HORIZONTAL DISTRIBUTION, GROWTH AND DYNAMICS OF DISPERSAL 403 



(b) Southern zone 

 (i) Although our observations are rather scanty there is evidence that here as in the northern zone 

 the staple whale food is represented principally by a heterogeneous assemblage of young 16-18 

 month old swarms together with, again apparently only in April, the older adult swarms that survive 

 the spawning. In this region of slow growth, it will be noted, exceptionally backward swarms may be 

 encountered that even as late as May have not yet quite outgrown their early adolescent (11-20 mm.) 

 state. 



(2) The only observation we have (at Station 1359) as to the condition of the 16-18 month old 

 females in these high latitude swarms suggests that in some, perhaps not infrequently occurring 

 instances, they may persist dominantly in stage i until well into May. There are no observations on 

 the condition of the females in the single adult swarm recorded. 



(3) The pattern of modal values displayed, by the young swarms at least, is heterogeneous. 



(4) In April and May the young swarms tend to fall mainly within a length range whose principal 

 limits lie between 25 and 36 mm., the most backward of them, even in May, still mainly in the 

 21-28 mm. range. 



Winter. The distribution and relative abundance of the staple whale food in winter (July-September), 

 when only a relatively narrow strip of the northern zone of euphausian abundance in the Weddell 

 stream remains open to investigation, is shown in Fig. 139. Now, as Fig. 140 shows, the older 

 krill are represented exclusively by maturing, although still largely adolescent, 19-21 month old 

 swarms, the previous summer's adult population having evidently completely died off. In the 

 Weddell zone, where the facts of the distribution can alone be determined with certainty, it will be 

 seen that while a moderately substantial population appears to exist in Weddell West and on the South 

 Georgia whaling grounds, the crowding of the positive occurrences in the latter area being doubtless 

 due to the close spacing of our observations there, elsewhere, notably in the rather heavily sampled 

 Weddell East, there is still every indication of the magnitude of the autumnal decline. It would appear, 

 in fact, that the northern decline in staple abundance resulting from the ravages of predators and the 

 dying off of the adult population is a phenomenon which must endure until the rising young genera- 

 tion of mixed larval and adolescent swarms (p. 367, Fig. 116) now, in winter, 7-9 months old 

 and massed in heavy concentration throughout the Weddell zone, eventually begins to outgrow its 

 early adolescent (11-20 mm.) phase. In other words, the depletion of the over 20 mm. class, so 

 evident throughout autumn and winter, cannot, in the northern zone at any rate, be made good 

 (p. 391, Fig. 133) until spring. 



Although not so numerous as at other times of the year, our winter observations in the West Wind 

 drift are enough to suggest the absence or great scarcity of the staple whale food there. It may be 

 noted, too, that all but one of the four negligible occurrences recorded are located in regions affected 

 by north-easterly outflow from the East Wind zone. 



The situation in the East Wind drift itself can only be surmised. It seems probable, how- 

 ever, judging from the situation there in autumn (p. 399, Fig. 137), that under the winter 

 ice-sheet in these high latitudes the 19-21 month old adolescent swarms representing the 

 staple class survive in a somewhat greater measure of abundance than they do in the northern zone. 



Winter then, as Figs. 115 and 122 show, is essentially a time of dual abundance with both larval 



and early adolescent (11-20 mm.) classes, the former in enormous numbers, dominating the northern 



zone of euphausian abundance to its farthermost geographical limits. It is a time, in the northern 



zone at least, of continued scarcity of the staple class. In the East Wind or southern zone, however, 



the larvae (pp. 338 and 376) are probably the dominant winter class, with the staple whale food of 



secondary importance and the early adolescents (p. 376) non-existent. 



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