REACTION OF KRILL TO SHIP AND NETS 259 



either directly or inversely, (i) with the intensity of the light or altitude of the sun, (2) with the degree 

 of turbulence of the sea, in so far as that may be said to unsight or confuse the surface individuals, 

 and (3) with the size and vigour of the krill themselves. 



Altitude of the sun 



The relationship of the sun's altitude to the gatherings of the larger krill we took at the surface in 

 our stramin nets is shown in Table 50 which reveals such a clear-cut inverse correlation between catch 

 and light values, that there can be little doubt that as the sun sinks and the strength of the daylight 

 wanes the krill find it more and more difficult to avoid the net, presumably because they find it more 

 and more difficult to see it. In broad daylight in fact they would seem to see it so clearly that avoid- 

 ance is virtually complete. It could of course be objected that when the sun was high, or at any rate 

 above the horizon, most of the krill had migrated to some deeper level and so could not in any case 

 have appeared in the surface catch, except perhaps in the negligible numbers recorded. A discussion 

 of this possibility is given at the end of this section, where it transpires that both avoidance and some 

 degree of vertical migration are perhaps equally involved. 



Table 50.* Varying light intensity and its relationship to the catches of the staple whale food in 



the horizontal {0-5 m.) stramin nets 



* Note this and Tables 51-6 which follow were drawn up before all the material, upon which, for instance, the vertical 

 distribution is based, was finally examined. 



f Excluding one enormous catch of over 200,000 at Station 2014. 



In so far as it affects the reliability of our data, evasion by sight operates principally in high lati- 

 tudes. In the neighbourhood of South Georgia, for instance, and in the Weddell drift generally, there 

 are always, both in summer and winter, ample dark hours to work in. Farther south, however, in the 

 coastal waters of the continental land, which are only accessible in summer or early autumn, much 

 of the work has to be carried out in more or less continuous daylight, the sun, particularly in January 

 and February, never setting or dipping only for short periods below the horizon. 



Turbulence of the sea 



The state of the sea, whether it be calm, slightly disturbed or boisterous, seems to have a marked 

 effect on the capacity of the krill to dodge the nets. This phenomenon is shown in Tables 51 and 52 

 in which the measure of turbulence is set down according to the Douglas Scale as follows : 



Calm to smooth Force o-i 



Slight to moderate Force 2-3 



Rough to very rough Force 4-5 



High to very high Force 6-7 



Catches where the real state of the sea was masked or damped down by the presence of pack-ice 

 have been excluded from both tables which refer exclusively therefore to the open ocean. 



29-2 



