ZOOPLANKTON IN THE SOUTHERN OCEAN 195 



that significant losses occur in the catches taken by this method. It is felt, however, that the conditions 

 which he describes do not fulfil the actual working conditions of the N70V both with regard to the 

 method and speed of hauling and to the length of water column sampled. (The net is hauled vertically 

 at a speed of 1 m. per sec. over a davit incorporating a spring accumulator, and the shortest closing 

 haul is from 100 to 50 m.) As 0stvedt (1955) points out, such losses, if they do occur, will only be 

 of significance if the plankton is very unevenly distributed in depth or if the haul is a short one, 

 i.e. 100-50 m. 



Nets of the dimensions given in Kemp, Hardy & Mackintosh (1929) have been used on the ships 

 of the Discovery Committee from 1927 until 1939. On the first post-war commissions of the R.R.S. 

 'Discovery II' (1950-51) and the R.R.S. 'William Scoresby' (1950) the nets were inadvertently 

 made to slightly different measurements, and there can be little doubt that, although the same grades 

 of silk were used, the filtering capacity of this net differed from its pre-war counterpart. In view of 

 this discrepancy preliminary comparative tests have been made which consisted of duplicate hauls 

 with nets of both dimensions. As a result of four paired hauls at each of six stations in the North 

 Atlantic and Mediterranean (i.e. 48 volume measurements), it has been found that there is a fairly 

 consistent difference, the pre-war net giving about one and a half times the catch (by volume) of the 

 post-war net. A factor of f has therefore been applied to the volume measurements of 203 samples from 

 the 33 post-war stations used in this work. This factor must be considered as provisional, since future 

 tests may allow the differences in filtering capacity and catching power to be more accurately gauged. 



It should be noted that even if uncorrected results are used they in no way alter the principal 

 conclusions drawn from the data, but it was deemed better to use a provisional correction rather than 

 to ignore the difference. 



At most stations there are samples from the following six routine depth intervals between the 

 surface and 1000 m: 50-0 m., 100-50 m., 250-100 m., 500-250 m., 750-500 m., and 1000-750 m. 

 Occasionally a deeper haul from 1500 to 1000 m. was worked, and at eight of the stations taken into 

 account in this paper (1675, 1 677-1682 and 1684) the normal shallow haul was done in two stages of 

 50-25 m. and 25-0 m. Here these volumes have been combined to represent a haul from 50 to o m. 



Hydrological data are available for each station at all depths sampled by the nets, so that it is possible 

 to view the standing crop of plankton in relation to the sea temperature and other variables and to the 

 distribution and movements of the main water masses of the Southern Ocean. (For full details of the 

 hydrology of the area reference should be made to Deacon, 1933, 1937; Clowes, 1938; and for a brief 

 account Mackintosh, 1937, p. 373.) 



The positions of stations from which the volumes of samples have been measured are given in 

 tables 1 1 a-f in the appendix, and it is necessary to explain the basis on which these stations have been 

 selected from the much greater number available in the Discovery collections. For various reasons 

 the work of the ' Discovery II ' was more concentrated in some areas and times of year than in others, 

 and for the present work it is not necessary to make use of all of them, since the labour of measuring 

 all the samples would be out of proportion to the objects in view. I have, however, measured 2185 

 samples from 366 stations, chosen so as to cover seasonal variations of the standing crop of zooplankton 

 in as many different regions of the Southern Ocean as possible. Certain meridional lines of stations, 

 which have been repeated in different months, are particulary suitable for this purpose. They cover 

 a period from December 1933 to November 1934 plus October 1932 in the meridian of 8o° W; April 

 1938 to March 1939 plus September 1936 in o°; and April 1938 to March 1939 plus June 1936 in 

 20 E. The nine lines of stations in both o° and 20 E cover all the seasons in the south-east Atlantic 

 sector of the Antarctic and sub-Antarctic zones, and the six lines in 8o° W (south-east Pacific) cover 

 more than half the year. 



