352 DISCOVERY REPORTS 



surface life at a marked disadvantage. However, from July, or at any rate August, onw^ards these 

 indifferently nourished, slowly developing swarms will continually and increasingly be augmented by 

 the arrival of older swarms from the west, swarms dominated by well-established Sixth Furcilias, or 

 by both Sixth Furcilias and young adolescents, stemming no doubt from the early January-February 

 risings that take place in the far western reaches of the current. Throughout winter, therefore, and 

 probably into spring, we should expect to find in Weddell East two distinct larval communities, a 

 younger representing the backward product of the March risings in Weddell Middle and the late 

 April risings in Weddell East, and an older representing the forward product of the January-February 

 risings in Weddell West. Moreover, with the older swarms continually coming in from the west we 

 should expect also to find that in each successive month of their appearance they would be more 

 prominently displayed in the western part of Weddell East, that is, near o°, than in the more 

 easterly reaches of this sector. As Fig. 105 shows both expectations have in fact been substantially 

 realised. 



Some estimate, too, can now be formed of the rate at which the larvae, developing as they go, 

 traverse the Weddell drift from west to east. If longitude 45° W in 60° S be taken as the mean 

 position around which they are rising in Weddell West from January to ApriV and 15° E in 60° S 

 as the mean position in Weddell East around which the Sixth Furcilias, or Sixth Furcilias and early 

 adolescents, are being encountered 180 days later from July to October, it could be argued that since 

 the distance from 45° W to 15° E in 60° S is approximately 1800 miles, the larvae move eastwards 

 at a rate of about 10 miles a day. A similar although slightly smaller figure is obtained if the March 

 risings in Weddell Middle be considered in the same way. If, for instance, 15° W in 60° S be taken 

 as the mean position for the March risings in this sector, and 15° E in 60° S the mean position of the 

 first major concentrations of Sixth Furcilias recorded in Weddell East 120 days later in July, then since 

 the distance to be covered is now 900 miles the larvae would appear to be moving at a rate of approxi- 

 mately 8 miles a day. Taking a more concrete case from Fig. 76 (p. 313) in which we see a rising 

 in Weddell Middle (Station 1144) in 18° W followed 40-42 days later by the appearance of surface 

 swarms in 01° E dominated by the Second and Third Calyptopis, it would appear that, the distance 

 from 18° W to 01° E in 60° S being approximately (p. 311, Fig. 75) 570 miles, the young krill 

 on their moving belt travel eastwards at about 14 miles a day or say at the rate of about J knot. 

 The average speed of the Weddell drift may in fact be somewhat higher than this. Sets of | knot 

 (p. 314, note I ) it is true are common, but stronger sets of i, i J and even 2 knots have been reported, 

 although the latter possibly may be of no more than local significance having been recorded perhaps 

 only when strong winds were blowing the right way. It need not, however, be supposed that the 

 larvae, planktonic though they are, move with precisely the same speed as the surface stream. It 

 might in fact be expected that they would move more slowly since for all we know the vertical move- 

 ments of the Calyptopis and Furcilia stages, somewhat restricted (p. 108, Figs. 9-1 1) though they 

 seem to be, may lead to some slowing up of the larval drift relative to that of the surface stream. 

 For although we know these movements take place for all practical purposes within the limits of the 

 Antarctic surface layer it is extremely unlikely that the whole 200 m. thickness of this mass of water 

 is moving bodily eastward at a uniform rate, the maximum rate of drift most probably being confined 

 to a relatively narrow surface stratum, the deeper layers moving more sluggishly, the deeper the 

 slower. Clearly then, such descent as the Calyptopes may undertake into the deeper water must 

 lead to some slowing up of their resultant drift to the east. The Furcilia stages on the other hand, with 

 their much more limited or negligible vertical movement (p. no. Fig. 11), possibly travel more 

 swiftly eastward than the Calyptopis forms. 



^ The actual mean position is 43° W in 59° S. 



