REACTION OF KRILL TO SHIP AND NETS 265 



the krill themselves. We know, however, that such action takes place, and takes place very rapidly, 

 and so the sundering of the swarm, or its sinking, must it seems properly be ascribed partly, and 

 perhaps mainly, to the mechanical thrust of the ship and partly to the capacity of the krill, whether 

 in unison or by individuals, to dodge, or swim away from, the advancing hull. 



General inferences to be drav^^n 

 In view of the enormous catches taken from the boom it is clear that even at night our stern nets, 

 whether horizontal or oblique, do not reveal anything even remotely approaching the natural density 

 of the krill in the sea, all our catch-figures showing that even when it is quite dark, whether voluntarily 

 or involuntarily, they retain an enormous capacity to elude them. Evasion by night, although the 

 natural light intensity is at its lowest, must also be regarded to some extent at least as evasion by sight, 

 since the wake in which the stern nets were fished, or broke surface, was illuminated by cargo cluster 

 or floodlight. During the dark hours, however, evasion by feeling or by purely mechanical scattering, 

 or both, must also be involved. However it may come about, the fact that we do capture the larger 

 krill on the surface by stern nets at night, and capture them sometimes in very substantial numbers, 

 shows however that evasion is far less effective by night than by day. This is only of course what 

 would be expected. 



Moore (1950) also refers to the extent of the avoiding action taken by surface-living euphausians 

 at night. He puts it remarkably high, stating that the surface density of certain highly luminescent 

 species has on occasion appeared to the eye to be as much as 75 times greater than the corresponding 

 density revealed by a net. Referring to the above Barham (1957) writes, 'Obviously these organ- 

 isms are able to avoid the nets even at night, and it would appear that some other sense besides 

 sight must come into play ' ; and he goes on to say (p. 262) that in his own experience euphausians 

 such as Thysanoessa spinifera appear to be extremely sensitive to vibration, particularly he supposed 

 to the vibration created by the rapidly moving propeller of a motor-boat. 



Referring to the observations of Boden (1950) on euphausians as sound scatterers, Emery (i960) 

 writes, ' It was believed that many of the euphausiids in the path of small nets may have avoided them 

 or escaped and that a higher concentration probably exists than was indicated in the sampling, even 

 with large nets '. And he was speaking, not in terms of a daytime surface community, that with all 

 its faculties alert might well indeed have been escaping, but of animals in the gloom or total dark- 

 ness of a deep environment. 



It seems that the short length of line on which our stern surface net was fished might after all 

 have a lot to do with the failure of this apparatus to capture the staple whale food in the enormous 

 numbers such as, for instance, were taken in the lateral net, in numbers such as might readily indeed 

 be expected if these large euphausians when swarming are packed as tightly as one to the cubic inch. 

 The swarming krill it seems, as Hardy (p. 154) with his walking-stick witnessed, when sundered or 

 scattered, will quickly re-form. Obviously, however, as our virtually negative daylight surface 

 observations show, when disturbed by so massive an obstruction as the hull of a vessel and the water 

 movements round it, they do not come together immediately in the wake, the low, by lateral standards 

 negligible, average night yield from the surface (p. 259, Table 50) suggesting they do not in fact close 

 up again with maximum effect until they have fallen astern of the surface net. The daylight results of 

 the 'consecutive net series' described by Hardy and Gunther (1935, p. 255) provide rather striking 

 evidence that this must in fact be what happens. In the operation of this series several turns of the 

 bight of a long manilla warp were taken round the warping drum of the main winch of R.R.S. 

 ' Discovery ', and with an identical net attached to either fall of this long line it was so arranged that 

 while the one net, paid away on the surface from the starboard quarter, was being slowly hauled in, 



30 DM 



