4IZ DISCOVERY REPORTS 



DISTRIBUTION OF SURFACE DISCOLORATION 

 AND ITS RELATIONSHIP TO THE GROSS DISTRIBUTION OF THE 



SURFACE POPULATION 



Since these investigations began many instances of discrete, characteristically reddish surface dis- 

 coloration have been reported from our vessels while in southern vv^aters. Investigation by nets, 

 such as that for instance (p. 152) undertaken at Station WS 540, and close-range observation (p. 151) 

 have together shown on many occasions that these discoloured patches may be caused by dense 

 concentrations or swarms of E. superba. While our reports, whether investigated or not, all refer to 

 these discoloured areas as krill patches, the possibility that other swarming or densely concentrated 

 animals may be involved in these phenomena must also be considered. The krill, although no 

 doubt largely responsible for it, are not the only Antarctic animals that can cause reddish patches on 

 the surface of the sea. They can be produced as my colleague, Robert Clarke, has observed,^ by swarms 

 of Calanus propinquus, or as Brachi (1953) has found, by dense concentrations of Salps, and doubtless 

 too by other organisms. However, the characteristically asymmetrical pattern of the circumpolar 

 distribution of the patches reported (Fig. 144) follows so exactly that of the major concentrations 

 of the total surface population revealed by the widespread sampling of our nets (Fig. 143) that 

 there would appear to be a strong likelihood that in a great many instances the discoloured areas 

 recorded were in fact phenomena associated with the swarming of the krill as we have always supposed 

 them to be. Comparing Figs. 143 and 144 in detail, and supposing all the discoloured areas 

 plotted in the latter represent major concentrations of the krill, it will be seen that both reveal a 

 pronounced concentration of the population in the East Wind-Weddell surface stream, both an 

 absence or virtual absence of any major concentration in the West Wind drift, except where it is 

 affected locally by East Wind influence, and both a major scarcity of this species in the Pacific sector. 

 In only one respect are these separate representations of the distribution at variance, the eastern, but 

 not the western, part of the supposed Atlantic backwater between 30° W and 30° E, a region our nets 

 reveal as very sparsely populated, carrying supposed krill patches in some considerable measure of 

 profusion. 



THE LARVAL POPULATING OF THE EAST WIND-WEDDELL 

 SURFACE STREAM AS AN ANNUAL EVENT 



The facts of the distribution and dispersal of the larvae of this species have of necessity (p. 285) been 

 worked out from monthly observations plotted regardless of the years in which they fell, and it is 

 realised that data so treated could lead here and there to some misinterpretation of the results owing 

 to the biased grouping of observations in certain parts of the circumpolar sea, to the neglect or exclu- 

 sion of others, at the right time of year. If it be expressed in terms of actual time, however, the great 

 summer larval outburst in Weddell West can be shown to be an annual event, not merely an appearance 

 resulting from an over-preponderance of observations in that region unsupported by simultaneous 

 negative observations elsewhere. In Fig. 145 I have plotted the principal concentrations of the 

 surface larvae according to the actual seasons in which year after year they were encountered in the 

 horizontal and vertical nets during our repeated cruises in these southern waters. The positive and 



^ Personal communication. The observation referred to, made during the whale-marking voyage of the ' Enern ' (Clarke 

 and Ruud, 1954), was upon a red surface patch suspected of being caused by E. superba, but which proved on investigation 

 by townet to have been produced by C. propmquus. 



