440 DISCOVERY REPORTS 



geographical range. The krill as they grow develop a mounting capacity to avoid the surface nets, a 

 capacity most conspicuous in full daylight, but decreasing as the daylight wanes. In view of this 

 capacity and because with a swarming, predominantly surface living, population such as this, a net 

 towed on the surface (especially at night) is manifestly more likely to strike a swarm than a net hauled 

 open to the surface obliquely from below, certain rather arbitrary corrections have to be applied to 

 the catch-figures of the stramin nets, corrections affecting principally the samples from the open 

 oblique nets fished in the loo-o m. layer (p. 258 and p. 278). 



32. A monthly survey of the distribution and relative abundance of the larval stages based on the 

 data from the vertical nets shows that in the oceanic water of the northern or Weddell zone they first 

 appear as Nauplii and Metanauplii deep down in the far western part of the Weddell drift in 

 December, reaching the surface there on a major scale as First Calyptopes by about mid-January. 

 Under the dual influence of the bottom water and the surface drift, and here to some extent also 

 of the warm deep water, they gradually spread to the east, being encountered in great abundance about 

 half-way along the Weddell drift in March and throughout the whole of this surface stream by the 

 end of April. From then until the last surviving Sixth Furcilias moult and become adolescent in 

 December they continue to fill the Weddell stream, the influence of which as a major instrument of 

 their dispersal appears to cease somewhere near 30° E. The whole of the development from Calyp- 

 topis I to Furcilia 6 takes place in the surface waters of this current and in the surface waters of the 

 South Georgia whaling grounds and Bransfield Strait affected by it, the first surface swarms appearing 

 to drift into the strait in March and to reach the South Georgia area on a major scale at the latest by 

 May. In the southern or East Wind zone the first deep larvae are encountered in oceanic water in 

 late January and early February, the first surface swarms appearing about the middle of the latter 

 month. It is not until March, however, that the completion of the developmental ascent becomes 

 general there (p. 284). 



33. In the northern zone the larvae grow rapidly, the first Sixth Furcilias appearing towards the 

 end of March, the whole surface life-cycle, Calyptopis i to Furcilia 6, probably being completed on 

 a major scale in from about 90 to 120 days. In the extreme cold of their southern environment they 

 grow more slowly, the surface life-cycle taking upwards of from 9 to 10 months to complete. Their 

 resultant rate of westerly drift in the East Wind zone and easterly drift in the Weddell is probably 

 of the order of from 8 to 14 miles a day (p. 349). 



34. A seasonal survey of the distribution and relative abundance of the massed surface larvae, the 

 adolescents and the adults, based on the material from the stramin nets, reveals, in an even more 

 striking manner than the vertical nets are able to convey, the magnitude of the larval outburst that 

 takes place in the western reaches of the Weddell stream in summer and the subsequent spreading 

 of these stages, developing as they go, to the east. Summer may be described as essentially a time of 

 single abundance when the staple whale food is at its peak and at that peak is spread throughout the 

 feeding-grounds to their farthermost geographical limits. It is a time, however, when the great mass 

 of the larvae, just launched on their surface existence, are confined to the western reaches of the 

 Weddell drift and when the small adolescent krill (the small whale food), shortly about to outgrow 

 themselves, survive principally in the higher latitudes of the East Wind zone. Autumn is essentially 

 a time of purely larval abundance, the young stages vastly outnumbering the older especially in the 

 Weddell zone. It is a time when, the surviving early adolescents having outgrown themselves in the 

 southern zone, the small whale food has virtually everywhere disappeared from the plankton and a 

 time too when the staple class, the paired and spawned adults evidently having died off, is practically 

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

 The autumnal scarcity of the staple class is very probably associated too with the magnitude of the 



