472 DISCOVERY REPORTS 



effect of varying intensity of whaling should show initially only at the ages when animals enter the 

 catches. It should take many years for these changes to work their way through the age structure of 

 the population, for the catches of fin whales may be drawn from as many as fifty year groups. Marks 

 have been recovered from antarctic fin whales as long as 26 years after marking (the first effective 

 marking was carried out in 1932/33). 



Thus even if the catch were a random sample of the stock in the sea important changes could take 

 place in the stock without being detected by the calculation of instantaneous mortality rates derived 

 from a regression line fitted to the catch curve. This method will give only a crude approximation to 

 mortality rates operating over a period of years, and mortality rates derived in this way may be mis- 

 leading when applied to current situations. 



The catch per unit effort gives an index of abundance of the stock. In estimating current mortality 

 rates the most satisfactory treatment would perhaps be to convert the sizes of year classes at two 

 known times to comparable values, by applying effort values so as to obtain density indices for different 

 age groups. Estimates of mortality can then be made from pairs of successive years for different year 

 classes fully recruited to the exploited part of the stock. Unfortunately, the unit of effort in whaling 

 is not stable owing to increasing catcher efficiency (see Laws, in press) and it has not yet been 

 possible to calibrate these changes in effort so that they can be used to estimate mortality rates. This 

 is potentially the most accurate approach, though sampling difficulties and changing recruitment 

 again pose serious problems. 



A method which will give us some indication of changing mortality rates is to construct time- 

 specific survival curves by smoothing the frequency distribution of corpora numbers (Table 31) more 

 or less heavily, according to the size of the sample. The resulting curve is converted by a graphical 

 method to an age frequency distribution by finding the values corresponding to 1-43, 2-86, 4-29 

 corpora up to 52-91 corpora (equivalent to post-pubertal ages of \ year, \\ years, z\ years and 

 37^ years). An additional 5 years must be allowed for the immature period, making the ages 5^, 6|, 

 7^ and up to 42^ years (Table 32). 



Direct observation of the age structure of juveniles is not possible by this or any other method 

 because of the minimum size limit, which means that early year classes are absent or not fully repre- 

 sented in the catches. An indirect method must, therefore, be used to estimate the recruitment to the 

 population. 



The data given in Table 32 show a total of 2681 females. For present purposes it may be presumed 

 that fertility does not change with age (see p. 454). The conception rate is taken to be 1-18 per 2-year 

 cycle, assuming 18% post-partum ovulations (p. 430), that is 0-59 per year. No allowance is made 

 for post-lactation conceptions, or for twins, because it is thought that post-natal survival of these 

 groups is low. On this basis 2681 adult females represent 1582 conceptions. The foetal sex ratio was 

 shown to be 52% male: 48% female so these females are estimated to carry 759 female foetuses. 

 Let us allow 10% prenatal mortality to cover foetal deaths from conception to birth and maternal 

 deaths from mid-pregnancy to parturition. This figure is based on an assumed 5% foetal mortality 

 and about 12% maternal deaths, the latter estimated from table 32. (An assumed stable population 

 of 2681 mature females has a recruitment at puberty of 325 or 12%, which should be balanced by 

 a corresponding number of deaths.) Then 683 female calves are expected to be born, the earlier 

 qualifying remarks about sampling being understood to apply. These data indicate that total immature 

 mortality assuming a stable population is 52-4%. If the population from which this sample is derived 

 was decreasing then the immature mortality would be higher than this value. 



This 'apparent' survival curve is plotted on a logarithmic scale in Text-fig. 58. The logarithmic 

 scale has the advantage that a straight line implies equal rates of survival (or mortality) with respect 



