VERTICAL DISTRIBUTION AND MIGRATION RECONSIDERED 



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EVENING AND DARKNESS 



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* STATION FALLING IN THE GATHERING DARKNESS OF MAY 



Fig. 62. Chart showing day and night vertical distribution of the staple whale food as revealed by the South Georgia 

 plankton surveys of 1926-7 (N 70 H net hauls). For further explanation see legend to Fig. 61. 



sets of observations (Figs. 61 and 62), though separate in time and space, lead to the same broad con- 

 clusions as have already been drawn from earlier presentations of the data. 



(i) There is a massive accumulation of swarms after nightfall in the surface (0-5 m.) layer, par- 

 ticularly conspicuous during the dark hours between 2200 and 0400, 



(2) The same zone in daylight appears to be largely, but not entirely, deserted by the older swarms, 

 suggesting rather strongly at first sight that most migrate by day to deeper levels. Clearly, however, 

 they do not all go down, or perhaps I should say do not always go down, the frequency of occurrence 

 of negative or negligible daytime subsurface gatherings being in fact so very high that we cannot it 

 seems ignore the possibility that many more perhaps than our nets reveal do remain on the surface 

 throughout the daylight hours, successfully avoiding, but again not always, the stern net that is fishing 



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