THE SEASONAL VARIATION IN PERCENTAGE INFECTION 



273 



This is clearly shown by the figures for 1928-9 (Fig. 3). The marked falling ofi^ in per- 

 centage infection of Fin whales in December represents the second wave of invasion 

 from the north. Examination of the maturity data for this season showed that an un- 

 usually large number of the December arrivals were mature, while it is evident from the 

 diatom data that some of the later immature arrivals had been within the Antarctic 

 Zone for some time. Kemp and Bennett's observations (1932, p. 180) make it seem 

 most probable that these infected immature whales were approaching from the west- 

 ward. In all other respects, the diatom data for this season is in accordance with the 

 general idea of whale movements gained from other sources. 



The figures for the following season (Fig. 4) do not agree so well. Little reliance can 

 be placed on the Blue whale figures for the later part of this season as the numbers taken 

 were very small. It is probable, however, that a majority of the Blue whales taken at 

 South Georgia throughout that season were recent arrivals in the Antarctic Zone on 

 passage to more southerly feeding grounds, and that but few of them followed the same 

 route on their return. The initial percentage infection of both species during this season 

 was high, which indicates that an unusually large nucleus of the stock remained south 

 during the preceding winter. This is well borne out by a study of the percentage having 

 thick or extensive film, figured in the next section (Fig. 6). The influx of immature Fin 

 whales after the new year is reflected in the drop in percentage infection during 

 January 1930. 



The figures for the third season studied, 1930-1 (Fig. 5), are somewhat vitiated by 

 the absence of observations in October, but ^, 

 suffice to show that conditions must have been 100 -, 

 somewhat abnormal. The Blue whale figures 

 follow a more or less normal course, with an 

 extremely high percentage infection at the end 

 of the season, when they were presumably ^ 

 travelling north. The Fin whale figures were 

 extremely high throughout, and such immature 

 whales as arrived after mid-season had evidently ^g _ ' 

 passed some time within the Antarctic Zone, as 

 most of them were infected. These conditions 

 again were probably brought about by an 

 unusually large number of whales remaining 25- 

 south throughout the winter. The latter part 

 of the preceding season had been unusually 

 mild, with poor plankton. In the spring of „_ 



1930-1 the pack-ice extended to the north of NOV DEC JAN FEB&MAR 



South Georgia and almost certainly caused Fig. 5. Seasonal variation in percentage infection 



the whales to accumulate locally, which would ^l .^°^'^ ^^^''^'^ '^"""g *he 1930-1 season. 



, ^ P . . . f. . rrii r 1 Solid line Fin whales, pecked line Blue whales. 



m Itself favour rapid miection. i hen, after the 



ice cleared away, it is at least possible that, owing to previous lack of food, persecution. 



4-2 



