F-8 



Sea (Hodgson 1902f Marr 1^62) and the Ker.^uelen Gaussberg Ridge area (iJonoto I968) a region 

 poorly sampled by Discovery Investigations (l6 stations total from November to Apri]). 



The catches by commercial type operations indicate a distribution very similar to that 

 described by Marr (I962) on the basis of plankton net samples (his Fig. 5 and 6), (see Fig- 

 ure 6. 4). The limited number of lar{;:o catches in xhe Nest Wind Drift zone can be attributed 

 to one of two factors. Either krill in this sono do not tend to swann and althou^i present 

 in large quantities do not occur in dense concentrations or their total quantity in the region 

 is lov;er. Mackintosh (1973) sug/rests that krill concentrations in Indian Ocean sector of 

 the Wcr-t Wind Drift are progressively /xa7,ed dovm by the vjhalcs as they migrate south. It 

 is not however clear whether the whale migration south is maintained because of food require- 

 ments. 



Theories of distribution control 



There are in the literature a wide variety of theories vihich attempt to explain the 

 reasons for the occurrence of the various life stages around the Antarctic continent. None 

 of these theories is totally proven. The more plausible will therefore be considered along 

 with the major evidence that is available in support amd against. 



Perhaps the simplest explanation is that throughout their life history krill remain more 

 or less in the same location (Marr 19'52). Mackintosh (1972) quotes an average surface flow 

 rate in the V/est Wind and Weddell Lrifts of lO era/sec. which is within the range of swimming 

 speeds determined by Semenov (1969) ^or adolescent and adult krill. Although! it is not known 

 for how long krill could maintain this speed Marr (1962, p. 155) quotes from some notes made 

 by E. R. Gunther who observed a swarm of krill swimming against an estimated /3 knot current 

 for several hours. It is also possible for krill to be maintained in more or less the same 

 locality by migrating between the warm deep water and surface water (they are known to occur 

 in significant numbers down to 4OO m (Shevtsov and Makarov I969)) althouf^ Fischer (1976) has 

 suggested that such a temperature discontinuity may represent a physical barrier. The eddying 

 effect around islands and submarine rises could also, as suggested by Makarov (1972) and 

 Khvatskiy (1972) hold krill in an area for a prolonged period. 



The remaining theories require that the krill are carried in a wide circulation pattern 

 and the most frequently considered area from this point of view has been the Weddell Sea. 

 Ruud (1932) basing his ideas on the circulation pattern of Meyer (1923) suggests that some 

 larvae and adults get carried back to the hif^i latitudes in the surface stream. Marr (I962) 

 whilst a.greeing that there is an undoubted circulation pattern of this typo believes that 

 the circulation involves an area far further east and that in addition in order to complete 

 the cycle the larvae would need to be carried south in the intermediate water during the 

 developmental ascent. The easterly spreading of early larvae in the surface waters is 

 clearly sliown in Marr's distribution maps (Fig. 64 onwards) which gives considerable weight 

 to the greater part of this circulation theory. 



Makarov (1972 ) using recent information from the Scotia Sea has taken the theory still 

 further and suggests that the main circulation pattern described above applies and the main 

 stock is maintained by spaivning in the vicinity of the South Orkney Islands. He suggests 

 that the larvae are carried around the Weddell Sea in a clock^irise direction to arrive back 

 at the South Orkneys a year later. Some of these krill are carried around for a second year 

 to spavm whilst the remainder drift out of this circulation pattern. Tliis theory requires 

 confirmation of the existence of the recurvature current (see Kumagori and Yanagawa 1958 and 

 also Fraser 1936, p I65) and also a better understanding of total circulation in the VJoddell 

 Drift (see Deacon 1976). 



In an extension of this cyclonic circulation theory Makarov (1972) has suggested that a 

 similar system to that in the V/eddell Zone could operate in the Ross Sea and Kcrguelen Gauss- 

 berg ridge areas. Treshnikov (l97l) describes six cyclonic g>'res produced by prevailing 

 cyclonic winds in coastal regions around t'ne continent. Thesn may represent areas of krill 

 concentration as s u gested by Beklemishev (I96O). 



