THE AGE OF FEMALE BLUE WHALES 263 



faded to extinction or grown to such enormous proportions as to have gained historic 

 notice. Furthermore we do not know that the incidence of the natural death rate is 

 constant at all ages, but analogy with other species gives us some ground for the belief 

 that mortality is roughly constant between adolescence and the onset of senescence. 



In the present instance adults only are being considered, and the age distribution of 

 adults indicated by the number of corpora lutea has been shown above (Fig. 9). The 

 actual numbers in each age class for the seasons 1934-5 an< ^ 1935^6 have been replaced 

 by the logarithms of these numbers in Fig. 13. If we consider these graphs in terms of 

 the principles advanced above, the most obvious feature is that the age distribution does 

 not fall on a straight line. It would appear therefore that the population from which 

 these samples were taken is very far from being a natural one. It is difficult to find any 

 explanation for this except on the grounds that the stock is being affected by hunting. 



It is impossible to say where a line representing a static population would lie in 

 relation to the existing curves. Supposing however we assume that it lies along the 

 line A- A' in Fig. 14, in which the two logarithmic curves of Fig. 13 have been amalga- 

 mated. This line does lie along a part of the curve which approximates to a straight line, 

 and which may conceivably be taken to represent what is left of a stock of whales which 

 lived under natural conditions. The shape of the left part of the curve with relation to 

 the straight line would indicate a heavy reduction in the number of young adults. If on 

 the other hand the natural line should lie in the position B-B' the indication would be 

 of an unnatural reduction in the number of the older whales. The latter position for 

 the line seems improbable since the gradient is so slight as to lead to the corollary that 

 whales under natural conditions should live to be well over seventy years of age. If this 

 were true, an occasional specimen would in all likelihood have appeared in these 

 collections with an age of at least forty years. 



It may be argued then that the line A-A' in Fig. 14 is more likely to be an indicator 

 of the constitution of a natural population. If we could be certain of this it might be 

 possible to calculate the date at which whaling first began to take effect on the composi- 

 tion of the adult stock, but this would lead us too far into the realms of speculation. All 

 that can be said with some degree of certainty is that a substantial reduction in the 

 adult stock has been caused, most probably among the younger whales. 



The influence of the killing of adults is twofold. Firstly, the stock is diminished by the 

 actual numbers taken. Secondly, and more important, the taking of female adults 

 diminishes the breeding potentialities of the stock and hence causes a decline in future 

 recruitment. The loss is greater or less according to the expectation of life which each 

 adult had at the time of its capture. If it were possible for only the oldest females to be 

 taken, the loss to the future stock would be slight, since these whales would have contri- 

 buted most of their quota of young. But if the killing applies to all ages and is excessive 

 the inevitable result will be a decline in recruitment. 



It is important to note that the effect of whaling would be cumulative, since adults 

 killed are a loss in terms of future recruitment as well as a diminution of the actual stock. 

 Continued whaling can therefore mean only a continued diminution of the breeding 



