47 4 DISCOVERY REPORTS 



against reading too much into the apparent survival curve and against describing the age structure 



by a single regression line. 



A third method of constructing an apparent survival curve is the dynamic method which assumes 

 that the sample represents the ages at death. In the present case the sample represents deaths due to 

 whaling and does not take into account natural mortality. However, the adult natural mortality rates 

 of fin whales should be very low to counterbalance the low reproductive rate and long life-span and 

 are probably constant over most of the age range. If we accept the fact that sampling difficulties 

 mean that estimates of absolute mortality rates derived from survival curves are unreliable and not 

 contemporary, we can still expect to obtain limited information on changes in the stocks by comparing 

 the shapes of apparent survival curves. While it is true that neither method is really satisfactory this 

 is thought to be preferable to calculating instantaneous mortality rates from the slope of a single 



IOOO 



500 



IOO 



O 

 > 



> 



a. 



IO 



AREA H 1939-1941 



IO 



- 1 — 

 30 



20 



AGE IN YEARS 



40 



50 



Text-fig. 58. Time specific survival curve for female fin whales. See text for method of construction. 



regression line. For this purpose dynamic survival curves appear to be more desirable. Bias towards 

 the catching of older animals (see above) will be shown in the dynamic curves as lower survival rates 

 or higher mortality rates (which is correct) rather than as higher survival or lower mortality as in time 

 specific curves. 



Dynamic survival data for the mature females is given in Table 32. These are derived from the 

 counts of corpora albicantia in the following way. There are 389 individuals in the sample and all of 

 these have survived to an age corresponding to one corpus luteum or corpus albicans; there are 

 39 individuals in the 1 -corpus group, from which it is inferred that only 350 survive to the 2-corpora 

 group; 21 individuals in the 2-corpora group are then subtracted to give the number which survive 

 to the 3-corpora group, in this case 329; and so on up to the 55-corpora group when there is no sur- 

 vivor. These data are converted to actual age data as before by finding the values corresponding to 

 1-43, 2-86, etc. corpora and the result is set out in Table 32 and Text-fig. 59. Recruitment at birth 



