EFFECTS OF WHALING ON BALEEN WHALES 



153 



In Fig. 7 the yearly percentages of mature females in different age groups 

 are presented in graphical form. In baleen group V all females are mature, 

 and in group IV, if we discount the 1945-6 season (see above), the percentage 

 of mature animals has been fairly steady at about 90 per cent during this 

 period. In group III, however, there has been a marked increase from 30 per 

 cent of mature females at the beginning of the period rising to a peak at 

 nearly 80 per cent in 1952-3 and then falling slightly to about 60 per cent 

 in 1955-6. Baleen group II shows a small increase in the proportion of 

 immatures, stabilizing at about 10 per cent from 1952-3 onwards. By 

 comparison it would seem that the apparent decline in the mature component 

 of age group III from 1952-3 onwards is not real. Probably stabiHty was 



lOO-i 



1945 



1947 



T 



1949 1951 



Years 



1953 



1955 



Pig. 7. — Changes in the percentage of sexually mature female fm whales in different 

 baleen age groups. 



reached in this group also in 1952-3 at 50-60 per cent mature. These trends 

 in the samples can best be accounted for in terms of a lowering of the age of 

 puberty. Converted to the average ages at puberty this shows a progressive 

 advance of about half a baleen group up to 1952-3, after which it appears to 

 have remained stable. 



(c) Accelerated growth 



The evidence for a steepening of the growth curve is mainly circumstantial, 

 because our material is not sufficient to show significant differences in length 

 at age values from year to year. 



It is suggestive that although the average age at puberty has decreased, 

 there is close agreement between the observed and estimated values for the 

 percentage of immatures (Fig. 5). If puberty occurs at an earlier age, but is 

 not associated with increased growth then the average length at puberty will 



