THE OLDER STAGES '71 



population there was of only local significance, or to suppose that our vessel was sampling and 

 observing a population any richer than, for instance (p. 61, Fig. 56), that of the East Wind zone. 

 The density estimates, therefore, could well I think be representative of the East Wind-Weddell 

 surface stream as a whole. It is interesting, therefore, to compare them with the corresponding figures 

 for whale flesh— 0-56 g./m.^ and 1908 kg. per square mile— recently given by Mackintosh and Brown 

 (1956) for their inner (1° C isotherm to ice-edge) Antarctic zone which itself coincides broadly with 

 the limits of the East Wind-Weddell stream. 



Table 36. Measurements of a sample of half -grown krill from a surface swarm at 



Station WS 558 



Length No. of 



(mm.) individuals 



25 23 



26 15 



27 12 



28 4 



29 3 



30 — 



The density of the krill then from these figures would appear to be about 4^ times greater than the 

 density of the whales, which does not in fact seem very much when one considers the incomparably 

 larger number of other animals that also prey upon this species. Our figure, however, although a 

 step in the right direction, may well fall far short of the natural density of the krill in the sea, it being 

 based on the average weight of a single sample of immature euphausians that would not in nature 

 be representative of the surface population as a whole. In February (p. 397, Fig. 136), when these 

 observations were made, the surface population of the staple class consists of half-grown and adult 

 krill assembled in a heterogeneous system of swarms in which the length frequencies of the individuals 

 vary greatly from one swarm to another, the variation even in adjacent swarms (p. 234, Fig. 44) 

 being sometimes quite pronounced. Among the half-grown swarms there would no doubt be many 

 in which the average weight per individual would be say o-o6 g., as in the sample from Station WS 558, 

 but among the older swarms there would be equally as many in which the corresponding weight would 

 be very much higher. Heyerdahl (1932) gives the following averages for krill ranging from 28 to 



65 mm. ^^^^^^ Average weight Length Average weight 



(mm.) (g.) {mm.) {g) 



28 0-19 50 074 



35 0-28 55 ^'12 



40 040 60 I "48 



45 0-56 65 i-6i 



From these figures, and from the figure provided by the sample from Station WS 558, the average 

 weight per individual of the summer surface population might, therefore, be put as high as 072 g. 

 and so D would equal 072 x 34 x 1-196 = 29-28 g./m.^ or approximately 50 times the weight of 



whale flesh instead of 4I. 



These are rough calculations and I present them with great reserve. Even so I believe the second 



equation, Weight of krill _ 



Weight of whale flesh ^ ' 



will in the long run prove to be the closer to reality. I believe, too, that the average weight of my 

 8800 half-grown krill may in fact be much too low, either because I over-dried my specimens or 



l8-2 



