78 DISCOVERY REPORTS 



Disregarding average catch-figures of less than lOO, what we may call the optimum seasonal 

 temperature ranges of the broadly grouped developmental phases shown in Table 4 may be sum- 

 marised as follows: 



Under 16 mm. group (vast majority larvae) 

 Autumn: -2-00° to + 1-99° C 



Winter: June —2-00° to +0-99° C; July and August -2-00° to -o-oi° C 

 Spring: —2-00° to -o-oi°C 

 Summer: Larvae virtually only taken in the vertical nets (see Table 10) 



16-20 mm. group (very young adolescents) 

 Autumn: Except sparsely in the East Wind drift, group virtually does not exist in the plankton (see Table 7) 

 Winter: Group does not exist in the plankton 

 Spring: -2-00° to -o-oi° C 

 Summer: Except sparsely in the East Wind drift, group virtually does not exist in the plankton (see Table 7) 



Over 20 mm. group (older adolescents and adults) 

 Autumn: -2-00° to +2-99°C 

 Winter: -2-00° to -o-oi° C 

 Spring: -2-00° to -o-oi° C 

 Summer: December and January -2-00° to + 1-99° C; February -2-00° to +3-99° C 



So far we have been considering in general terms the thermal environment of the krill throughout 

 its entire circumpolar range. Let us now examine the matter from a regional point of view and con- 

 sider in turn the conditions in the principal water masses, (i) of the South Georgia whaling grounds 

 (Table 5), (2) of the Weddell and East Wind drifts (Tables 6 and 7), and (3) of the Bransfield Strait 

 (Table 8), in which this species is known (p. 61, Fig. 56) to reach its maximum abundance. 



A glance at this presentation of the data reveals at once that throughout the whole vast extent of its 

 circumpolar range E. siiperba is nowhere, not that is to say in any measure of abundance, to be found 

 living in the warmest upper hmit of the absolute temperature range (namely, between 3-00° and 

 3-99° C) except, as might be expected from its relatively low latitude, in the neighbourhood of South 

 Georgia, and that even there it is only encountered in these conditions in February and then only in 

 its older or adult state. In the main east-flowing part of the Weddell stream by contrast (Table 6) 

 and in the East Wind drift (Table 7) it will be seen that the entire surface population throughout its 

 development is confined to water which except in February and March, and then only in the Weddell 

 drift, never gets warmer than i -99° C, and that it is only in March in the Weddell drift that the early 

 larvae, and they alone, are found in some minor measure of abundance in the range 2-00° to 2-99° C. 

 In the East Wind zone the absolute upper limit of the temperature range even in summer is only 

 1-99° C, and it is clear from the average catch-figures in Table 7 that the krill live and develop there 

 in the coldest physical environment to be found anywhere in the Antarctic seas, the vast majority of 

 them in surface temperatures which for 10 months of the year, from March, when the young ice first 

 begins to form, until December, when the first signs of the spring break-up appear, must be below 

 zero C. The prolonged winter cold that exists in the East Wind drift is only to be expected, since in 

 the high latitudes in which this coastal current flows the sea freezes earlier and remains frozen longer 

 than it does in the more northerly regions of euphausian abundance. 



The observations in the Bransfield Strait (Table 8), although not so comprehensive as in the South 

 Georgia or Weddell regions, are nevertheless enough perhaps to indicate that the krill there exist 

 in sub-zero temperatures from April right through to November. They show too that the optimum 

 temperature range of the older adolescents and adults in February lies between o-oo° and 2-99° C, 

 a summer range that approaches more closely to the conditions on the South Georgia whaling grounds 

 than to those in the East Wind-Weddell stream. 



