268 DISCOVERY REPORTS 



Einarsson (1945) also refers to the extent of the avoiding action taken by adult euphausians, 

 ' especially during daylight hours ', and calls attention to the ' inadequacy of the sampling ' that results 

 from this phenomenon. The difficulties of sampling the large Meganyctiphanes norvegica, a very 

 pronounced swarmer, are mentioned too by Bigelow (1926), while Zelikman (1958) calls attention to 

 the ' procedural difficulties involved in the quantitative estimation of the very mobile adult euphausians, 

 which remain in schools ', meaning simply how difficult they are to catch. 



Half a century ago Franz (19 10) cast serious doubt on the reality of the vertical migrations said to 

 be undertaken by larval eels, suggesting the possibility that such creatures could see an approaching 

 surface net by day and avoid it by rapid movements, Russell (19276) acknowledging that to 'a certain 

 extent he is justified in his arguments, especially when very small nets are employed '. 



Robert (1922), working with freshwater plankton, calls further attention to the advantages of high- 

 speed towing by day, concluding that animals not far from the surface, particularly Crustacea, are able 

 to avoid the net when towed at slower speeds. Southern and Gardiner (1926), also working in fresh 

 water, found that the catch of nets hauled vertically by day was less than the corresponding catch by 

 night, the difference they say being due to ' the capacity of the Crustacea to avoid the net in the upper 

 illuminated water layer ' during the daylight hours. 



Gushing (1951) also cites Tattersall on Thysanoessa inermis, recording many other instances of day- 

 light surface swarming by plankton animals, particularly during the northern summer. In every 

 instance he mentions, however, he records no more than that the swarms were seen on the surface. 

 It would have been doubly interesting to know in these cases if attempts had been made to sample 

 them, and if so, with what success they had been sampled there. 



Simpson (1956), referring to the work of Russell (1928), Johansen (1925) and others on the vertical 

 movements of fish larvae, observes that the diurnal movements that seem to take place among certain 

 species cannot be regarded as proved in view of the doubt that exists as to whether vertical migration 

 or net avoidance explains these appearances. Further investigation, he adds, is required. Golton 

 (1959) is more emphatic. 'For some years', he writes, 'we have been concerned with the problem 

 of sampling larval and juvenile fish. This problem is complicated by the marked vertical and horizontal 

 patchiness of postlarval and juvenile fish^ and their ability to avoid slow moving nets during daylight 

 hours'. 



VERTIGAL DISTRIBUTION AND MIGRATION RECONSIDERED 



Professor Hardy tells me that while he readily believes there must be some measure of avoidance 

 at the surface by day, especially among the older and more active animals, he also believes that the 

 extreme poverty of our daylight surface gatherings of the older krill must to a large extent be caused 

 by the vertical migration of the older swarms away from the surface by day. His view, like mine, is 

 that most of such swarms are congregated at or near the surface when it is dark, to the manifest 

 advantage of the surface net. He thinks, however, on the evidence of the three-level horizontal nets 

 used in the South Georgia surveys of 1926-7 (p. 54), that most of them, perhaps still as swarms, 

 must go down by day to lower levels, and since there is a distinct possibility that they would not all 

 go to the same depth, they would no longer be disposed, as they are at night, along the same narrow 

 horizontal layer. 



Owing to their vertical scatter the day oblique net would now be offered a lesser chance of hitting 

 a swarm than the night surface net is, since the latter fishes among swarms disposed in a relatively 



^ A patchiness I would emphasise that suggests strongly that, like the Antarctic krill and perhaps many other plankton 

 animals (p. 236), they live discretely in swarms. 



