THE AGE OF FEMALE BLUE WHALES 267 



The tables above show that the rise in the percentage of immature females in the 

 catch has been very considerable and has more than doubled itself in four years. 



In the issue of Hvalradets Skrifter from which these figures were taken, the authors 

 suggest that the rise in the number of immatures in 1934-5 is due to some extent to the 

 time limit and later season. In this year for the first time the Norwegian and most of the 

 British fleet did not start fishing till the beginning of December instead of in October 

 as normally in the past. There may be a tendency for more immature whales to be 

 killed in the latter half of the season, as their data go to show, but it is extremely 

 doubtful whether the shifting of the centre of gravity of the season could produce such 

 a marked difference as shown above. 



Not only is the rise in immature captures a sign of depletion of the stock, but the 

 practice of taking any immatures at all must be deprecated, since future breeders are 

 being removed from the stock before they have reproduced. The decline in the recruit- 

 ment of the adult stock which was discussed in a previous section (pp. 261-5) will have 

 been greatly accelerated in late years by the killing of immatures. Killing of adults, as has 

 been said, reduces the birth rate, in any case, and if those whales that do succeed in 

 being born are intercepted before they reach maturity the recruitment is bound to 

 decline even further. 



SUMMARY 



1 . Investigation into the life history of Antarctic Blue whales has been made both by 

 personal examination of Blue whales taken in pelagic whaling and by inspection of 

 ovaries imported from the southernmost whaling grounds. Use has been made of 

 statistics supplied by the factory ships sending ovaries and of summarized statistics 

 prepared by Norwegian writers on the subject. Material collected at South Georgia 

 and Saldanha Bay from 1925-31 by the staff of the Discovery Investigations has also 

 been utilized. 



2. The conditions of modern pelagic whaling are shortly reviewed in so far as they 

 affect the composition of the catch on which conclusions may be based. 



3. There seems every reason to believe that Blue whales taken by the pelagic factories 

 in the Weddell, Bouvet and Kerguelen Areas are identical in species and reproductive 

 habits with those taken at South Georgia and off the African coast. Sexual maturity 

 appears to be attained at the same age and length, and the pairing season and period of 

 gestation seem to be identical. 



4. The onset of physical maturity in Blue females coincides with the accumulation 

 of eleven corpora lutea in the ovaries. Length above 81 ft. is in general no guide to 

 age. Blue females become physically mature at a minimum length of 86 ft., though 

 many grow much beyond this length. 



5. Detailed inspection of the percentage of pregnant and resting whales taken in 

 pelagic whaling and at South Georgia and Saldanha Bay suggests that while Blue 

 whales can breed once every two years at best, a substantial proportion of the adult 



