144 DISCOVERY REPORTS 



this region, based on the plankton gatherings of the ' Discovery ' (1901-4), the 'Terra Nova ' (1910-13), 

 the floating factory 'C. A. Larsen' carrying a Discovery Investigations observer (1928-9) and the 

 'Discovery II' (1936), is shown in Fig. 13, the combined observations of these vessels, so far as 

 I know, providing the only data available from net-hauls relating to the euphausian population of this 

 region of which, whether published or not, there is any record. Not one of these expeditions it will be 

 seen records the krill in any measure of abundance southward of a line in approximately 73° 30' S, 

 which, our hydrological observations between 175° E and 175° W show, appears to be the limit of 

 major southerly penetration of the shelf by warm deep water in this particular locality, and none 

 record it at all southward of a line in 74° 45' S between the same meridians where the last most 

 southerly traces of a warm deep penetration can be detected. All to the southward, between 175° W 

 and the Victoria Land coast and up to the Great Ross Barrier, the water is pure shelf water, cold to 

 the bottom, the krill that have been recorded in it being represented exclusively by that other large 

 southern euphausian, E. crystallorophias, which in January 1936 was seen from 'Discovery II' 

 swarming in great profusion near the barrier face, particularly in the neighbourhood of the Bay of 

 Whales. No doubt it is this species that is the food of the minke whales that frequent the barrier 

 region and doubtless too it was this that Lindsey (1937) found in the stomachs of the Weddell pups 

 he examined near Little America and not as he records (p. 134) E. superba. 



In a preliminary note on the biological collections of the 'Discovery' (1901-4) Hodgson (1905) 

 refers to the 'profusion' in which the large E. australis (= E. superba) was encountered between 

 latitudes 66° S and 72° S, but he does not mention it farther south. 



The combined observations of four well-equipped expeditions operating at intervals over a 

 period of 35 years point fairly conclusively it would seem to the absence of E. superba from the greater 

 part of the shelf water of the Ross Sea region, and it seems likely that the factors principally contri- 

 buting to this interesting distributional phenomenon are (i) the failure of the warm deep current, 

 carrying the ascending larvae, to penetrate more than a short distance on to the shelf,^ and (2) the 

 absence of any influx of krill, whether larval, adolescent or adult, west-borne upon the surface stream 

 that enters this region from the east and flows clockwise round it. 



Between 175° E and 175° W the warm deep current overflows on to the shelf as a narrow stratum 

 averaging less than 150 m. thick, penetrating as such as far as 73° 30' S, where, as a major carrier of 

 the larvae, its influence must virtually cease. It continues, however, to the southward as a barely 

 detectable trace to about 74° 45' S where, as Fig. 13 shows, the last, virtually negligible^ and most 

 southerly occurrences of the krill have been recorded. It seems likely too that the majority of such 

 south-borne larvae as do get carried on to the shelf and subsequently rise to the surface there must 

 quickly be carried off it again by the cyclonic movement of the surface stream. 



The non-recruitment of the high southern reaches of the shelf by surface-borne influxes from the 

 east is probably associated with the impenetrable and more or less permanent nature of the pack-ice 

 that for long has been known to exist in that direction. Although so far only its northern outskirts 

 have been explored it seems that the high latitude Pacific ice-cover is very compact and thick, 

 probably continuous over vast stretches of ocean and merging imperceptibly with the continental 

 ice-sheet, presenting in all it would appear such an effective barrier to the direct passage of sunlight 

 so essential to the existence of a diatom flora that no euphausian population it seems could exist. 

 Mawson (1928) has already called attention to the probable conditions below the shelf-ice bordering 



1 Riedel (1958) has just called attention to the possibility that the absence of certain cosmopolitan Radiolaria from 

 Antarctic shelf stations south of AustraUa may also be due to the failure of the warm deep water to penetrate on to the shelf. 



* The total yield from thirteen net hauls in these, the highest known latitudes of the krill's occurrence, was only fifteen 

 individuals. 



