F-6 



is directly associated vrith feeding (although no indication is given of how widely applicable 

 these observBtions are). VJhon the krill are feeding actively they rise to the surface and 

 are dispersed. They come together as swarms when they are replete, stop feeding and then 

 descend. Pavlov suggests that the onset of swann formation and the duration of the swarm is 

 directly related to food availability. Nakaraura (1974) described a similar type of vertical 

 migration although he does not link this to feeding pattern directly. Assuming that availa- 

 bility of food is a major factor in controlling sv;arm formation it would be expected that 

 swarms would occur more frequently during the summer when primary production is at its maximum 

 than during the winter. 



Feeding studies by Pavlov (1971 » 1974) have shown that detritus plays an important part 

 in the diet of E. superba which would allow than to feed all year round in deeper water. 

 Although Pavlov~~(l969) describes the guts of krill cau^t at 300 m as being empty and there- 

 fore not feeding, the fact that their main lipid store is not in the form of waxes (Bottino 

 1974) suggests they do not go v;ithouL food for long periods (see Sargeant and Lee 1975)* Also 

 Mauchline and Fischer (19^9) suggest by comparison with other euphausiids that Ej_ auporba 

 althou^ feeding on phytoplankton by preference will also feed on detritus and animal material. 



In addition to this information re.-^rding migration in the surface water Shevtsov and 

 Makarov (1969) also describe significant concentrations at over 200 m depth although it is 

 not clear whether these wore detected by echosounders or solely by fishing at predetermined 

 depths. Confirmation of this observation may be inferred from Permitin's (1970) observation 

 that krill forms an important componont in the diet of several demersal fish species in the 

 shelf area (see also section on Fish in this paper). More recently significant deep water 

 concentrations of krill have been identified in shelf areas of the Scotia Arc where there was 

 no marked temperature discontiniiity (Fischer 1976), 



The fact that those recent observations conflict with those of Marr can be attributed 

 to either a changing behaviour pattern by the krill or else due to the different techniques 

 involved in estimation. Ifarr, in considering reasons for the disparity in the daytime and 

 ni^t time catches finds some evidence that the difference is not due to a diurnal vertical 

 migration beyond the normal range of the nets, but that it is more likely to be due to avoid- 

 ance or the fact that subsurface swarms represent a poor target for obliquely hauled nets. 

 The difference may also be due to the methods employed. There is clearly a need for stand- 

 ardised sampling in conjunction with acoustic detection devices in order to quantify the 

 krill concentrations in both the surface water and also in the intermediate water. 



Horizontal distribution of later stages 



Prom the time tiiat they arrive in the surface waters until they spawn two years or more 

 later, a significant proportion of the krill must move, or be carried, to an area from which 

 spawning will result in maintenamce of the stock. Over most of the Southern Ocean the surface 

 water has a mainly easterly and to a lesser extent northerly set. The result of this is that 

 krill which occur in that area vri.ll be naturally carried away northwards. 



Within the general area of the Ekst Wind and Weddell Drifts, which contain the major 

 concentration, the distribution is very patchy. In addition to this the habit of E^ superba 

 in forming swarms has made detailed distributional study liable to considerable error. Recent 

 fishing expeditions have, however, identified concentrations regularly in clearly defined 

 areas. These are in the area of mixing of the circumpolar and Weddell currents (Cershanovich 

 and Lyubimova 1971) particularly to the north of the South Orkneys (Burukovskiiand Yaragov 

 1967), on the loe side (in terms of dominant wind and current) of islands and submarine ridges 

 and also in areas of descending currents bordered by ascending currents (Elizarov 1971) and 

 off South Georgia (e.g., Elizarov 1971, Makarov, Naumov and Shevtsov 1970, Bogdanov and 

 Solyanik I970). In the Ea,ct V/ind Drift zone concentrations have been found around lOO^E 

 (Hasu 1974) and from I30OE to 170°W ^Ozawa et al. I968). All of these observations were made 

 during the summer months and also in areas of svjarms. The only major concentrations that 

 occur outside the East VJind and Weddell Drift zones are reported from the north of the Ross 



