F-9 



In his review of distributional control one of the mechanisms that Marr (1962) postulates 

 is the possibility that a population of krill in the Bellingshausen Sea continually releases 

 larvae and adolescents that seed the East Wind Drift. During the course of successive gener- 

 ations this process is continued around the continent (see section on zooplankton in this 

 paper). Such a theory althanf^ possible with water circulation pattern may not seem very 

 likely in view of the known higher density of krill at the end of the cycle (Weddell Sea). 

 However, this could be explained by the Weddell Zone itself .being a self maintaining aysteni 

 as described above. I-Iarr also mentions the possibility that larvae could be carried from 

 the Weddell to the Bellingliausen Sea via the Branafield Strait. Althougii there is no evi- 

 dence for a water movement the full distance, there is evidence that water does enter the 

 Bransfield Strait from the Weddell Sea (Clowes 1934 )• It has also been suggested (Everson 

 1976) that the Bransfield Strait itself might contain a self maintaining system. 



The only area containing large concentrations of krill that has not so far been consid- 

 ered in this review is South Georgia. This has been considered as an area in which there is 

 no successful spawning (Marr 1962) and as a result it is suggested that the krill population 

 there has its origins in other regions. Because VJeddell Sea water has been considered to 

 flow along the northeast coast of the island (Deacon 193?) and also because the Southern 

 Scotia Sea has been shown to contain large concentrations of krill the Heddcll Sea is often 

 considered as being the origin of most of the South Georgia Krill (Marr I962). Recent 

 research has suggested that the frontal zone between IVeddell Drift and Wf^st Wind Drift now 

 passes well to the south of South Georgia (Bogdanov _et al. 1969» see also section on Hydro- 

 graphy in this paper). Under normal conditions therefore it would seem unlikely that Weddell 

 Drift krill could be carried to South Georgia. Bogdanov and Solyanik (1970) in an attempt 

 to explain how VJeddell Drift krill could get to South Georgia show a good correlation bet- 

 ween whale catches and the mean annual air temperature at South Georgia and Laurie Islands 

 (South Orkneys). They suggest that Weddell Drift surface water is carried to South Georgia 

 as a result of wind action on the surface. Unfortunately the cold years (which should have 

 produced largest krill concentrations and thus vjhales)were linked to low whale catches. 

 Maslennikov (1972) considers that krill concentrations in the vicinity of South Georgia are 

 present in the boundary region between a north westerly flowing coastal cvirrent and a south 

 easterly flowing current further offshore. In years when this circulation pattern is ill 

 defined, commercial krill catches have been poor (Maslennikov jet al. 1971 ). The impression 

 is given in their paper that the water in this area is from the Antarctic circumpolar cui^ 

 rent and that Weddell surface water is mixed in the manner described by Bogdanov and Solyanik 

 (1970). It has also been suggested that the South Georgia Krill have their origins in the 

 Bransfield Strait or Bellingshausen Sea area (discussed by Everson 1976) although there is 

 only circumstantial evidence to support this idea. 



It is an interesting point regarding all of the proposed theories of ' distributional 

 control that they all rely on fairly precise limitation in timing of development and -flow 

 of water masses. This is in complete contrast to the mechanisms of distributional control 

 proposed for other members of the macroplankton which in general rely on the southerly move- 

 ment of the warm deep water and northerly movement of the surface water. (Mackintosh 1937» 

 Foxton 1956, Voronina I968). For a mechanism such as this to be applied to krill it is 

 clearly necessary to prove the existence of a seasonal vertical migration. Recent evidence 

 has shown the existence of significant concentrations of krill dovm to 400m althou{^ as yet 

 there is no year round coverage of observations to suggest that a seasonal pattern of verti- 

 cal migration exists. 



Mention has already been made of the possibility that krill could feed in deeper water 

 on detritus. The fact that the main lipid store is not waxes (Bottino 1974), indicating 

 continuous feeding (Sargeant I975), would require the krill to be in deeper water during the 

 winter. (With almost zero primary production in the surface water in winter the only food 

 at that time is likely to be detritus at greater depth). 



Clearly there is a need for more research to identify which of these mechanisms are of 

 major importance in maintaining the enormous standing stocks of krill. 



