475 



AGE-DETERMINATION BY MEANS OF THE OVARIAN CORPORA 

 is calculated as before to be 857 indicating a total immature mortality from birth to 5 years of about 

 53'3%> a ver Y similar result to that given by the time specific method. 



This apparent survival curve again approximates to the diagonal type (Text-fig. 59). Mortality 

 appears to be higher in the early segment of the curve and increases again after 25-26 years. Between 

 9 and 26 years the survival curve plotted on a logarithmic scale is almost exactly linear, that is to say 

 the decline in numbers is exponential. For this segment of the curve the apparent rate is e 00973 or 

 90-8%, corresponding to an annual mortality rate of 9-2%. 



One of the central problems of whale research is to define and distinguish the different populations 

 being sampled ; a complementary problem concerns the extent to which the samples from the catches 

 are representative of the actual population in the sea. Until more progress is made in the solution of 

 these problems it would seem that population studies will lack precision. 



1000 



500 



100 



a. 

 o 

 > 



> 

 a 



m 



20 30 



AGE IN YEARS 



Text-fig. 59. Dynamic survival curve for female fin whales. See text for method of construction. 



Growth curves 

 The relation between length and corpora number for area 11 females has already been discussed 

 (Table 6, Text-fig. 25). The average length at physical maturity was there taken to be 73 ft., which 

 was the average length of 187 females which had more than 20 corpora in the ovaries. Nishiwaki, 

 Ichihara and Osumi (1958) suggest that after the attainment of physical maturity there is a slight 

 decrease in body length and the present material also suggests this, although the shrinkage is evidently 

 slight and there is no statistical support for it. The relative growth curve in Text-fig. 25 was, therefore, 

 fitted to the average lengths at different corpora numbers so that it attained 73 ft. at 14-3 corpora. 



This smoothed curve may be converted to a length-at-age curve in the same way as the age structure 

 of a sample was obtained (p. 466). The lengths corresponding to 1-43, 2-86, 4-29 corpora, etc., are 

 equivalent to post-pubertal ages of about \ year, i\ years, 2 \ years, etc., and as before an additional 

 5 years is added to allow for the immature period (Table 33). 



