142 DISCOVERY REPORTS 



The consumption by whales of such small euphausians may well be of commoner occurrence than 

 is generally supposed, for as Ruud and others have noted the younger krill are digested, and therefore 

 become unrecognisable, far more rapidly than the more heavily chitinised adults, the latter persisting 

 longer in a recognisable state in the stomachs. Even when fresh very small krill, especially those as 

 small as the Sixth Furcilia, might readily indeed escape the notice of the casual observer since in a 

 well filled stomach in which the young and old broods are both represented the smaller animals tend 

 to be masked by the sheer bulk of the larger. 



It is evident then that numerically at least, if not in bulk, krill of hitherto unsuspected smallness 

 may form an important constituent of the diet at certain times of the year. Nevertheless, in so far as 

 sheer bulk of intake is concerned it is likely that, as Peters (1955) has also remarked, it is upon the 

 older euphausians over 20 mm. long that the whales mainly depend for their annual fattening up. 

 During the feeding process the baleen plates must behave selectively, retaining the large euphausians 

 more readily than the small. At the same time it is difficult to avoid the conclusion that the supply 

 of young krill, the staple whale food of the future, might become seriously depleted if the whales were 

 to frequent the feeding-grounds in large numbers at a time when older krill was scarce (see 

 charts on pp. 363, 366, 375, 380, 399 and 404) and there was little else for them to eat but the 

 late larval and early adolescent brood. Fortunately, when such a situation arises, as it does during 

 autumn and winter, the whales have largely deserted their southern haunts and dispersed to warmer 

 waters to breed. Thus as they do not return to their feeding-grounds until spring, the rising generation 

 of whale food is granted annually some three to four months' respite from the ravages of its largest 

 predators. 



That the baleen plates of both blue and fin whales should be capable of dealing effectively with 

 euphausians as small as say from 1 1 to 20 mm. long is not surprising when it is recalled that in northern 

 waters, as CoUett (1886 and 191 2) and Milais (1906) have shown, the sei whale, although the fringes 

 of its baleen plates are admittedly of a finer texture than those of the larger rorquals, is well known 

 to feed quite commonly on much smaller organisms such as the copepods Calanus finmarchicus and 

 Temora longicornis. 



In view of its abundance and pronounced tendency to occur in surface shoals (Hardy and Gunther, 

 1935, Fig. 133) it is surprising that the large pelagic amphipod, Parathemisto gaudichaiidii, is so 

 seldom found in the stomachs of the large southern baleen whales. The seis that penetrate into 

 the Pacific sector (Nemoto 1959), are so far the only recorded substantial consumers. At South 

 Georgia the seis examined by Discovery observers were feeding on krill (Matthews, 19386). In the 

 stomachs of the other well-known krill-eaters, blue, fin and humpback,^ this swarming amphipod 

 has not been recorded except as an occasional and evidently fortuitously swallowed morsel (Mackintosh 

 and Wheeler, 1929). As Mackintosh and Wheeler remark it is so abundant in the plankton that the 

 whales ' can hardly help swallowing a certain quantity '. If blue, fin and humpback whales preyed 

 deliberately on the Parathemisto swarms, as well as on E. superba, we should expect to find them not 

 only in conspicuous masses in the stomachs but like the krill (p. 148) in discrete masses, and this has 

 never been recorded. In northern waters Bigelow (1926) calls attention to the same phenomenon in 

 the Gulf of Maine, noting that 'the large, easily recognized, pelagic amphipod Euthemisto, locally 

 and temporarily so abundant, has never been recognized in the stomachs of any of the whalebone 

 whales. Is it eaten? And if not, why not?' I would suggest that the most likely answers to these 

 questions are that in southern waters the whales concerned are able to distinguish between the bright 

 red of the krill swarms and the blackness of the Parathemisto swarms and that for some unknown 



1 Little is known about the southern right. At South Georgia, from a single record, it would seem to be krill-feeder 

 (Matthews, 1938 a). 



