RHINCALANUS GIGAS 315 



ever, shown that R. gigos exhibits no diurnal vertical migration, but that there seems to 

 be a tendency for the species to occur at a higher level during the afternoon towards the 

 end of the hours of daylight (p. 242, fig. 112). 



SUMMARY 



1 . The area of greatest abundance of R. gigas, in which the species was most abundant 

 both numerically and proportionately, in the Falkland Sector during the summer 

 seasons 193 1-2 and 1932-3, lay in the Drake Passage and in the South Atlantic Ocean. 

 In 193 1-2 it occupied the Bellingshausen Sea current south of the Antarctic convergence, 

 but in 1932-3 it extended north of the convergence into sub-Antarctic water in the 

 Drake Passage and between the Falklands and South Georgia (pp. 294, 310). 



2. This area of abundance is in striking contrast with the comparative scarcity of the 

 species in the Weddell Sea current (pp. 296, 297). 



3. In the South Atlantic the limits of the area of greatest abundance were the Ant- 

 arctic convergence on the north and the 0° C. isotherm, calculated from the average 

 temperature of the surface 100 m., on the south. In the Scotia Sea, where the influences 

 of the Bellingshausen Sea and Weddell Sea currents mingle, the southern limit of the 

 area of abundance was less certainly fixed, but lay somewhere between the o and i-o° 

 isotherms. In the Drake Passage the southern limit of the area of abundance was not 

 defined in the season 193 1-2. There was a great abundance of the species in water colder 

 than — 1-5° C, which was perhaps due to the upwelling of warm deep water along the 

 Antarctic continental shelf. In the season 1932-3 the northern limit of the area of 

 abundance was not strictly defined in the Drake Passage and South Atlantic but seems 

 to have been more or less coincident with the 5-0° isotherm. The southern limit in the 

 Drake Passage was the — i-o°C. isotherm (pp. 294, 310). 



4. The species occurred in fair abundance at the stations taken in sub-Antarctic 

 water in the South Atlantic during both seasons. At the end of both seasons, however, 

 it became restricted to the Antarctic Zone, and was taken in very small numbers in sub- 

 Antarctic water (pp. 298, 311, 312). 



5. In the winter months around the Antarctic Continent a progressive diminution of 

 the catches in the surface 250 m. was found, together with the restriction of the species 

 to the Antarctic Zone. By mid-winter R. gigas had almost disappeared from the catches, 

 and in September in the South Pacific it had disappeared completely. Observations for 

 July and August are lacking. The species reappeared in the surface 250 m. in October 

 in the western Drake Passage (pp. 299, 301). 



6. A study of the proportion of the combined catches taken in the upper and lower 

 nets strongly suggests that R. gigas spends the summer months within the surface 

 100 m. and descends in about April to a level between 250 and 100 m., while in May it 

 sinks below the 250-m. line and remains there until October, when it reappears between 250 

 and 100 m. In November it regains the surface. Thus this species undertakes seasonal 

 vertical migrations similar to those found for several species in the northern hemisphere. 

 Its habitat during the summer months is thus the northward-flowing Antarctic surface 

 water and during the winter months the southward-flowing warm deep water (pp. 301-2). 



