RHINCALANUS GIGAS 359 



5. The catches became progressively reduced in size during the winter months, and 

 after midwinter in the South Pacific Ocean, the species had practically disappeared from 

 the surface 250 m. (pp. 299-301). 



6. Examination of the percentage of the total catch taken in the upper and lower nets 

 in the summer in the Falkland Sector, and in April in the South Indian Ocean, strongly 

 suggests that R. gigas undertakes seasonal vertical migrations similar to those of the 

 species investigated in the northern hemisphere. It spends the summer months within 

 the surface 100 m., and at the end of the summer descends below this level. At the end 

 of April it had sunk out of range of the 250-100 m. net. It reappears again in the surface 

 250 m. in the spring, and in November returns once more to the layer between o and 

 100 m. (pp. 301-4). This theory has been confirmed by the work of the ' Discovery II ' 

 on her third commission (1933-5). Thus, while R. gigas spends the summer in north- 

 ward-flowing Antarctic surface water, it spends the winter months in the southward- 

 flowing warm deep water. 



7. The species has two spawning periods during the year. One takes place in the 

 summer in Antarctic surface water from mid-December onwards (in the Falkland 

 Sector) — although the exact range of time covered by the spawning has not been fixed 

 (pp. 323-30). The summer generation produced by this spawning descends into the 

 warm deep water and spawns there probably in late May and the first half of June (pp. 

 338-40). This spawning produces the over- wintering generation which reappears in 

 October in the surface 250 m. mainly in stages iv and v (pp. 323-6, 341). 



8. It seems that the over-wintering generation spends the months July, August and 

 September in warm deep water mostly in stages iv and v, but the stage reached by the 

 spring seems to depend on the temperature of the water in which development occurs, 

 since in the spring the winter generation reappears in sub-Antarctic water largely in the 

 adult condition and in stage v. In Antarctic water, however, it reappears largely as 

 stages iv and v, and in the coldest water, in the spring of 193 1-2, comparatively large 

 numbers of stage iii were found (p. 324). The over-wintered generation which re- 

 appeared at the surface in the western Drake Passage in the warmer spring of 1932-3 

 was in a more advanced condition than that which reappeared in the colder spring of 



1931-2 (p. 341). 



9. The over-wintered generation comes to maturity in late November and December 

 around South Georgia (pp. 326, 343), and the approach of the spawning period is 

 heralded by the appearance of ripe eggs in the ovaries of the adult females (pp. 316-19) 

 and by an increase in the proportion of adult males in the lower nets (pp. 320-2). 



10. In the Falkland Sector the summer spawning takes place in water of Bellings- 

 hausen Sea origin in the South Atlantic, western Scotia Sea and Drake Passage, within 

 an optimum temperature range not exactly fixed but probably between i-o and 4-0° C. 

 In the season 193 1-2 it began in December in waters east of South Georgia and spread 

 later into the Scotia Sea and Drake Passage. In the season 1932-3 spawning probabl} 

 began earlier than in 193 1-2 (pp. 326-30, 345). 



11. No spawning at all took place, in 193 1-2, in Weddell Sea water before the end of 



