402 DISCOVERY REPORTS 



surprising, for whereas the greater part of the Weddell system is open to major depredation for 

 6 months or more, and the remainder, being permanently ice-free, to depredation in some form or 

 another all the year round,^ in the East Wind zone the period of major depredation, in so far as it is 

 effected by whales, seals and penguins,^ must be cut to three months or less because of the prevailing 

 ice conditions there. In other words this high latitude region of euphausian abundance may be con- 

 ceived as being a relatively untapped reserve, in which the krill of all classes, larval, adolescent and 

 adult, stand a greater chance of survival and reproducing their kind than they do in lower latitudes. 

 It is a region above all where the generation of one year's status, benefiting from the long protection 

 it has enjoyed below the winter ice-sheet, survives in relative immunity to breed the following year. 



Autumn, then, is essentially a time (Fig. 112) when the massed surface larvae everywhere dominate the 

 southern feeding-grounds to their farthermost geographical limits, most conspicuously, but only because 

 it is so readily accessible to vessels, in the Weddell stream. It is a time, on the other hand, when the small 

 whale food in the natural order of things is everywhere absent, or virtually absent, from the plankton of 

 the circumpolar sea, and when the older adolescents and adults of the staple class are practically 

 everywhere, except possibly in the East Wind zone, at a low, perhaps their lowest, level of abundance. 



The developmental condition and length frequencies of the principal concentrations of the staple 

 whale food encountered in autumn are shown in Fig. 138. Taking in turn {a) the situation in the 

 northern zone, and (b) the situation in the southern zone, the principal facts thus presented may be 

 summarised as follows: 



(a) Northern zone 



(i) The staple whale food in autumn is represented principally by a heterogeneous assemblage 

 of 16-18 month old swarms together with (in April only) the few remaining adult (28 month old) 

 swarms that survive after spawning. The younger (16-18 month old) swarms have now completely 

 outgrown their early adolescent (11-20 mm.) phase. 



(2) In the young swarms the females are dominantly in stages i , 2 and 3 in April, those in stage i 

 at Station 207, however, probably being backward swarms of East Wind rather than of Weddell j 

 origin. In May and June they are dominantly in stages 2 and 3. In the adult (paired and spawned) 

 swarms that survive into April the females are dominantly in stage 8. There is, however (Station 2346), 

 an interesting late April exception from Weddell East. There 41% of the males were in stages 6 and 7 1 

 and both they and the females had only just begun to pair, the vast majority of the latter, 85%, being! 

 still in stage 5. Clearly, in view of its high modal value, this swarm must be regarded as representing 

 an instance of the attainment of adult (third year) stature with failure to ripen to maturity, and since! 

 Bargmann's elaborately worked out life-history suggests each developmental phase to last 2 months 

 in the male and 2^ months in the female it would appear distinctly possible that in certain, perhaps 

 very rare, instances E. superba does not spawn until it is fully 3 years old or more. 



(3) The pattern of modal values displayed by both young and old swarms is heterogeneous through- J 

 out the season. 



(4) From April to June the young 16-18 month old swarms tend to fall mainly within a length] 

 range whose principal limits lie between 25 and 48 mm., the more backward in the 25-36 mm., the] 

 more forward in the 36-48 mm. range. 



(5) The few remaining adult swarms that survive the spawning into April fall dominantly within! 

 a length range of 44-56 mm. 



'■ On Elephant Island it will be recalled (p. 43) Gentoo penguins are recorded feeding heavily on krill in the autumn and | 

 winter of 1916. 



^ See, also, the section beginning on p. 423 on the coastal topography of Antarctica as an additional factor affecting the j 

 depletion and conservation of the euphausian stock. 



