,^35 DISCOVERY REPORTS 



tions, (2) they are going down by day in isolated patches or pockets, chiefly it would seem to levels 

 between 10 and 40 m., and, becoming scattered about in their patches over a moderate vertical 

 horizon, are presenting a more difficult target for our horizontal nets than they do when massed at 

 the surface at night, all along it seems, the same relatively narrow horizontal stratum, (3) there is 

 definite avoidance by the older animals of the surface net by day. (i) I think can be ruled out, our 

 deep observations with obUque nets below the East Wind-Weddell stream (Table 32) having failed 

 conspicuously to reveal any substantial daytime concentration below 250 m. The scarcity of the 

 staple whale food at the surface during the daylight hours would therefore appear to be due to a 

 combination of (2) and (3). I return to this matter on pp. 268-78, where the whole question of the 

 nature and extent of such vertical migration as may in fact be going on, and the behaviour of the 

 krill that take part, is reviewed. 



Table 32. Daytime vertical distribution of the staple whale food at depths below 250 m. 



Depth Total Number Average 



(ffi.) catch of hauls per haul 



500-250 38 50 076 



1000-500 12 ao o-6o 



2000-1000 



I 8 012 



If E. superba were in fact a rhythmic diurnal migrator, with a pronounced vertical movement such 

 as for instance Hardy and Gunther (1935, p. 240, Fig. 125) have demonstrated in E. frigida,^ we 

 should expect to find it during the daylight hours in far greater abundance and far more frequently in 

 such abundance at depths of say between 500 and 100 m. than our 243 day observations in this 

 particular horizon (Table 31) reveal it to be. Of these 243 net hauls only six in fact produced catches 

 with even moderate claim to significance (that is catches of between 100 and 1000), the largest of them 

 numbering 510 individuals, the remainder, 97% of the total, producing an average yield of a fraction 

 over two euphausians per net. 



Mackintosh (1934) refers to the diurnal variation in numbers of E. superba taken in the upper 

 (loo-o m.) oblique nets in the following passage: 



This is a difficult species to deal with owing to its extreme patchiness and tendency to form shoals. Many of the 

 big shoals have been seen at the surface during both the night and the day, but the deeper shoals and the more 

 scattered individuals might undertake vertical migrations. For the estimation of the average per haul at different 

 hours, samples containing over 1000 E. superba have been disregarded. This should ehminate the disturbing influence 

 of heavy shoal catches and give some idea of the general behaviour of the species. The resuking curve suggests only 

 a minor degree of diurnal variation.^ At St. 461 the majority seemed to remain near the surface, while those living 

 at greater depths gave some signs of moving up and down. At Sts. 618-25 there was quite a marked diurnal variation. 

 The explanation would seem to be that while a section of the population of this species undergoes some vertical 

 migration, the greater part remains at the same level [near the surface], especially perhaps when forming shoals. 



Even if a section of the population does so behave I question if such a movement could be described 

 as an aspect of the general behaviour of this species. For in view of the lifelong swarming habit^ to 

 which the krill (pp. 219-40) seem prone, a habit involving not merely isolated or occasional groups of 

 individuals but it seems the entire euphausian population, it is now clear that to ignore the shoal 



1 Even this movement Gushing (1955) suggests may prove to be more apparent than real, it being possible that this species 

 also avoids the surface net by day. 



* Referring to various papers by M. E. Vinogradov, Bogorov (1957) states that 'it may be considered as determined that 

 species of the surface zone do not perform diurnal vertical migrations, or their migrations are very limited'. 



' I have already called brief attention to the swarming habit and suggested that it may persist from hatching onwards 

 (Marr, 1955). 



