EFFECTS OF WHALING ON BALEEN WHALES 151 



adult females which are pregnant. Because only adult females are considered 

 the figures are independent of changes in the age at puberty. 



Another factor influencing the apparent 'percentage pregnant' is the 

 mortality rate. Non-pregnant mature females taken in antarctic waters are 

 on average about a year older than pregnant females, because most females 

 become pregnant at the first ovulation at puberty and do not join the non- 

 pregnant group until a year later. The succeeding cycles also average about 

 two years. Thus, even if lactating females were fully represented in the 

 samples and the actual pregnancy rate for individual females was 50 per cent 

 (that is, one pregnancy every two years), the apparent 'percentage pregnant' 

 of the population as a whole, associated with a total mortality rate of, say, 

 20 per cent should be 55' 5 per cent. (For every fifty pregnant females there 

 should be only forty non-pregnant, because ten die in the intervening year.) 



However, even if these factors are allowed for, the relative changes in the 

 'percentage pregnant' are considerable and probably represent a real change 

 in fertility. In the fm whale this might be caused by increased fertility at 

 post-partum or post-lactation oestrus cycles and by reduced prenatal 

 mortality. In the blue whale post-partum conceptions are rare. I have also 

 examined the incidence of pregnancy in various age groups and it would 

 appear that the youngest mature females have a slightly lower fertility rate, 

 that fertility is maximal at about 15-25 years of age, after which it appears 

 to fall slightly (Laws, in press). If the age composition of the population 

 has changed then this would affect the 'percentage pregnant', but only to a 

 very slight extent. This effect, if appreciable, is in any case likely to result in 

 a lowering of the 'percentage pregnant'. 



With these reservations in mind the variations from year to year in the 

 'percentage pregnant' may be briefly described. In Fig. 6 I have plotted the 

 values for blue whales, combining samples from all areas because individual 

 areas show a similar pattern. This indicates an apparent increase in fertility 

 in pre-war years, levelling off at about 80 per cent pregnant. The great 

 reduction in the intensity of whaling during the war years is apparently 

 correlated with a fall in the percentage pregnant to the original low level of 

 the early 1930's. Then, in post-war years, there was a similar increase in 

 fertility up to the 1 950-1 season, though it seems that the maximum fertihty 

 is lower than in pre-war years. Here again sampling difficulties are probably 

 responsible, because pregnant females are more abundant in catches in the 

 early part of the season (Mackintosh, 1942), and the later season for blue 

 whales should result in a potentially lower 'percentage pregnant'. Unfor- 

 tunately, with the decline in the catches of blue whales in post-war years, 

 samples after 1954-5 have been too small to use. 



The data for the fm whale follow a similar pattern and I have plotted the 



