428 DISCOVERY REPORTS 



by them can continue to live and develop unaffected in association with the forms producing them. It may be that some 

 benefit from such associations, but if they are not at least tolerant, then they must either migrate or slowly succumb. Only 

 those which are tolerant, too, can succeed after the production of such substances, whilst such tolerant forms must 

 quickly be overtaken in the succession by any forms which have developed favourable adaptations to the conditions. 



The onslaught of predators upon this species is a phenomenon perhaps without parallel in marine 

 ecology and yet in its untold millions it survives. What, however, if it had no predators, or at any 

 rate suffered vastly less severely from them than it does? It seems inevitable that with its enormous 

 fecundity it would multiply enormously, in the end reaching such fantastic numbers that even the 

 immense wealth of pasture of the southern seas would be unable to support it, and, as happens 

 with fishes (Rounsefell and Everhart, 1953) when their predators have been removed, there would 

 be overcrowding of the polar sea by undersized,^ slow-growing krill, and ultimately a density- 

 dependent mortality would set in and the population would decline. Lack's work (1954, 1955) on 

 animal numbers and the factors that regulate them suggesting it might decline enormously. 



In the face of such staggering odds the ability of E. superba to maintain itself so successfully passes 

 comprehension. We may well indeed wonder whether in fact it has not already reached a point where 

 in its fantastic numbers it is competing for life on such a massive scale that even now it is bringing about 

 a displacement of its competitors in the same ecological niche and (Hardin, i960) perhaps threatening 

 their very existence. 



Besides disparity in numbers E. superba and E. triacantha present an interesting contrast in circum- 

 polarity, the larger species, in the major aspects of its distribution, showing a marked asymmetry, the 

 smaller, spread round Antarctica with far less latitudinal variation, a distinctly more symmetrical 

 pattern. The distribution of E. triacantha is marked too by a seeming intolerance of the cold surface 

 current system in which the whale food occurs in such profusion. In fact only in the extreme north, 

 round South Georgia, where hydrological conditions are complex and waters of very mixed origin 

 converge, is it regularly found in this system at all. In short, E. triacantha is essentially a creature of the 

 West Wind drift. Is it right, however, to suggest that it is ' intolerant ' of the somewhat colder, although 

 not all that colder, environment in which we find the vast bulk of the congregating krill ? It may be 

 that it is the way of life of this species, its behaviour, vertical movements and so on, and the behaviour 

 (or dynamics) of the water masses in which it lives from hatching onwards, that in general combine 

 to prevent its co-habitation with its southern neighbour and mixing freely with it. The dynamics] 

 of the environment clearly lead to free mixing near South Georgia, and again it seems, although 

 perhaps in a lesser degree, south-east of Kerguelen and south-east of New Zealand, wherever in factj 

 there is local penetration of the West Wind zone by the branching of the East Wind stream. However, I 

 w^here their major concentrations are concerned, the two species obviously occupy distinct 'ecological] 

 niches', being segregated, as so many other Antarctic plankton animals are, in a seemingly uniform! 

 environment in which as Mackintosh (i960) remarks 'there are few tangible barriers to dispersal'. 



THE FEEDING MIGRATIONS OF THE BALEEN WHALES. 

 THEIR POSSIBLE ORIGIN AND HISTORY 



In most baleen whales in both northern and southern hemispheres there is a breeding-feeding rhythml 

 involving them in extensive movements from subtropical and temperate breeding grounds into Arctic] 

 (or Arctic-Boreal) and Antarctic waters to feed. It is worth while speculating as to how this remarkable] 

 rhythm evolved, and how the whales came to undertake such enormous journeys as they accomplish ] 

 today. 



^ A parallel and well known phenomenon is the stunting of fish in overstocked lakes and ponds (MacGregor, 1959). 



