a population, expressed graphically with the numbers 

 of animals in each age group given in logarithms, is represented by a straight line. The 

 gradient of the line from the beginning of life to the end is an indication of the mortality 

 from various causes. 



We cannot say that the stock of whales has ever been exactly in accordance with the 

 assumptions made above, but it may be argued that over long periods of pre-whaling 

 history the stock must have shown approximate constancy, as otherwise it would have 



