232 DISCOVERY REPORTS 



From the data collected in our present survey we have been able to determine in 

 broad outline the vertical migrational behaviour of the more important plankton animals 

 in the South Georgia area. Whilst a few species show little or no such migration it may 

 be said that in general vertical migration is a marked characteristic of the Antarctic 

 zooplankton. The possible ecological significance of this vertical migration will be 

 discussed in Part V, pp. 332-356; here the data relating to the different species will 

 be described in detail. 



Methods of investigation 

 The knowledge of the vertical migrational behaviour of the different organisms has 

 been obtained in three ways. 



At St . 4 1 , 1 6 miles off the north-east coast of South Georgia , the ' Discovery ' anchored 

 in 272 m. of water in an attempt to carry out repeated observations on the plankton 

 throughout a period of 24 hours. The object of this attempt was twofold : to study the 

 diurnal changes in the vertical distribution of the plankton if any, and also by repeating 

 hauls at intervals to obtain a "control" on the validity of the single observations made 

 at other stations. The "control" results will be discussed in a subsequent section, here 

 we will deal with the vertical changes. N 70 V nets were used at the following depths : 

 265-150 m., 150-100 m., 100-50 m. and 50 m. to the surface. The first series of hauls 

 were made at 1300-1345 o'clock and repeated at 1 540-1605 o'clock, 1700-1728 

 o'clock, 1910-1955 o'clock, and 2105-2140 o'clock. The series began in almost perfect 

 weather, with the sea calm, but by 1530 o'clock the wind had freshened and there was 

 a slight northerly swell, and 6 hours later the wind was blowing with a force of 5-6 and 

 the sea had risen to 4-5 with a heavy northerly swell. A full gale rapidly developed, so 

 that the experiment had to be abandoned. Another opportunity for a similar experiment 

 did not occur. Our information regarding the diurnal migration of the zooplankton at 

 this station is then based upon these five repeated series of hauls from soon after midday 

 until some 3 hours after darkness. This information by itself is of limited value since 

 we know from the results of former workers that the vertical migrational behaviour of 

 animals is usually by no means fixed, but varies under different conditions; the results 

 however are valuable when taken in conjunction with those obtained by the other two 

 methods. 



In the second method the average percentage occurrence of different species at 

 different levels in the series of vertical hauls taken at stations throughout the survey 

 between the hours of 0600 and 1759 o'clock were compared with corresponding average 

 percentages for stations taken between the hours of 1800 and 0559 o'clock. Only those 

 stations were included which had a sufficient number of the species in question. 1 The 

 data for shallow water stations, i.e. stations in water of depth 200-300 m., approxi- 

 mately equal to that of St. 41, were separated from the data for deeper water stations. 

 This method gives a more general indication of the behaviour of different species, since 



1 A minimum number of 100 per 50 m. haul was chosen for this purpose, except in the case of Calanus 

 simillimus where a minimum number of 50 was used. 



