EFFECTS OF WHALING ON BALEEN WHALES 



155 



the number in the sample with only one corpus in the ovaries is subtracted 

 from the total to give the number surviving to accumulate two corpora, 

 and so on. It is in effect a form of smoothing. These data are then plotted 

 on a logarithmic scale and converted to age data by interpolation, assuming 

 that on average i • 43 corpora accumulate per year. For the later samples 



955-7 



1000 



100 



10; 



10 20 30 40 50 O 10 



Age in years 



Fig. 8. — 'Survival curves' for female fin whales, area IL 



20 



30 



the average age at puberty is taken to be five years from ear-plug data 

 (Laws, in press), and for the pre-war sample puberty is assumed to be half 

 a year later. The part of the survival curve relating to adult females therefore 

 begins at five and a half or six years. 



There is no reliable material for immatures but the number of female 

 calves has been estimated from the total number of adult females. The 

 conception rate is taken to be 60 per cent; a foetal sex ratio, referring to 

 mid-pregnancy, of 48 per cent females (Laws, in press) is assumed to 

 apply at birth and total foetal mortality is estimated at 20 per cent. This 

 mortality estimate is the only assumption not based on firm evidence, but is 

 probably realistic and in any case variations in this parameter have a relatively 

 small effect on the estimates of recruitment. The recruitment is then plotted 

 and the whole curve adjusted so that it represents the survival of a cohort of 

 1,000. 



This method gives hypothetical survival curves, and for the reasons I have 

 given they are probably biased and not applicable to the stock as a whole, 



