THE OLDER STAGES '57 



in the microneckton, a group intermediate between the thrusting neckton and the feebler swimming 

 plankton '. And where indeed but high among such a near-nektonic community are we to place the 

 full- and even half-grown southern krill? 



Uniformity of individual action may well be fundamental to the existence of the swarm, for it is 

 difficult to see how it could remain as such unless every member of it were behaving in exactly the 

 same way — heading in the same direction/ changing or reversing direction in unison as witnessed by 

 Hardy, and moving at the same speed. Without some such pattern of behaviour it is conceivable 

 that the strong swimming krill would scatter and disperse in all directions so that the swarms would 

 cease to exist and the whales dependent on them would either go hungry or be hard put to it to obtain 

 an adequate meal. 



Vertical distribution and migration 

 While it has been established through direct observation, going back (p. 42) a long way in history, 

 that dense concentrations of the older stages of this species, variously described as patches, shoals, 

 ' rafts ' or swarms, occur at or very close to the surface of the sea, there is no means of telling, except 

 through the medium of the data provided by the catches of townets covering a wide bathymetric 

 range, whether this congregating of the krill is a phenomenon exclusive to the surface zone, or 

 whether it be one that may extend into the deeper waters beyond the range of human vision. These 

 are important questions, for if it can be shown that the main concentrations of the whale food are 

 confined to a relatively narrow surface zone, then we should have reasonable grounds for supposing 

 that the whales themselves must be largely, if not exclusively, surface feeders, or at any rate that there 

 would be little point in their ranging far into deeper water in quest of an adequate meal. 



The material available for study is derived from 2960 net hauls of which 774 were horizontal hauls 

 ranging from the surface down to 250 m. and 2186 oblique hauls covering a bathymetric range 

 extending from the surface down to a maximum of 2000 m. The majority of the observations were 

 taken with the lOO-cm. diameter stramin net. In exploring the deeper strata, however, more especially 

 the depths below 250 m., we made extensive use of the large 200-cm. diameter TYF, an apparatus 

 with four times the aperture of the N 100 and one besides that quite often was fished for as much as 

 up to twice as long or more. To express the catch of the large net, therefore, approximately in terms 

 of that of the smaller, the following equation has been used, 



c = cTm, 



where C is the corrected catch, c the catch of the TYF, t its fishing time and T the standard duration 

 of haul of the Nioo which (p. 59) was 30 min. All the nets concerned, with the exception of the 

 uppermost (loo-o m.) oblique nets and the horizontal nets fished in the immediate vicinity of the 

 surface (0-5 m.), were closed at the end of their period of tow. 



In working out the vertical distribution of the staple catch I have discarded as before (p. 65) all 

 data derived from horizontal nets towed on or very close to the surface in daylight, since obviously 

 such data if used would have led to a gross underestimate of the surface density of the older stages 

 of the euphausian population. 



Surface nets excluded, in the several presentations of the data that follow no attempt has been made 

 to show the actual depth (accurately measured for all nets below 5 m. by Kelvin tube or depth gauge) 

 at which each individual net was fished. There has instead been a grouping of depths of tow into 

 selected broad horizons. Thus, taking the oblique hauls as an example, nets fished at say 400-200 m., 

 390-250 m., 520-300 m., 480-300 m. and so on have been grouped together as having fished within, 



1 An underwater photograph recently taken from the French bathyscaphe F.N.R.S. 3 (Houot and Willm, 1955), depth 

 and date not stated, shows a tight group of about 100 euphausians, all but two pointed in the same direction. 



