RHINCALANUS GIGAS 307 



volves no marked upward movement of a mass of water such as could carry the plankton 

 to the surface, but it does effect a raising of the level at which the temperature, or some 

 other factor, is an optimum and might thus bring about the active concentration of the 

 plankton at that level. 



In the season 1932-3, during November and December, at all the stations in Weddell 

 Sea water, except St. 1052 which was taken in Weddell Sea water of the " oldest " type, 

 the majority of the catch was in the upper nets. At St. 1052 there was an approximately 

 equal proportion in the upper and lower hauls. Thus, as in the previous season, the 

 population of Rhmcalantis in the "younger" water flowing out of the Weddell Sea 

 (water colder than 0° C.) belongs to the Weddell Sea stock and was found at the surface, 

 while the population at the only station taken in the " oldest " type of Weddell Sea water 

 (water between o and i-o° C.) was found largely in the lower haul and part of it at any 

 rate must have come from farther north in warm deep water. On the line into the Weddell 

 Sea taken during March 1933, Sts. 1138, 1 142 and i i44were taken inwater flowing north- 

 eastwards out of the Weddell Sea (Fig. zb), while Sts. 11 48 and 11 50 were taken inwater 

 flowing westwards into the Weddell Sea. At St. 1 142 the entire catch of over a thousand 

 individuals occurred in the lower net. This station, as the loo-m. temperature indicates, 

 was taken in a tongue of warmer water possibly moving southwards from the South 

 Atlantic. The Rhincahnms population here is probably, therefore, of definitely South 

 Atlantic origin, carried southwards in warm deep water. At St. 1138, at which most of 

 the catch was in the lower nets, part at least of the population belongs to the warm deep 

 water and part to the Weddell Sea itself, since this station was taken near the boundary 

 between the Bellingshausen and the Weddell Sea currents. 



At Sts. 1 148 and 1 150, in water flowing westwards into the Weddell Sea, the majority 

 of the catch was in the upper hauls, although the proportion in the lower nets was high. 

 At the stations taken in this water in January of the previous season (Sts. 816 and 817) 

 the majority of the catch was in the lower nets and it was assumed that the population 

 which enters the Weddell Sea in this current originates from the warm deep water of the 

 South Indian Ocean. If we compare these two stations with Sts. 1148 and 1150 it is 

 noticeable that the discontinuity which marks the upper limit of the westward-flowing 

 deep water lies between 70 and 80 m. at St. 1 150 (Table lib), while St. 1 148 lies on the 

 boundary between the westward- and eastward-flowing water, where, as already explained, 

 warm water wells upwards in the lower layers. At Sts. 816 and 817, on the other hand, 

 the discontinuity is found at a depth of about 150 m. (Table lla). This might well 

 account for the fact that a comparatively high proportion of the catch of Rhincalarms 

 was taken in the upper nets at Sts. 1148 and 1150, but in the lower at Sts. 816 and 



817- 



On the evidence available it is not possible to generalize much about the vertical dis- 

 tribution o{ Rhincalmms in the Weddell Sea. It will be shown later that the population in 

 this area is probably not endemic (pp. 330-2) and must originate from waters outside the 

 Weddell Sea area. From the foregoing account of the vertical distribution it may be said 

 that the population of the Weddell Sea is derived from two sources — from the current 



