1,8 DISCOVERY REPORTS 



to the south and, therefore, could have been carried, as First Calyptopes, for many days in the surface 

 stream. I would point out too that I have nowhere suggested that the 'climbers', once they have 

 reached the surface, behave in any way differently from the 'resident' population. What I have in 

 fact been suggesting all along is that both climbers, their journey accomplished, and residents, behave 

 in the same way, undergoing the somewhat restricted vertical movements to which I have already 

 called attention in (8). 



(lo) I agree of course that in view of the mortality factor we should expect to take fewer late 

 Furcilias than Calyptopes. Yet rafting I think cannot be ignored. It is so strongly suggested by the 

 enormous gatherings of Furcilias 4-6 we make with the horizontal (0-5 m.) stramin nets fished in 

 both daylight and darkness at the same stations as the vertical nets are used. Such gatherings at 

 times run to tens and even hundreds of thousands. At Station 2300, for example, there were only 

 261 late Furcilias in the 50-0 m. vertical net. In the horizontal net there were over 600,000. Many 

 other instances could be given where the horizontal net took thousands or tens of thousands of speci- 

 mens whereas the vertical net took only two or three or none at all, and the general impression we get 

 is that the older Furcilias, especially Furcilia 6, are massed at the surface, both night and day, in an 

 extremely narrow stratum, no more perhaps than a metre or two thick. This in fact is what might 

 be expected from what we know (p. 149) of the shallow draught of the older swarms we so often see on 

 the surface. 



Finally, if the Calyptopes and Furcilias did in fact have a daily rhythm, involving mass descent of 

 the total surface population into the warm layer, then, assuming both surface stream and deep current 

 to be travelling at approximately the same speed, with compensating northerly and southerly com- 

 ponents, the larvae would not, as they manifestly are, get carried readily to lower latitudes in the 

 surface drift but would tend to remain where they were ; and so the populating and annual recruitment 

 of distant outlying districts of the krills' geographical range, such as for instance the South Georgia 

 whaling grounds, where spawning (p. 190) is not a successful event, would become impossible to explain. 



Some biological implications of the developmental ascent and its influence 



ON THE distribution AND CONSERVATION OF THE EUPHAUSIAN POPULATION 



It follows from the restricted vertical movement of the larvae just described that once the larva, as 

 the First Calyptopis, reaches the Antarctic surface layer it goes through the whole of its subsequent 

 development there without ever, at any rate on any massive scale, becoming involved again in the 

 warm, mainly south-flowing, deep water. In other words it is only during that period of its existence 

 when, having left the cold bottom water, it changes from the Second Nauplius to the First Calyptopis 

 form, that the larval population as a whole can be said to be moving towards the south. And this being 

 so it is obviously a matter of major importance to determine, if it can be determined, how long the 

 larvae actually spend in their warm deep environment and to what extent their sojourn there may 

 affect or control the distribution of the whale food in the Antarctic seas. The role of the warm current 

 as an agent of distributional control was originally postulated by Fraser (1936, p. 167) who in the 

 concluding paragraph of the summary of his results writes, ' ... the continual abundance of E. superba 

 in Antarctic waters and the replenishment of the stock of adolescents at the ice-edge is brought about 

 by the rotary movement resulting from the assemblage of the earlier developmental stages chiefly in 

 the southward flowing warm deep water and that of the later stages in the northward flowing 

 Antarctic surface water '} 



1 Mackintosh (1950), in a paper referring to some of my early findings, speaks in similar terms, noting that the adults live 

 very close to the surface while the ascending larvae are carried southwards in the warm intermediate layer, to the benefit of 

 the stocks in higher latitudes. 



