260 DISCOVERY REPORTS 



be more properly investigated in detail when the long-range oceanographical data 

 accumulated by the ' Discovery II ' have been more fully worked out (see p. 199). In the 

 meantime it is appropriate to draw attention here to certain broad features, and to indi- 

 cate in general terms the connexion between the distribution of whales and their physical 

 environment. 



Most, if not all, truly oceanic organisms in the Southern Ocean have a circumpolar 

 distribution and tend to occupy a more or less well-defined zone, the boundaries of 

 which are liable to shift with seasonal changes in the environment. Since whales are 

 migratory animals this zonal distribution applies only to their summer habitat in the 

 Antarctic. Here the greater part of the stocks of Blue, Fin and Humpback whales 

 occupy a circumpolar belt varying from about 200 to 600 miles wide, and bounded to 

 the south by the pack-ice edge. Hansen's Atlas over Antarktis og Sydishavet (1936), 

 gives an instructive picture of the extent of the zone within which whales are taken by 

 the factory ships. It probably gives a better impression of the distribution of Blue 

 whales than of Fin and Humpback whales, for at that time far more Blue were taken 

 than other species. 



The Antarctic convergence, the point at which the cold Antarctic surface water sinks 

 abruptly below the warmer sub- Antarctic water (see Deacon, 1937, p. 22), is a line 

 running roughly round the middle of the Southern Ocean (see Fig. 2). Although it lies far 

 to the north of the Antarctic circle it is, from the hydrological point of view, the northern 

 boundary of the Antarctic waters. Immediately to the south of the convergence in a 

 zone which is completely, or almost completely, devoid of the krill on which the whales 

 feed. It appears to be identifiable by its plankton population (see Mackintosh, 1934, 

 p. 150, and Hart, 1942), and it is not normally occupied by whales except when they 

 cross it in the course of their migrations. Beyond this is the zone occupied by the 

 krill and mostly covered by pack-ice in winter (see Mackintosh and Herdman, 1940). 

 With the retreat of the ice during the summer the great feeding grounds of the Antarctic 

 are opened up and the whales penetrate into higher latitudes. The majority are then 

 found between the 2° C. isotherm and the edge of the pack-ice. Although Blue, Fin and 

 Humpback whales may anywhere be found together it is well known that Blue whales 

 have a tendency to reach higher latitudes and colder water than Fin whales. Some im- 

 portant changes take place during the summer in the composition of the Antarctic whale 

 population, the main herds of Fin whales for example arriving in high latitudes rather 

 later than the Blue whales, but these are dealt with below (pp. 269-78). 



RELATIVE ABUNDANCE OF THE DIFFERENT SPECIES 



Although it is at present difficult to calculate even the order of magnitude of the actual 

 numbers of the species of whales in the Southern Ocean, it is possible to gain some idea 

 of their relative numbers. For this purpose the Discovery Committee's data, though 

 limited, are more instructive than the statistics of catches, for the proportions of species 

 in the catches are strongly influenced by selection. Thus one species may be caught in 



