Feeding of krill around South Georgia. I: A model of feeding activity in relation to depth and time of day

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Abstract
Antarctic krill Euphausia superba were sampled from 4 depth horizons from a small area off South Georgia every 4 h for 6 d [days]. Stomach, hepatopancreas and gut fullness of > 9000 specimens were assessed visually. Plant pigment levels were measured in the stomachs and hepatopancreata of a subsample of the krill: the visual classification distinguished significantly different levels of plant pigment in both organs. Multivariate analysis of the number of krill in each depth, time and feeding category was used to generate a simple model of the relationships between stomach, hepatopancreas and gut fullness and to investigate how they varied with both depth and time of day. Both field techniques and analytic procedures represent an improved approach to the problem of understanding the simultaneous effects of environmental and physiological pocesss on krill feeding. Neither stomach, hepatopancreas nor gut fullness varied with depth, and only stomach fullness showed any significant variation with time of day. This is in contrast to previous studies which were based on smaller samples and less rigorous analyses. Although depth distribution of krill is known to vary with time of day, our results suggest that the link between feeding, swarming and vertical migration may be more tenuous than has previously been supposed.