VERTICAL MIGRATION 233 



it averages the results of a number of stations taken under different conditions. The 

 results by this method are shown alongside those from St. 41 for comparison in a 

 number of figures, of which Fig. 101 is an example. 



The third method was to construct charts showing the depth distribution of different 

 species in the series of horizontal hauls with the N 70 H and N 100 H nets at all 

 stations in the survey, such stations being arranged in a line in order of the time of day 

 from noon, through the afternoon, night and morning and back to noon again. Figs. 99 

 and 120 show examples of such charts for the N 70 H and N 100 H nets respectively. 

 These nets, it will be remembered, were towed horizontally in a series, one at the 

 surface, one at approximately 60 m. and another at approximately 120 m. The actual 

 depths were determined by the reading of a Kelvin tube fastened to the warp above the 

 stream-line lead at the bottom of the warp. The depths recorded varied considerably, 

 often due to the difficulty of regulating the slow speed of the ship in rough weather, 

 and occasionally the bottom net would reach 200 m. On the chart below the appropriate 

 station number the depth of each net haul is shown by the position of a figure indicating 

 the number of the particular organism in the haul, or of a dot if no such organisms 

 were present. This method shows in considerable detail the general vertical migrational 

 behaviour of the different animals. It shows the actual times of their rise and fall, 

 whether this rise and fall is gradual or rapid, and whether it Occurs in some species with 

 regularity or in others irregularly. Like the second method this has the merit of pre- 

 senting a picture based upon a large number of stations taken under different conditions 

 throughout the area, and so gives an indication of the average or normal behaviour of 

 the animals. The series includes stations taken at different seasons ; whilst most of them 

 are taken in December and January, some are taken in March and a few in May. This 

 accounts for the, at first sight, curious separation of St. WS no (late May) and St. 25 

 (March) from the rest of the stations taken in darkness in the 24-hour scale ; the parts 

 of the chart representing hauls taken between sunset and sunrise are shaded. 



Copepoda 



Calanus simillimns shows a most marked and rapid vertical migration coming to the 

 surface from depths greater than 50 m. between the hours of 1700' and 1800 o'clock. 

 The results from St. 41 (Fig. 101) and those from the N 70 H nets at all stations (Fig. 

 99) confirm one another. From the latter observations it would seem that the upward 

 movement through at least 50 m. of water may be carried out in less than an hour's 

 time, indicating an upward speed of at least a metre a minute and probably very much 

 faster. It comes to the surface earlier in the evening than does any other copepod, 

 except perhaps Drepanopus pectinalus which also makes a very rapid ascent. The be- 

 haviour of these two copepods is very similar, as will be seen by comparing the figures 

 which are placed together for this purpose. Calanus simillimns may remain in moderate 

 numbers at the surface until between 7 and 8 o'clock in the morning, but the bulk of 

 them appear to sink at the approach of 2 or 3 o'clock. 



