Male krill grow fast and die young

The size-differentiated sex ratio (proportion of males, PM) of Antarctic krill Euphausia superba was examined with an extensive dataset derived from scientific surveys in the Indian Ocean sector and the southwest Atlantic sector, and from the krill fishery in the Southern Ocean. The percentage of males in size classes of adult krill was generally high in krill of 30 to 35 mm total length, always low in 38 to 42 mm krill, sometimes showed higher values in 45 to 50 mm krill, but always decreased in the largest krill (>50 mm). This pattern was reproduced by a model simulation that assumed faster growth and a shorter lifespan for males when compared to females. These results suggest that the numbers of males should decline with time unless new recruits enter the population. Indeed, interannual variations in PM from the field (net collected data and penguin diet data) showed a decline in the proportion of males when several years of low recruitment followed a recruitment pulse. These results lead us to conclude that male krill grow faster and have a shorter lifespan than females in the natural environment.