458 DISCOVERY REPORTS 



80% pregnant. The great reduction in the intensity of whaling during the war years is apparently 

 correlated with a fall in the percentage pregnant to the original low level of the early 1930's. Then, in 

 post-war years, there was a similar increase in fertility up to the 1950/51 season, though it seems that 

 the maximum fertility is lower than in pre-war years. Sampling difficulties may be responsible for 

 this change, because pregnant females are more abundant in the catches early in the season (see above) 

 and the later season for blue whales in recent years would be expected to result in a potentially lower 

 ' percentage pregnant '. The catches of blue whales have declined greatly in post-war years and samples 

 after 1954/55 have been too small to use for this purpose. 



The data for fin whales follow a similar pattern and the values for area 11 and areas m and iv have 

 been plotted separately in Text-fig. 53. Areas m and iv have a similar history of exploitation, beginning 

 long after area 11 (Text-fig. 54). 



The 'pregnancy rate' for the South Georgia fin whale catches rose from about 50% in 1925/26 to 

 70% or more at the beginning of the 1930's. At this time the annual catches of both fin and blue 

 whales in area 11 were increasing (Text-fig. 54) (geographically South Georgia is within area 11). The 

 pregnancy rate for area 11 fin whales had risen to about 80% by 1940/41, after some years of high 

 catches in this area (Text-fig. 54), but in 1945/46 after the greatly reduced catches of the war years 



I960 



Text-fig. 54. Antarctic catches of fin whales from 1909/10 to 1957/58 according to the whaling areas. 



had fallen to 32%. This season was rather an abnormal one and the sample may have been less 

 representative of the stock than in later seasons, but bearing in mind the additional evidence from 

 blue whales, it does seem reasonable to conclude that the real pregnancy rate was low. By 1950/51, 

 after several seasons of sustained heavy exploitation (Text-fig. 54), the 'percentage pregnant' was 

 above 80%. This would appear to be the maximal response of which the fin whale was capable, for 

 in later seasons (1955/56-1958/59) even after larger catches the percentage pregnant was stable at 

 about 80%. The apparent decrease in the pregnancy rate in 1958/59 is disturbing, but may not be 

 real, and it is too early to assess its full significance. Owing to differential sampling as mentioned 

 above, a 'pregnancy rate' of 80% is probably equivalent to a real pregnancy rate of about 60%. 



Similarly in areas in and iv the 'pregnancy rate' appears to have stabilized in post-war years at 

 rather more than 80%. A very small sample from area m in 1932/33 suggests a very much lower 

 pregnancy rate, but little confidence can be attached to this figure. 



In area 1 the ' percentage pregnant ' was round about 70-80% from 1955-58 when very large catches 

 were being made in this area. Area I was a sanctuary up to 1955/56, so that according to the hypothesis 

 that fertility is lower in natural conditions and rises with increasing fishing mortality, the ' percentage 



