DISTRIBUTION IN OUTLINE 57 



DISTRIBUTION IN OUTLINE 



Although the main section on the distribution does not follow until later it is necessary at this stage to 

 anticipate it with a short account of its major aspects and their relationship to certain physical and 

 geographical features of Antarctica and its surrounding waters to which repeated reference will be 

 made in the text which follows. 



The principal water masses and surface movements of the circumpolar sea and the physical and 

 geographical features of Antarctica with which the krill are associated are shown in Fig. 4, the 

 hydrological boundaries^ and surface movements following Colbeck (1905), Deacon (1937, pp. 14-20, 

 Fig. 4), Clowes (1934), Herdman (1959), Mackintosh (1946) and Nasu (1959), the mean position of the 

 summer ice-edge following Mackintosh and Herdman (1940, Pis. Lxix and Lxx). 



It has long been known that euphausians on the surface in daylight are difficult to catch, that with 

 rapid movements they can dodge out of the way of an approaching net or react by mass sinking when 

 disturbed by the hull of a moving vessel. Tattersall (1924) seems to have been the first to call serious 

 attention to this phenomenon and since then many authorities, notably Bigelow (1926), Mackintosh 

 (1934), Hardy (1935), Einarsson (1945), Moore (1950), Barham (1957) and Zelikman (1958), have 

 called further attention to how readily they can avoid capture either as individuals or by concerted 

 action. At an early stage in my study of our material I found that our stramin nets, above all our 

 surface stramin nets, were consistently yielding smaller, in most instances far smaller, gatherings of 

 krill in the East Wind zone than they were in lower latitudes. I began at once, however, to suspect 

 something wrong, for in these high latitudes the whales have for long been known to be plentiful 

 and today yield a rich and profitable harvest to the whalers. Obviously then there must be enough for 

 them to eat. This anomalous situation, abundance of well-fed whales, but an apparently indifferent 

 food supply, first of all led me to suspect that in this summer sunlit zone our nets could not have been 

 behaving with the same efficiency as they were in lower night-dark latitudes, and eventually led me 

 to go into the whole question of the relative efficiency of our variously used apparatus and of the 

 relative success with which in diflFerent places and under various conditions they were sampling the 

 swarming krill. A full account of the results of this investigation is given on pp. 258-68. Meanwhile, 

 it is necessary to call attention to two of the more important facts that have emerged, (i) Our nets on 

 the surface in daylight, that is, our i-m. diameter (0-5 m.) stramin nets, are so easily avoided by the 

 older animals once they have grown to about 20 mm. long that such data as they provide as to the 

 presence or absence, abundance or scarcity, of these stages becomes for all practical purposes valueless. 

 (2) The same nets fished obliquely from about 100 m. to the surface, although producing by and large 

 quite adequate samples of the total surface population, do not in general by any means sample it with 

 the same success, either by night or by day, as the stramin nets on the surface do when fished in 

 darkness, or, if the escaping over 20 mm. population be ignored, in daylight as well.^ 



In the East Wind zone a very large number of the surface nets we fished were fished in the prevailing 

 summer daylight of these high latitudes, and throughout this period of absence of darkness, or at best 

 twilight, their gatherings were negative or consistently low; so low in fact that in most instances I was 



^ Wexler (1959) proposes that the classic Antarctic Convergence of Deacon should now be referred to as the Antarctic 

 Divergence, arguing, from a few widely scattered bathythermograph records, that it is a region of cold upwelling induced 

 by horizontal divergence of the surface layer. His data, however, seem not to be enough for this to be conclusive and in any 

 case Garner (1958), also using the bathythermograph, finds no evidence of such upwelling along what has for long been 

 recognised as a convergence zone. 



^ While there is good evidence that the larger and more active animals at times remain on the surface by day and at such 

 times readily avoid the surface net, there is also evidence (p. 273) that some desert the surface in daylight and, sinking to 

 slightly deeper levels, contribute further to the poverty of our daytime gatherings of the older stages of the surface population. 



