DISCOVERY REPORTS 



3 "4 



HORIZONTAL DISTRIBUTION OF ADOLESCENTS AND ADULTS 

 Range in Antarctic and Subantarctic water 

 In the preparation of the chart (Fig. 2) and other figures showing the horizontal distribution of 

 adolescent and adult E. triacantha the data used have been limited to oblique hauls made at night 

 between approximately 100 m. and the surface with the 100 cm. diameter net. There are two reasons 

 for this limitation of the data. First, although the N70V hauls provide valuable information on the 

 vertical distribution of larval stages, the numbers of adolescents and adults taken in these nets are 

 small where a comparison of the populations of different regions is required. Secondly, as mentioned 

 above, E. triacantha performs a marked diurnal vertical migration and consequently hauls are strictly 

 comparable only when taken through the same depth range at the same hour of the day or night. 

 In selecting the stations for plotting in Fig. 2 it was considered sufficient if they had been worked 

 during the hours of darkness. However, owing to the routine adopted for station work, over 75 % 

 of the hauls were, in fact, taken between 2100 and 0100 hrs. 



In Fig. 2 the number of E. triacantha at each station is shown according to the scale on the chart. 

 North of 50° S between 30° W and 60° W no NiooB hauls are available. In this area, therefore, 

 oblique hauls with the young fish trawl (TYF) have been examined, but only to determine the 

 presence or absence of E. triacantha, since the quantities sampled by this larger net are not comparable 

 with those from the i m. net. Presence at these stations is indicated by a cross. Absence is shown by 

 an open circle in the usual manner. 



The Southern Ocean consists of a continuous circumpolar belt of deep water in which the environ- 

 ment is more or less uniform from east to west and in which changes take place from north to south. 

 Broadly speaking this results in the distribution of the planktonic species in circumpolar belts of 

 greater or lesser width. It has been shown (Baker, 1954) that within these belts there is no longitude 

 in which the commoner macroplankton species, including E. triacantha, may not be found (Fig. 2). 



While considering the distribution of E. triacantha, John (1936, p. 232) states that 'it occurs 

 throughout the ice-free water of the Antarctic Zone '. The present more detailed study shows this to 

 be true, as will be seen from the distribution chart. However, there is a very marked concentration 

 in the region of the Antarctic convergence, and where it has been taken in the higher latitudes the 

 catches tend to be small. It also occurs consistently to the north of the Antarctic convergence and this 

 southern zone of the Subantarctic water must be considered as part of the normal habitat. 



Except possibly in the south-western Atlantic, E. triacantha shows a close association with the 

 Antarctic convergence throughout its circumpolar range. Because of this association the latitudinal 

 distribution in relation to the convergence can be examined by using it as a datum line. 



In Fig. 3, six meridional lines of stations are plotted according to the distance of the stations north 

 or south of the Antarctic convergence. The positions of the convergence shown in the figure are from 

 observations made at the time of crossing, and have been taken from Mackintosh (1946, Table 9). 

 The number of animals taken at each station is shown as a percentage of the total number of animals 

 taken along each line of stations. The lines have been selected so that different longitudes and months 

 are represented. It will be seen from this diagram that, although the total range of E. triacantha may 

 extend from 450 miles to the north of the convergence to 650 miles to the south, in all these lines of 

 stations at least 60% of the population in the surface waters at night occurs in the region 150 miles 

 north to 200 miles south of the convergence. 



It is generally accepted that the Antarctic convergence can act as an important faunistic boun- 

 dary (Mackintosh, 1934, p. 83; Ommaney, 1936, pp. 298-9). David (1955) found that, although the 



