IMPACT OF THE SWARMS ON THE OCEAN PASTURES 241 



A parallel phenomenon is presented by the impact of ctenophores on copepods and other plankton 

 animals. Oi Pleurobrachia pileus, for instance, Bigelow (1926) has written, 'Wherever these cteno- 

 phores swarm they sweep the water so clean and they are so voracious that hardly any smaller 

 creatures can coexist with them. Copepods in particular are locally exterminated in the centers of 

 abundance for Pleurobrachia, though in their own turn they may swarm nearby'. 



In his recent studies on the distribution of the standing crop of zooplankton in the Southern 

 Ocean, Foxton (1956), working with the material from our vertical nets, arrives at the somewhat 

 unexpected result that the crop (excluding that of the staple whale food which cannot, or virtually 

 cannot, be sampled by the vertical nets) reaches its maximum in the krill-poor West Wind drift, 

 being actually from two to three times heavier there than in the East Wind-Weddell zone where the 

 krill are so abundant. What can lie behind such a remarkable difference? It could, I suggest, spring 

 directly from the impact of the multitudes of grazing swarms on the pastures of the circumpolar 

 sea, at its massive maximum in the East Wind-Weddell stream, where surely it must be operating 

 to the disadvantage of other herbivores, at its minimum in the West Wind zone where equally it 

 must be operating to their advantage. As Graham (1956) has said of the 'world of plankton', it is a 

 ' dynamic world, in which the plants are as directly removed by the animals as is the grass by herbivores 

 of the plains. Within that world there are side-relationships, one species depending on another or 

 adversely affecting another, perhaps by competition for nourishment, perhaps by secretion of harmful 

 or beneficial metabolites '. 



HAPPENING OF THE WHALES UPON THE SWARMS 



In krill-rich regions such as the Weddell drift, where as we have seen (p. 148) the swarms may be 

 encountered every two or three hundred yards or so, or even closer together, and may be scattered 

 thus in all directions over hundreds of square miles of sea, it might be supposed that the questing 

 whales simply come upon their food by chance, for in the midst of such profusion they could hardly 

 it seems fail to strike a swarm, or at any rate come close enough to see one, in whatever direction they 

 moved. In other words, as Beklemishev (i960) suggests, having come upon this richly spread 

 table, they probably start seeking their food at random. Obviously, however, though chance must 

 play a part, equally obviously, the swarms being often massed at the surface and distinctly 

 visible there, in daytime at least, eyesight must also come into play. For as Eraser and Purves (1959) 

 observe there is evidence that the visual sense can be normally developed and even adapted to 

 underwater vision in conditions of good visibility.^ Hjort (1933), too, remarks on the keen eyesight of 

 fin whales, observing that no sooner do they catch ' sight of a patch of food than they are upon it in 

 a flash'. 



It may be, too, that the incessant movement of the limbs, pleopods and mouthparts of the concen- 

 trated myriads of a swimming and feeding swarm creates a vibrational disturbance in the water^ to 

 which the whales are acutely sensitive, the wax plug in the external auditory meatus, as Eraser and 

 Purves (1954) and Purves (1955) have recently discovered, being a good conductor of sound, ' especially 



^ MacGinitie and MacGinitie (1949) remark on the poor quality of whales' vision, stating that 'most whalebone whales 

 have poorly developed eyes and have little use for sight'. Breathnach (i960), after a comprehensive review of the literature, 

 states that until much further work is done ' it is not possible to form a final opinion on the status of vision in Cetacea. Observa- 

 tions from life, Mann's findings [see p. 143], and the condition of the subcortical centres suggest, however, that it is much 

 more acute and discriminatory than previously thought'. 



^ Strange echo records have recently been detected in the North Sea (Gushing and Richardson, 1956). They are described 

 as 'noisy' and seem to have been produced by densely packed euphausians (Nyctiphanes couchii) just below the 

 surface. 



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