VERTICAL DISTRIBUTION AND MIGRATION RECONSIDERED 271 



abundant.! Moreover, it has been reported so often by so many different voyagers from the earhest 

 times that it can scarcely, I think, be such a rare occurrence as Professor Hardy suggests. The fact, 

 too, that baleen whales have so often been seen feeding on the surface, not only in the Antarctic, 

 but wherever the fishery has been, or is now, carried on, provides further evidence that it is there that 

 the daytime swarms of these and many other congregating animals must often be. It will be recalled 

 too (p. 151) that congregating birds have commonly been reported feasting on surface swarms by day. 

 I do not of course suggest that the mass of the krill population is gathered in visible swarms at the 

 surface at any one time, but simply that surface swarming can be seen as a not uncommon pheno- 

 menon, at any hour of the day or night, if a careful watch is kept. 



I would finally call attention to one important aspect of the behaviour of the swarms that must some- 

 times I think contribute substantially to the efficiency of the oblique net as a daytime sampling instrument. 

 Surface swarms of this and other species (p. 262) have been seen to sound (sink bodily out of sight) when 

 disturbed by an approaching vessel. When this happens they must sometimes I feel sure sink directly into 

 the path of the oblique net rising from the ill-lighted depths below, and, taken unawares, be readily 

 captured. This too could be described as vertical migration, but of a limited and specialised kind. More 

 correctly perhaps it should be regarded as avoidance. The same thing, it seems, occasionally happens at 

 night, to the advantage of the oblique net (p. 28 1 , Table 59, lower part) and the disadvantage of the net on 

 the surface. In fact it is obvious that sounding surface swarms, at any time of the day or night, would be 

 sampled much more effectively with an oblique than with a surface net since the latter would be passing 

 over, not through them, and might be missing them altogether or at best getting only the stragglers. 



To sum up, it is clear that the more strictly planktonic section of the surface population, the larval and 

 early adolescent swarms, tends to be massed at or near the surface both by night and by day, and in con- 

 sequence can generally be sampled with conspicuous success (Tables 57 and 58) by a surface net at any hour 

 of the day or night. It is clear, too, that the younger swarms are not very successful daytime escapers. With 

 the older swarms (Table 59), which are obviously so heavily massed at the surface at night, the daytime 

 position is not so clear. There is indisputable evidence that when they are on the surface by day the older 

 and more active krill, both as individuals and swarms, readily avoid the conventional stern net. It seems 

 distinctly likely, however, that some of them desert the surface by day and, sinking to lower levels, become 

 distributed haphazardly, chiefly it would seem between 10 and 40 m. This movement contributes further 

 to the poverty of our daytime surface gatherings and in a very large number of instances leaves us with the 

 oblique ( 1 00-0 m.) net as the only instrument capable of sampling the older section of the surface popula- 

 tion during the daylight hours. The horizontal subsurface closing net used in the South Georgia surveys of 

 1926-7 is, of course, another instrument with which we have sampled the population by day, but 

 owing it would seem to the random vertical subsurface scatter of the daytime swarms it does not sample 

 it with the same success as the oblique net does^ fishing all the way up from 100 m. to the surface. 



Shortly before going to Press I looked again at Hardy and Gunther's original bathymetric data^ 

 for the older krill taken in the three-level horizontal (NiooH) nets used in the early South Georgia 

 surveys, replotting their results in a somewhat different form from that in which they originally 

 presented them. In doing so I corrected all catch-figures for duration of tow* (p. 283) and used only 

 material obtained, in so far as subsurface levels were concerned, in nets that were closed before 



1 The vast majority of the surface patches plotted in Fig. 144 were reported by observers on whale-marking cruises. 

 This is not really surprising since they were constantly on deck and constantly on the look-out for whales. 



^ Compare, for instance, the average catch-figures for the loo-o m. layer given in Tables 27, 28 and 31. 



^ Hardy and Gunther (1935, Appendix n, Zooplankton Table iii). 



* Hardy and Gunther plotted their results on a basis of length, not duration, of tow. The South Georgia horizontal nets 

 were fished for one mile, but owing to varying conditions of wind and sea the time taken to traverse this distance, especially in 

 the old 'Discovery', varied enormously. 



