PHYSICAL ENVIRONMENT 75 



Stations of this winter voyage lay well north in the West Wind drift where the krill, the adults 

 especially, are now known to be very sparsely distributed. 



To sum up, of the two environments, the open sea and the pack, the krill are to be regarded neither 

 primarily as creatures of the one nor primarily as creatures of the other, but essentially as creatures 

 of both, their presence or absence, their abundance or scarcity, in the one or the other, being 

 entirely dependent on the time of year or locality in which one happens to strike them. For 

 clearly their abundance near pack or among icebergs, or at the edge of a freezing sea, is purely 

 a matter of the chance or natural obtrusion of these encumbrances upon the open water of regions 

 such as the Weddell drift, where in any case, in one condition or another, E. superba is extremely 

 abundant all the year round. In winter, perhaps more so than at any other time of the year, the 

 whole of this great surface stream from west to east (p. 366, Fig. 115) seems literally to be 

 teeming with larval whale food, carrying vast numbers not only along the northern periphery of the 

 pack, but also in wide stretches of the unfrozen drift to the north, the dense concentrations at the 

 ice-edge being simply attributable to the fact that it is the southern part of the current that in the 

 natural order of things periodically becomes frozen over in winter. In fact it is just as misleading to 

 say that E. superba is a creature of the Antarctic pack as to say that certain fishes are creatures of the 

 freshwater ice merely because of the annual freezing over of the lakes they live in. 



From the earliest times, it is true, an abundance of krill has been reported in the pack and we are 

 repeatedly left with the impression that it is there rather than in the open sea that the whale food 

 occurs in greatest profusion. But I am sure we get this idea simply because in high latitudes in the 

 polar summer surface life of all kinds is so very easy to observe. Vessels working through heavy ice 

 are often stopped in water that as a rule is flat calm and crystal clear, and observers, even casual 

 observers, can readily see what is going on, not only at the surface, but quite a few metres below. 



Surface temperature 

 For all that has been published on the systematics and general biology of E. superba it is a little 

 surprising to find that what may well be ^ne of the most important aspects of its physical environment, 

 the thermal conditions of the surface waters in which for the greater part of its life it is known to live, 

 have been largely neglected in the literature or at any rate given little more than cursory attention. 

 For instance, Tattersall (1924) remarks that it is 'a purely Antarctic species, circumpolar in distribu- 

 tion and not yet recorded north of the mean isotherm of 6° C, while Ruud (1932) states that it 'has 

 only been caught with absolute certainty in temperatures between —1-50° (or below) and +4° C. 

 Mackintosh (1934) places it among his 'cold water' group of plankton animals, a group including 

 species that may occur in large numbers anywhere south of the 3° isotherm. John (1936), while in 

 general agreement with both Ruud and Mackintosh as to the temperature range, is more incHned 

 to the view that its normal habitat is in the coldest Antarctic water ranging from the coastal regions 

 of Antarctica to the northern periphery of the sea ice and not very far beyond. Bargmann (1937) 

 and Sheard (1953) repeat this view, the former stating it is 'found in the colder water of the Antarctic 

 zone, and along the edge of and under the pack-ice '. 



From the mass of data bearing on this question that has now been examined it can be shown that 

 the relationship of the krill to surface temperature is a matter of considerable complexity — that the 

 temperature range as might be expected not only varies from month to month and from season to 

 season, but also from place to place, and that as a result of the constantly changing physical environ- 

 ment in which the krill are growing certain developmental phases, owing to the time of year in which 

 they appear in the plankton, spend virtually the whole of their surface existence in distinctly colder 

 conditions than those encountered by other phases. 



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