148 



DISCOVERY REPORTS 



Patchiness 

 Although known to occur over vast areas of the Antarctic seas it is clear from the evidence of plankton 

 nets, from eye-witness accounts of its occurrence at the surface and from the many observations that 

 have been made on the stomach contents of the whales themselves, that E. superba is far from evenly 

 distributed throughout its range. In recent times^ indirect evidence of what appeared to be a tendency 

 for it to occur in circumscribed shoals or swarms was first obtained by Mackintosh and Wheeler (1929) 

 during the 1926-7 whaling season at South Georgia. Referring to the diet of the whales that season, 

 they say, ' The krill differed from that of other seasons in the fact that there was in most cases a 

 noticeable mixing of Euphausiids of different sizes. These were not ahvays mixed indiscriminately in 

 the stojnach.^ Large or small individuals might be found together in different parts of the mass of 

 stomach contents, or patches of large ones might occur in a mass of smaller forms suggesting that the 

 whale had been feeding on separate shoals which differed in respect of the sizes of the individuals '. 

 In the ' Vikingen' stomach samples Ruud (1932) also found a tendency for the small and large indivi- 

 duals to 'keep apart in separate shoals '? Since 1927 continuous sampling of Antarctic surface waters 

 for distances of up to and over 30 miles by repeated series of consecutive nets has shown conclusively 

 that patchiness or unevenness in distribution, suggesting a tendency to congregate in dense swarms 

 or patches, is one of the most remarkable features of the krill in the sea. From the results of one such 

 net series worked off South Georgia in January 1927 Hardy and Gunther (1935, pp. 256-7 and p. 353) 

 estimate that the patches, assuming them to be circular, are probably not larger than some 200 yards 

 across and probably not more than about | mile apart. This concentration of the whale food into 

 relatively small densely crowded areas they remark, 'helps one to understand how the large rorquals 

 are able to collect sufficient to form an ample meal'. Indeed, when one thinks of the vast body of 

 the whale and the relatively minute creatures on which it feeds it is difficult to conceive, were the 

 habit of the krill otherwise, how the great southern whale population could even exist. 



Direct observation on the whale food at sea largely confirms the conclusions reached by Hardy and 

 Gunther both as to the order of magnitude of the patches and the degree of their separation one from 

 another. During a cruise undertaken by R.R.S. 'William Scoresby' into the Weddell Sea in January 

 and February 1931 an area of profuse patchiness was encountered and observed in great detail by 

 Gunther who was on board at the time. The patches were particularly abundant in the neighbourhood 

 of Station WS 540 in 57° 55' S, 21° 21' W where they were seen continuously over an area estimated 

 to be at least 150 square miles in extent. They were separated from each other by short gaps at most 

 a third or a quarter of a mile wide and although they showed considerable variation in size none was 

 exceptionally large, an average-sized biggish patch covering an area of about 2500 square yards, or say 

 60 by 40 yards. The biggest seen was a narrow belt 150 yards long by about 20 wide.* They varied 

 greatly in shape (Fig. 15). Some were irregularly circular, some oval, some irregularly oblong, others 

 occurring as ribbon-like behs which sometimes had narrow tributary out-streamers resembling the 



^ See, however, early records, pp. 40-42. 



^ The itahcs are mine. 



» The same phenomenon has been found in feeding sahnon, Aron (1958) recording an instance where the stomach con- 

 tained amphipods and euphausians in almost 'pure cultures, with a sharp division between the two groups'. While allowing 

 that the fish might have selected first individual euphausians and later only amphipods from among the plankton, he adds, 

 ' But it is easier to believe that the stomach contents represent actual pure cultures as they occur in nature and that the salmon 

 fed first on a swarm of euphausiids and then on a swarm of amphipods'. 



^ Some patches, however, I suspect may be much larger than this. An entry by Gunther in one of our rough deck logs for 

 18 February 193 1 (time 1700) reads as follows: 'A large extent of krill (suspected) imparting a dull plum hue to the green 

 water around. The patch extended towards South Georgia as far as the eye could see. Birds present. No net shot. Sea 

 calm with long easy swell. ' 



