424 DISCOVERY REPORTS 



toll of the krill in the neighbourhood of South Georgia, would be unlikely to occur. Thus, it would 

 appear, the krill in these high latitudes, already enjoying at least some measure of immunity (p. 402) 

 from the major depredations of whales and seals, exist in some measure of immunity too from penguins 

 and possibly coast fishes, an immunity it seems that might be particularly high on the Atlantic or 

 Weddell side. And so, enormous as must be the mortality suffered annually by this species through 

 the ravages of its northern predators, in the East Wind zone both pack-ice and barrier contribute 

 something to its protection, combining to create in these high latitudes something of a natural reserve. 

 The impenetrable core of the pack-ice in the heart of the Weddell Sea, if the krill are there (see 

 pp. 71-3), must also contribute to its protection. 



EUPHAUSIA SUPERBA AND E. TRIACANTHA 



The gross distribution and relative abundance of (i) E. superba (over 20 mm. group), and (2) the 

 smaller circumpolar E. triacantha (all stages excluding larval), based on the data from the upper 

 (loo-o m.) oblique stramin nets, is shown in Fig. 157, the data for E. triacantha having kindly been 

 provided by my colleague Mr A. de C. Baker, who has recently published a comprehensive report 

 upon it (1959). This figure is presented principally to show the enormous disparity in abundance 

 exhibited by the two species, the larger a highly concentrating swarmer, the smaller, judging from 

 the remarkably low order of abundance in which it persistently occurs in our samples, an organism 

 scattered broadcast more or less uniformly throughout its circumpolar range. The disparity in 

 numbers, enormous even when only the older stages of the krill are considered, would of course be 

 vastly accentuated by including the post-larvae of this species in the construction of the chart. 



Although other factors as yet obscure must also be involved, it is possible that the great depths at 

 which the krill eggs hatch contribute something to the immense scale on which this species exists. 

 For at these depths other pelagic animals are scarce and the resultant larvae it seems might begin 

 their existence in some measure of immunity from depredation. Moreover, such relative immunity 

 as they may enjoy at the outset of life does not end there, for the ascending larvae it seems in most 

 regions are being carried towards further immunity, the southward movement of the warm layer 

 through which they pass bringing some of them in the end into the high latitudes where major depreda- 

 tion is at a minimum. Or again, if the krill spawn on the bottom, and we have some evidence that they 

 might, it could be supposed that the eggs so laid would enjoy a greater measure of protection than 

 they would if drifting about on the surface at the mercy of the myriads of plankton animals that 

 doubtless prey upon them. Singer (1959), referring to the spawning of sand eels {Ammodytes spp.) 

 in the relative security of the shallow sandy bottoms of the North Sea, suggests that this might 

 contribute at least a little to the immense scale on which these heavily preyed-on fishes exist. 



In his studies on the reproductive and larval ecology of bottom-living invertebrates, Thorson (1950) 

 emphasises that it is the early part of the life-cycle that is most vulnerable and that the abundance 

 of older individuals will to a large extent be dependent upon the numbers of larvae that survive to 

 settle down to their benthic existence. In so far as it may affect the annual recruitment of the teeming 

 krill population the relative immunity its immensely deep early developmental phase enjoys could well 

 it seems therefore be critical. As Thorson has stressed, a fundamental knowledge of the factors 

 governing the numbers in a population is only to be obtained through a consideration of the limiting 

 values, not the average values, of the ecological factors, and we must, therefore, focus our attention 

 he says on the weakest link in the life-cycle, normally to be found during the breeding period and 

 larval development, 'when the requirements of the organisms from the environment are often much 

 more definite ' than at other times. Thus in a demersal fish such as the haddock the relative numerical 



