Differences between swarms of Antarctic krill and some implications for sampling krill populations

Antarctic krill, Euphausia superba Dana, were sampled from 28 discrete swarms in a small area south west of Elphant Island, South Shetlands, over a period of 14 d (24 February-9 March 1985). Four biological characteristics of the krill (length, sex, moult, gut fullness) were examined in a study of variation between swarms. Analysis of these characteristics indicated extensive differences between swarms and no single characteristic, or combination of characteristics, emerged as consistently accounting for the observed heterogeneity. There was no relationship between the degree of heterogeneity and the physical or temporal proximity of swarms. The variability between even close swarms strongly suggests that swarms are the basic unit of organization of krill populations. This conclusion has important implications for sampling programmes aimed at estimating population parameters. Thus, in the population studied here, we calculate that it was necessary to sample 23 swarms to estimate mean length and 10 swarms to establish a reliable estimate for the proportion of females in the population.