THE OLDER STAGES 163 



catches is to ignore a fundamental characteristic of its habit in the sea. Such catches in fact, far from 

 being a ' disturbing influence ', are on the contrary normal, representing as they do the closest approxi- 

 mation to the fiatural abundance of the krill in the sea our nets can provide. Clearly they cannot be 

 disregarded, but must be accepted universally as the cornerstone of any presentation of the data that 

 may be devised to show the vertical movements of the population as a whole. 



During the South Georgia plankton surveys of 1926-7 Hardy and Gunther (1935, p. 237) found 

 that in general, although by no means always, far more krill were taken on the surface by night than 

 by day and concluded that this species was in fact a vertical migrator although its behaviour in this 

 respect seemed very erratic. 



In summer and autumn dense swarms of Meganyctiphanes norvegica commonly appear on the 

 surface in the Gulf of Maine and Bigelow (1926) remarks that this phenomenon does not ' correspond 

 to the diurnal vertical migrations shared in by many copepods, because the appearances of Meganyc- 

 tiphanes at the surface appear to be independent of the time of day. Therefore, the actual captures so 

 far recorded do not indicate any definite phototropism on its part, positive or negative, although it is 

 doubtful whether it could long survive the full illumination of bright sunlight'. Paulsen (1909) con- 

 cludes that since Meganyctiphanes is a microplanktonic feeder, when it comes to the surface it does so to 

 seek food, not, as Bigelow (1926) comments, as a direct response to temperature or the state of the light. 



Bennett (1931) remarks that in bright daylight the whale food lives 'at some distance down in the 

 water, but in dull weather and at night-time comes up nearer the surface, often to the top. At times', 

 he continues, 'large patches of "whale food" appear on the surface, giving the sea a red colour'. The 

 last sentence, of course, refers to daylight patches and it cannot perhaps be too strongly emphasised 

 that it is not only in dull weather that such patches have been recorded, but also in brilliant sunshine 

 (p. 155) to which the krill show no aversion, but on the contrary seem partial to a degree. It may be 

 observed, too, that ' some distance down ' unless it be a statement based on assumption, would imply 

 that the euphausians are visible at that level and therefore however bright the sun must still be rather 

 close to the surface. 



A Norwegian gunner of long Antarctic experience once told me that, although he too had never seen 

 krill on the surface in brilliant sunshine, when the sky was overcast he had often seen patches there. 

 It may be, therefore, that in general the patches are easier to see when the weather is dull and that 

 reflection of bright light from the surface tends to mask their presence there. ^ As Hart (1959) has 

 recently said of blooms and surface discoloration in general, the overall visual effect is modified by 

 the quality and direction of the incident light (sun's altitude) and by prevailing weather conditions 

 such as cloud and state of the sea. 



While serving in U.S.S. ' Henderson ' in the New Zealand-Australian sector of Antarctica in 1947-8, 

 Dietz (1948) found that the Deep Scattering Layer disappeared completely for 3 days after this 

 expedition, southward bound, had crossed the 62nd parallel, and 'was rarely ever well developed in 

 Antarctic waters'. In the same region a year later the French expedition in the 'Commandant 

 Charcot' (Douguet, 1950; Tchernia, 1950) lost all trace of the layer after crossing the same latitude, 

 repeated observation to the southward failing to reveal ' d'autres echos que I'echo du fond '.^ If the 

 scattering layer, as has so often been suggested, is in fact caused by vertically migrating marine 

 organisms these observations would seem to provide further evidence that the krill are never very far 

 away from the surface of the sea. For if they were to sink daily in their massed fortnations to some deep 

 subsurface level surely a pronounced indication of this phenomenon would have been detected by 

 now on a modern echo trace. We cannot, of course, as yet say for certain whether these animals, how- 



1 Gunther (p. 150) also records how conspicuous they can sometimes be under a grey sky when the sea is grey too. 



2 LaFond, it will be recalled (p. 73), calls attention to the absence of acoustic scattering layers below the Arctic pack. 



17-2 



