26o 



DISCOVERY REPORTS 



Table 51. Varying turbulence of the sea and its influence on the catches of the horizontal {0-3 m.) 

 stramin nets. Night-time data in roman type, daytime data in italics 



* Note this number throughout is restricted to the periods during which the three broadly grouped developmental phases 

 severally have their optimum range in the plankton. Thus the under 16 mm. group ranges from March to December, the 

 16-20 mm. group from September to December and the over 20 mm. group all the year round. 



It is clear from both tables that the surface population, whatever the state of its development/ 

 can be sampled more effectively when the sea is rough or disturbed than when it is flat, calm or smooth. 

 This is particularly evident for the daylight hours but is also, and perhaps unexpectedly, true for the 

 dark hours as well, although in a slightly lesser degree. Apparently, therefore, because their vision is 

 blurred, or perhaps simply because they are confused, or again perhaps because even although they 

 may see the net turbulent eddies carry them into it, the krill, both young and old, and by night and 

 by day, are far less able to avoid the nets when there is surface disturbance than they are in calmer 

 conditions. The extreme poverty of the material taken in high to very high seas is not, however, to be 

 ascribed to any particular action on the part of the krill themselves, but rather to the decreased 

 efiiciency of the net when used under such rough conditions. For these were the conditions which 

 marked, and often indeed went beyond, the extreme limit of endurance of our apparatus, conditions 

 with winds at gale or near gale force when the nets would become split, or get torn bodily from their 

 rings, or get lost, complete with bridles, altogether. In such conditions, even if they survived intact, 

 they would generally be moving about so violently that such krill as they might chance to capture 

 would no doubt largely get lost through wholesale spilling of the catch. Normally we fished the stern 

 nets with the wind on the port beam, the beam wind filling the nets, and, blowing them to leeward, 



1 The apparent diminution of the oblique night catches of the 16-20 mm. class with increasing turbulence (Table 52), 

 which in view of the day and night data provided by the horizontal nets (Table 51), and the day data provided by the oblique 

 nets (Table 52), is obviously anomalous, can be attributed to the fact that the night stations at which these particular oblique 

 gatherings were obtained chanced to fall in places where 16-20 mm. euphausians were scarce. 



