88 DISCOVERY REPORTS 



summer's spawning. In the Weddell drift the larvae exist in a negative-positive environment from 

 January to June and in an all-negative environment for the rest of the year. Exclusively positive 

 temperature ranges for the larvae are only in fact encountered in the Bransfield Strait and on the 

 South Georgia whaling grounds, in the former locality in March alone, in the latter throughout the 

 six months December to May. In the Bransfield Strait the young stages endure an all-negative 

 temperature range from April to November and on the South Georgia whaling grounds from July 

 (probably) until November, the conditions in June in the latter locality most likely being positive- 

 negative as they are then in the Weddell drift. Of the three developmental phases the very young 

 adolescents, in all four regions of euphausian abundance, emerge as the group that endures the coldest 

 physical environment of all, the vast majority of them spending their entire existence in the plankton 

 in water that never rises above zero C. As for the older adolescents and adults it will be seen that in 

 the East Wind drift they exist in negative-positive conditions from January to February and in all- 

 negative conditions for the rest of the year, and that in the Weddell drift their monthly ranges are 

 the same as for the larvae. It is again only in the Bransfield Strait and on the South Georgia whaling 

 grounds that the older stages of the whale food encounter exclusively positive surface conditions, in 

 the former locality from January (probably) to March, in the latter, like the larvae, from December 

 to May. In both localities the periods of sub-zero existence for the older stages are as described for 

 the larvae. 



Finally, it may be observed that although the krill, both as larvae and adults, are manifestly tolerant 

 of surface temperatures which in summer and autumn (Table 4) range between — 2-00° and +3*99° C, 

 it need not be supposed that this absolute range is a major factor determining the distribution of this 

 species in the circumpolar sea. It must always, it seems, be to the movements of the East Wind- 

 Weddell surface stream, and the farthermost limits to which it penetrates, that we must turn to find 

 the ultimate determining factors. It is in this current system that the krill both young and old are 

 found par excellence in their maximum abundance and if both it and the tongue of water flowing from 

 it towards South Georgia be regarded as one great surface stream, as in fact it is, it is clear that the 

 temperature range of the euphausians it carries must itself ultimately be dependent upon whatever 

 measure of warming up this current may receive as it moves from high to low latitudes. It is con- 

 ceivable then that if the flow of this great polar stream were stronger and it were to find its way into 

 still lower latitudes the temperature range of the krill might be appreciably wider than it has been 

 shown to be. As Deacon (1957^) has said, it would be 'useful to be able to form a precise idea of how 

 the movements of water may facilitate dispersion of eggs or larval stages from successful breeding 

 grounds and the subsequent replenishment of the stock. There seems to be growing evidence that 

 such considerations may be more important than the eff"ect of a simple temperature difference '. 



THE LARVAL STAGES 



Vertical distribution 



In the spring and summer of 1929-30 a series of fourteen vertical plankton stations was worked from 

 the Norwegian floating factory ' Vikingen ', the observations being spread over the main east-flowing 

 path of the Weddell drift from longitude 40° W to the meridian of Greenwich. Two standard hauls, 

 from 450 to 200 m. and from 200 m. to the surface, were made at each station. In the latter part of 

 January and in early February large numbers of larval E. superba, principally the Calyptopis stages 

 but including very small numbers of First Nauplii and Metanauplii and a few adults, were collected 

 by this expedition and in the summary of his report on this material Ruud (1932) writes, ' It {E. superba] 

 is a surface species, and both the adults and larvae in our material were taken, almost without 



