Variation in reproductive status of Antarctic krill swarms; evidence for a size-related sorting mechanism?

Length, sex and maturity stage of Antarctic krill Euphausia superba Dana occurring in discrete swarms were measured. Over a 14 d period 38 swarms were sampled in a small area southwest of Elephant Island, South Shetland Islands. Analysis indicated that the composition of the swarms in terms of the sex ratio and maturity stage was very variable. Swarms ranged from equal mixtures of male and female krill to nearly all one sex; furthermore krill of similar maturity frequently occurred together. The mean length of each maturity stage also varied between swarms; in some swarms the size of each maturity stage was bigger than average while in others each stage was smaller than average. The range of krill sizes in a swarm was often restricted, indicative of some type of size sorting. However size sorting alone could not account for the observed variation in sex ratio and maturity stage and it is likely that in addition, differential growth and active behavioural responses contribute to the observed variation.