THE STOCK OF WHALES AT SOUTH GEORGIA 367 



by Harmer (193 1). It is shown that definite incursions take place but that the time of 

 their advent is variable and usually divergent in the two species. In certain seasons the 

 incursions are indicated in the graph of the catch as a single peak, showing that the 

 supply increased for a time, reached a maximum and then declined ; in other seasons 

 there are two maxima separated by an interval of several weeks. As indicated by Harmer 

 (1931, p. no) certain localities are characterized by one or other type of season-graph, 

 the study of which suggests that there are definite migration routes, probably not the 

 same for both Blue and Fin whales, on which under certain conditions, again specific, 

 the moving herds are delayed and the concentrations so profitable to the whalers 

 occur. 



The difference in the relative abundance of Blue and Fin whales is sometimes very 

 sharply marked, and gives rise to the terms " Blue " season and " Fin " season, according 

 to whether there is a great preponderance of one species or the other in the season's 

 catch. These differences do not always occur, of course; sometimes both species are 

 plentiful, sometimes both are scarce. The cause of these fluctuations is not thoroughly 

 known, nor is it certain that the predominance of one species in the catch always indi- 

 cates the true state of affairs in the catching area. If one species concentrates nearer the 

 island than the other, the former will be the predominant one although the latter may 

 be superior in numbers, because the catchers are, after all, concerned only with taking 

 as many whales as they can as quickly as possible. The concentrations and movements 

 of Fin and Blue whales have lately been the subject of a report by Kemp and Bennett 

 (1932). The results obtained are based on returns made by the catchers, and, as the 

 authors remark (p. 169) : " When whales are plentiful in inshore waters the catchers will 

 naturally not go farther afield, and we have evidently no means of knowing how 

 abundant whales may be in the unexplored parts ". It is shown that the main concentra- 

 tions of Fins and Blues as plotted from the combined data of eight seasons (loc. cit., 

 pi. xxiii) are in remarkable agreement and that this agreement is correlated with an 

 abundant food supply; but that in different individual seasons the positions of the 

 centres of the concentrations differ greatly, some part of which is attributed by the 

 authors to irregularities in the time of arrival of different schools of whales. 



For many years the whalers have noticed that some form of correlation can be traced 

 between the presence of Blue whales and the ice conditions. Risting (1930, pp. 56, 93 

 and 97) definitely connects the action of warm and cold currents upon the drifting pack 

 ice with the growth of plankton and the occurrence of whales. Harmer (1931) discusses 

 this question fully and correlates the September mean air temperature at South Georgia 

 (closely connected with the events leading to the melting of the ice) with the order in 

 which the two species reach their maximum in the catches of the season immediately 

 following (p. 131); and by an examination of the records he shows that the species which 

 is first in excess nearly always maintains its superiority in both halves of the season and 

 in the total catch (p. 146). 



We have then evidence from the catches that concentrations of one or other or both 

 species are formed off the island, which vary both in position and in time, such varia- 



