2 8o DISCOVERY REPORTS 



The same authors have given much attention to changes in the average size and the 

 percentage of immature whales in the yearly catches of Blue and Fin whales. Although 

 the average sizes of immature and mature Blue whales calculated separately has re- 

 mained more or less constant, the proportion of immature whales in the catches has 

 shown a significant increase, and for this reason the average size of all Blue whales has 

 declined. The percentage of immature Blue whales, according to Bergersen, Lie and 

 Ruud's figures, increased from 15-9 % in 1930-1 to 36-3 % in 1935-6. This increase 

 was arrested in subsequent years, but would no doubt have continued had it not been 

 for the imposition of minimum lengths below which whales could not be taken. In the 

 same period the average length of immature and mature whales combined declined 

 from 78-6 to 76-6 ft. in males and from 82-4 to 79-0 ft. in females. This must be the 

 result of a reduction of the stock of Blue whales. It can be inferred that the average age 

 of the catch has declined, but it does not follow with certainty that the average age of the 

 stock has declined, for if Blue whales are scarcer there may be less selection of the larger 

 (and on the whole older) individuals. The Norwegian authors emphasize that a fall in 

 the average length of the mature whales would be of greater significance. Changes in 

 the composition of the catches must in any case be interpreted with caution, for as 

 Ottestad (1938) has pointed out, they may arise in part as a result of fluctuations in the 

 year classes of whales. However, when the data from the Discovery Committee's col- 

 lections of ovaries, which cover a number of years, have been fully worked out (see p. 216) 

 it should be possible to decide whether there has in fact been a reduction in the average 

 age of the adult whales, for this would be reflected in the yearly average numbers of 

 corpora lutea in the females, proper allowance being made for the locality and time of 

 year in which the specimens were obtained. In adult whales length is only in a small 

 degree indicative of age (see p. 225), and it seems quite possible that the average age of 

 mature whales could undergo a reduction without any change in the average length of 

 mature whales. 



Although we are not yet in a position to make a quantitative estimation of the rate at 

 which the stock of Blue whales has been reduced, it seems at least certain that depletion 

 has gone far enough already to have an important effect on the whaling industry, even 

 though the level of production has perhaps been maintained by larger catches of other 

 species and by technical improvements. 



For a detailed comparison of the signs of depletion in the separate areas reference 

 should again be made to Hjort, Bergersen, Lie and Ruud, who have shown that the 

 percentage of Blue whales in the catches has declined to a lower point in area II (16 % 

 in 1937-8) than in areas III and IV in which whaling has not been conducted for so 

 long. Similarly, the percentage of immature Blue whales was at first highest in area II ; 

 but in later years area II was overtaken in this respect, first by area III and then by 

 area IV as the intensity of whaling spread eastwards. 



There seems to be no obvious indication that the stock of Fin whales has yet been 

 seriously affected by the whaling industry, though there appears to be a tendency for the 

 percentage of immature whales to increase if the yearly figures are considered separately 



