Age structure and cyclical annual size change in the Antarctic krill, Euphausia superba dana

Summary

The traditional method of studying age structure of krill populations through length-frequenc histograms is negated by laboratory and field observations that krill revert to an immature form and regress in size after spawning-indeed, this is part of their over-wintering strategy. Study of the age pigment lipofuscin reveals that, in the Prydz Bay population, five age classes of adult krill were present. It is suggested that these classes correspond to year cohorts. The size as measured by the length from the rostrum to the rear of the cephalothorax, remained relatively constant. However, when length data for a complete year of samples from the South Atlantic was re-analysed, evidence was found in support of the size regression hypothesis.