264 DISCOVERY REPORTS 



The Antarctic population, shows a similar rise and fall in numbers, but the peak appears to be later 

 in the year, and the summer and autumn averages are almost the same. The low average in February 

 may be due to the comparatively small number of hauls available in this month, and if this is so then 

 possibly the summer total is rather too low. 



The extent of the winter migration in the Antarctic can be seen by comparing the totals and averages 

 for the months during which the animals are in deep water (May-September) with the totals and 

 averages for the remainder of the year (Table 136). 



Table 130. Monthly and seasonal totals and averages per 20 min. haul zvith N100B 



in the surface layers 

 (For explanation of figures in brackets see p. 263.) 



Table 13ft. A comparison of winter and summer totals from 1 m. oblique net hauls in Antarctic water 



From this it can be seen that the surface population is three times larger in summer than in winter. 

 Although this is due in part to the presence of many young broods, it must be remembered that there 

 are very large numbers of young hatched in the autumn, and therefore a major part of the difference 

 between the two seasons is due to migration. 



The numbers of S. gazellae decrease in the vicinity of the Antarctic Convergence, and although this 

 is not very apparent in Fig. 17, Fig. 18 shows some North to South lines with the stations plotted 

 according to their distance in nautical miles from the Convergence, which demonstrate the point more 

 clearly. 



The position of the Antarctic Convergence fluctuates slightly, and it is likely that the sudden and 

 violent changes in physical conditions brought about by these fluctuations are unfavourable to 

 S. gazellae (Eukrohnia hamata is numerous in this area and is presumably not so sensitive to environ- 

 mental fluctuations). 



