THE OLDER STAGES 177 



Time of spmoning 



The northern or Weddell zone. From the occurrences of the eggs in the Bransfield Strait and in the 

 adjoining waters of the western Weddell drift Fraser (1936, p. 113) has shown that spawning in this 

 locality must last from November to March and is not the ' spontaneous ', that is short-term, outburst 

 of Ruud who in the year of his expedition to the Weddell Sea (p. 44) supposed it, as a major event, 

 to have been confined to the first half of January. The material examined since Fraser wrote, again 

 yielded eggs throughout the period November to March and it seems clear that in the northern zone 

 at any rate this is the maximum extent of the spawning season, the large numbers of spent and gravid 

 females recorded in February and March (p. 180, Table 37) suggesting it is probably at its cUmax 

 there in February. 



The southern or East Wind zone. Although little information about the spawning in the East wind 

 drift can be gathered from the occurrences of the eggs themselves, their numbers (p. 182, Fig. 19) 

 being generally so small, the occurrences of Nauplii and Metanauplii, stages which do not appear 

 (p. 301, Fig. 70) in substantial numbers until February, suggest that in this relatively narrow 

 coastal belt spawning does not begin on any appreciable scale until well into January, is probably in 

 full swing by February and lasts as in the northern zone until March. It may be, however, that in 

 these high latitudes it continues into April, for as late as the 20th of that month, at a station worked 

 within sight of the Enderby coast, I found a number of recently spent females in which there were a 

 few unshed, unsegmented eggs still clinging to the walls of the otherwise empty ovary. The later 

 spawning characteristic of the East Wind zone and its compression in time is no doubt associated with 

 the relatively short and generally unpredictable period there during which a blooming phytoplankton 

 is offered to the gravid or near gravid females, the large-scale production of plant life in these high 

 latitudes being much curtailed and hampered by the ice-cover that extends over them for the greater 

 part of the year. As Hart (1942, p. 314) remarks, 



From the known climatic and ice conditions it is obvious that large-scale production [of the phytoplankton] can 

 only begin when the first large areas of open water are formed in January, and as new ice begins to form in March it 

 follows that the annual production must be crowded into three summer months with no possibility of a secondary 

 autumnal increase. Our observations fully bear this out, the main increase evidently begins very suddenly in January 

 and rises to a high maximum (as the oceanic values go) in February. A few moderately high values have been recorded 

 in the early days of March, but taking that month as a whole the falling off was most marked. 



In far northern waters a blooming phytoplankton seems to offer a marked inducement to spawning 

 among certain phytoplankton feeding bottom invertebrates, Thorson (1946) calling attention to the 

 large-scale spawning of these animals that coincides with a sudden and heavy increase in plant 

 production in the East Greenland fjords, Grainger (1959) to a similar phenomenon among certain 

 herbivores of the plankton, especially copepods, of Arctic Canada. 



Law and Bechervaise (1957) give a vivid description of the brief opening up of the East Wind ice- 

 field (oflF Mawson in 64° E) engendered by the onset of the ' sudden summer ' in these high latitudes 

 and the almost continuous sunshine that goes with it. 



Blizzards had tailed off very suddenly in early December and, with the period of the midnight sun, long successions 

 of peerless, clear days had arrived. Sometimes a whole week passed with no moment, day or night, when the sunlight 

 was not gleaming somewhere on the great seaward icebergs. . . . 



Under these circumstances the heat radiated from the dark rock is astonishing. Men may sunbathe as in the high 

 alps, even when the air temperature is below freezing. Lichens make colourful growth and all minor snow drifts 

 rapidly melt away. For this one brief period in the year, liquid water is not a phenomenon. 



The disappearance of the sea ice may occur with dramatic suddenness. There comes a day when the sound of the 

 sea lapping on the rocks is first audible. Any considerable swell is now able to crack the rotten ice floes and almost 

 overnight the whole scene changes. The sea is mobile. 



19 DM 



