2ye DISCOVERY REPORTS 



observations were made, the stock on those grounds must have been almost equally- 

 composed of individuals that had already spent some months in Antarctic waters and 

 the forerunners of the great second wave of (mainly immature) arrivals from the north. 



CORRELATION WITH FATNESS 

 It is not possible to demonstrate the correlation between heavy diatom infection 

 and fatness by the most obvious methods, for the following reasons: if the fatness of 

 whales bearing thick film is plotted against that of whales upon which no film was 

 seen, the numbers in the latter group are usually small and unrepresentative of the 

 stock as a whole. If the uninfected whales in any given month group happen to 

 include a few pregnant females or newly weaned calves, the fatness ratio of which 

 is always high, the figures tend to be entirely misleading, while, if such whales are 

 disregarded, the numbers may be so small that it is certain they cannot give a true 

 mean. The more obvious alternatives: plotting fatness of all infected whales against 

 uninfected whales, or heavily infected whales against uninfected and slightly infected 

 whales also fail, the first owing to the rapidity with which a slight degree of infection 

 may be attained, and the second because it is not possible to be certain how much 

 of the film has been lost after death in a whale which shows diatoms but is not heavily 

 infected. These complications appear to be chiefly due to the constantly changing 

 character of the whale population off South Georgia. Nevertheless, we have found 

 that the gunners at South Georgia hold by Bennett's early observation, that whales 

 upon which the infection is heavy tend to be in better condition than the others. This 

 would no doubt be more readily apparent on the South Shetland grounds, which lie 

 farther within the Antarctic Zone, and at which, therefore, the differences in condition 

 between early and late arrivals from the north are likely to be much greater. 



With these considerations in view it was decided that the only satisfactory way in 

 which the correlation could be tested with regard to the South Georgian whales was 

 to take the mean fatness of the total Fin whales in each month group as the nearest 

 possible approach to a figure representative of the condition of such a mixed population, 

 and to plot against them the mean figures for the heavily infected Fin whales. This 

 has been tried with considerable success with the records of the 1928-9 and 1929-30 

 seasons, when as shown below the heavily infected Fin whales were fatter than the 

 average in no less than ten out of the twelve month groups studied. The number of 

 Fin whales taken during the very poor 1930-1 season was insufficient to test the corre- 

 lation satisfactorily, and for the same reason little reliance could be placed on the Blue 

 whale figures obtained at Grytviken during 1928-31. 



The figures obtained by this method from the data of the 1928-9 season are plotted 

 in Fig. 7, the solid line represents the mean fatness of the total Fin whales measured 

 during each month, and the pecked line that of the Fin whales upon which heavy 

 diatom film was observed. It will at once be seen that the heavily infected whales were 

 fatter than the average except for the first month of the season. The most reasonable 

 explanation of this exception lies in the probability that some of the whales remaining 



