374 DISCOVERY REPORTS 



or other causes, the stock that would normally have invaded South Georgian waters 

 from the north followed some other route, and were replaced by a small number of 

 whales moving in a west to east direction. 



The general agreement between the seasonal variation in the stock of Fin whales at 

 South Georgia and the inferences that can be drawn from diatom infection receives 

 striking, if unconscious, confirmation by some of the earliest observers. Captain Bryde 

 and Mr Henriksen, in 1913, informed Major Barrett-Hamilton of the existence of 

 "separate schools that do not mix, of a larger grey or yellowish^ form, and a smaller 

 black form, of South Georgian Finners" (Hinton, 1925, p. 129). Since the large whales 

 tend to arrive on the grounds early, they are naturally more liable to diatom infection 

 than the smaller whales, mainly immature, which are known to arrive in well-segregated 

 schools about mid-season. The yellowish colour produced by diatoms can be so marked 

 as to have led in the past to the mistaken recognition of the "sulphur-bottom" as a 

 distinct species, so that considered in this light the observation becomes readily under- 

 standable. 



The fairly good agreement between the percentage infection and the general know- 

 ledge of whale movements gained from other sources indicates that it is a useful aid to 

 the study of migration, provided that a sufficiently large number of observations are 

 available. By itself the method would be unreliable except in so far as it seems certain 

 that an infected whale has passed at least a month within the Antarctic Zone. 



THE SEASONAL VARIATION WITH THICK FILM 



The percentage of Blue and Fin whales upon which thick and extensive film was 

 observed during the three seasons studied is shown in Fig. 6. It will be seen that 

 the figures bear out the impressions gained from the study. of percentage infection, 

 some features, indeed, being very strongly accentuated. Chief among these are the 

 large number of whales that must have been a long time in Antarctic waters and 

 yet were captured at the beginning of the 1929-30 and 1 930-1 seasons, and the return 

 of Blue whales to South Georgia from more southerly waters in March and April 1929. 

 During the 1928-9 season the influx of Fin whales in December is clearly shown by the 

 low proportion with thick film ; the February figures were similarly low, though, as we 

 have seen, the percentage infection showed but a slight falling ofli^. Thus the impression 

 is gained that while the influx of immature whales in February cannot have come 

 directly from the north, a majority of them had not been very long within the Antarctic 

 Zone. 



The steady increase in the percentage of Fin whales with thick film during the latter 

 half of the 1929-30 season indicates that the whales were remaining on the grounds, as 

 the percentage infection never became very high owing to the arrival of immature 

 whales from the north. 



The figures for the abnormal 1 930-1 season indicate that some Fin whales moved 



^ Italics mine. 



