MULTIPAROUS FEMALES 449 



waters in January, and entered the pelagic catches in peak numbers in February. The ascending limb 

 of the pelagic curve actually appears to be about one to two months later than the ascending part of 

 the curve for South Georgia. In drawing this figure the scale, on which the estimated numbers of 

 lactating females have been plotted, has been so arranged that the descending limb of these curves 

 is in advance of the descending limb of the upper curve (which shows the approximate expected 

 frequency of weaning), by about 2 weeks to a month. This is in order to allow for the fact that lactating 

 females taken in antarctic waters are on average estimated to be only 2-4 weeks in advance of weaning. 

 The high value for March in the South Georgia curve reflects the high percentage frequency of 



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MONTHS 



Text-fig. 51. Above: A, estimated frequency of weaning; B, monthly incidence of lactating females in South Georgia 

 catches, and C, in pelagic catches. Below: average monthly sea surface temperatures in South Georgia waters and on the 

 pelagic whaling grounds. 



lactating females in the sample for this month (Text-fig. 50) ; it may well be too high. Even so the 

 slope of the descending limb of the South Georgia curve is fairly close to that of the expected curve. 



In explanation of these results, it is suggested that the southward migration of females with suckling 

 calves is dependent on, and limited by, seasonal changes in the temperature of the sea. In latitudes 

 from 56 S. to 66° S. the average sea surface temperature is at a maximum in February (Mackintosh, 

 1946, fig. 11), when it is 1-2 C. higher than in December or earlier. The average surface temperature 

 for March is also much higher than the December value and somewhat higher than the January surface 

 temperature. Also, the summer rise in the surface temperatures appears not to begin until early 

 January in higher latitudes (64°-66° S.) owing to the presence of sea ice. 



In Text-fig. 5 1 the average monthly sea surface temperatures for South Georgia and the pelagic 

 whaling grounds are plotted for comparison with the curves showing the relative frequency of 



