262 DISCOVERY REPORTS 



adults. It is clear from this presentation of the data that while the older krill in the over 20 mm. 

 class practically elude the surface net altogether during the day, the younger and not so active animals 

 do not by any means so readily escape capture. In fact the surface larvae and very early adolescents 

 in the under 16 mm. class can it seems be taken in more or less equal abundance whatever the condi- 

 tion of the light, although the early adolescents in the 16-20 mm. class already clearly seem to have 

 developed a distinct capacity to evade. The turbulence data given in Tables 51 and 52, notably the 

 daytime data, also reveal a tendency for the older krill to be the more successful escapers, those in 

 the over 20 mm. class tending to elude capture, in calm to moderate seas, in greater numbers than 

 the smaller animals are evidently able to do. 



Table 53 . Mounting capacity to evade developed by the krill as they grow, from data based on the catches 

 of the horizontal {0-5 m.) stramin nets. Night-time data in roman type, daytime data in italics 



* Excluding one enormous catch of over 600,000 at Station 2300. 



■j- Including total catch and net hauls from second column of Table 50. 



Table 54. Varying light intensity and its relationship to the catches of the largest (over 30 mm.) 



krill in the horizontal {0-5 m.) stramin nets 



Clearly then, as they grow, the krill develop a mounting capacity to avoid the nets, the largest 

 individuals, all for instance over 30 mm. long (Table 54), eluding them as might be expected, through 

 daylight, twilight and darkness (cf. Table 50), most effectively of all. 



Capacity to act in unison 

 Hardy's observations at the jetty (p. 154) reveal a pronounced ability in the individuals of a swarm, 

 or at any rate those of a small swarm, to act in unison, Barham (1957) recording a similar capacity 

 in tightly packed shoals of Thysanoessa spinifera, which in daylight would ' sound rapidly ' whenever 

 they were approached by a boat with the motor running. Further evidence of this phenomenon was 

 obtained during the cruise of 'William Scoresby ' into the Weddell Sea (p. 152), when it was observed 

 that time and again when sampling was attempted in daylight, surface swarms would sink bodily 

 away, or, splitting asunder as when disturbed by the dropped lead and line (p. 156), would leave an 



