VERTICAL MIGRATION RECONSIDERED 325 



the surface. Day after day we found that the two closing vertical nets hauled up from twenty to ten 

 fathoms were of a brownish-green colour, and contained (especially the Nansen) an abundant gather- 

 ing of diatoms. The surface nets during this time contained more Copepoda. On April 15 and 19, 

 however, when the change in plankton was taking place, the diatoms were found to be mainly on the 

 surface and the Copepoda below. As an example of wide distribution I may cite April 10, when the 

 nets gave consistent results all the afternoon at three localities north of Port Erin, the diatoms being 

 in all cases more abundant at the bottom, and the Copepoda on the surface. 



In our present survey we have unfortunately no knowledge of the depth distribution 

 of the phytoplankton, our samples being obtained from a single column of water from 

 100 m. to the surface. At some stations the phytoplankton may have been near the 

 surface, at others lower down. Nevertheless we must examine our material and see if 

 it offers any direct evidence as to whether or not organisms which undertake vertical 

 migrations are in fact to be found higher in the water in daytime in regions of lower 

 phytoplankton concentration, and deeper in the water in regions of denser phytoplankton. 

 The data are by no means ideal for this purpose. At each station three N 100 H nets 

 were towed horizontally for a mile at the same time, one below the surface, and the 

 other two at depths which, although the length of towing rope was kept constant, varied 

 considerably from station to station due to differences in the speed of the ship under 

 different and difficult weather conditions. Tow-netting in the Antarctic seas, particularly 

 in a square-rigged auxiliary steam vessel, is not so easy as it is in temperate waters. The 

 depths were determined by Kelvin tubes. Some of these stations again were taken in 

 comparatively shallow water, so that the lowest net was not far from the bottom ; thus 

 at some stations an organism would be free to migrate to a considerable depth below 

 the lowest net, whilst at other stations this would not be possible. 



In considering the position of organisms in the daytime we must be sure that the 

 diurnal vertical migration has ceased or that it has not begun; for this reason we will 

 first of all consider only those stations which lie in times between 0800 and 

 1600 o'clock. This curtails our number of stations very much, but we shall use all 

 the stations available between these times for which there are phytoplankton records, 

 except those taken outside the Antarctic Zone beyond the line of convergence. 

 There are twenty-two such stations at which N 100 H nets were used, including two 

 taken in March and three in May; eleven have phytoplankton values of less than 0-5 

 million cells per N 50 V haul (0-003 to 0-187 cells), and eleven have phytoplankton 

 values of over 0-5 million cells (0-55 to 531-2 cells). There are nineteen such stations at 

 which N 70 H nets were used, including three in May ; nine have phytoplankton values 

 of less than 0-5 million cells per N 50 V haul (0-006 to 0-358 cells), and ten have phyto- 

 plankton values of over 0-5 million cells (0-55 to 531-2 cells). In Table LXVI the num- 

 bers of the more important organisms which appear to exhibit animal exclusion are 

 given for these stations in ascending order of phytoplankton values from left to right 

 for surface, middle and lower depths. Numbers over 50 are shown in heavy type, as 

 also are shown the means. The stations taken in March and May are marked by asterisks 

 and dagger marks respectively; the remaining stations are taken in December and 

 January. The stations for March and May are included because, granted that the move- 



D XI 42 



