PHYSICAL ENVIRONMENT 83 



The monthly surface temperature range of the krill in the sparsely populated West Wind drift, in- 

 cluding such circumscribed parts of it as are affected by local encroachment of cold water from the East 

 Wind zone, is shown in Table 9. Small though the average West Wind catch-figures generally are they 

 are, nevertheless, enough to indicate that the scattered population in this wide circumpolar belt exists 

 throughout the year in conditions broadly comparable with those on the South Georgia whaling 

 grounds. There are several instances it will be seen where the average catch-figures in Table 9 point 

 to some, if only minor, measure of West Wind abundance. Such instances are provided by the figures 

 for the larvae and early adolescents (the under 16 mm. and 16-20 mm. groups) in December and 

 again for the larvae in March, June and July. Strictly, however, these figures are not typical of the 

 West Wind zone, for they are based almost exclusively (i) on observations in the north-going outflows 

 of cold East Wind water that occur in the neighbourhood of the Cape Adare — Balleny Islands and 

 Kerguelen-Gaussberg submarine ridges (p. 58, Fig. 4), and (2) on observations so close to the only 

 approximately known northern boundary of the Weddell drift that for all we can tell they may have 

 been located in the more northerly reaches of this krill-rich stream. 



Temperature range figures for the exclusively larval gatherings from the vertical nets are given in 

 Tables 10 and 11, Table 10 covering the principal region of larval abundance in the East Wind- 

 Weddell surface stream. Table 1 1 the West Wind drift, the South Georgia whaling grounds and the 

 Bransfield Strait, three regions where our vertical gatherings of larvae in the vast majority of instances 

 have been of very small or virtually negligible size. In both tables the average monthly catch-figures 

 are based on the material obtained in the combined hauls of the three uppermost nets traversing the 

 250-100 m., 100-50 m. and 50-0 m. layers, and as might be expected with nets of such small size 

 hauled for such short distances the figures in general are small and, except for from January^ to 

 June in the Weddell drift, do not carry the major significance provided, for instance, by the much 

 more voluminous data from the stramin nets. Small, however, though they are they are nevertheless 

 of sufficient significance, having regard to the size of the vertical net and the circumstances in which 

 it was fished, to be used in certain instances to augment the data from the stramin nets, particularly 

 where such data are scanty as, for example, in the Weddell drift in January and February (Table 6, 

 under 16 mm. group) and in the East Wind drift from February to May (Table 7, under 16 mm. 

 group), and it is accordingly as such that they have been used in the final summary of the temperature 

 range data that follows on p. 87. The pronounced falling off of the vertical catch-figures from July to 

 November it may be noted is primarily to be ascribed to the vertical distribution of the larvae during 

 that period. For whereas from January to June the young epiplanktonic population ranges in great 

 abundance from the surface down to 250 m. (p. 90, Tables 13-15), and in consequence can readily 

 be captured by any or all of the three vertical hauls made between those levels, from July to November 

 (Table 15 and pp. 331 and 335), it tends to be massed permanently very close to the surface and so 

 can only be sampled momentarily by the uppermost 50-0 m. vertical net. 



The average monthly catch-figures for the surface larvae we have occasionally recorded in the 

 vertical nets outside the main East Wind- Weddell stream (Table 11), although on the whole very 

 smalP are, nevertheless, again perhaps enough to indicate broadly that, the Bransfield Strait apart 

 (Table 8), such larvae as are encountered in these northerly latitudes must develop in distinctly 

 warmer summer and autumn temperature ranges than anywhere else in the circumpolar sea. 



1 The Discovery expeditions surprisingly recorded very few surface larvae in the Weddell drift in January. Norwegian 

 observations, however (p. 88), not included in Table 10, reveal heavy January concentrations there, indicating at the same 

 time that the absolute surface temperature range of the Weddell larvae then lies between — 2-oo° and + 1-99° C, the optimum 

 between —2-00 and +0-99° C. 



2 A single, and exceptional, catch of 707 Calyptopes was recorded on the eastern side of South Georgia in March 1937. 

 Without this gathering the average March catch-figure for the 3-00° to 3-99° C range would be nought instead of loi. 



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