Recruitment of Antarctic krill Euphausia superba in the South Georgia region: adult fecundity and the fate of larvae

The high concentration of adult Antarctic krill Euphausia superba Dana around South
Georgia, Antarctica, is a product of immigration and not local recruitment. We investigated whether
reproduction and early larval development are the cause of local recruitment failure. It was found
that the majority of adult females were reproductively active in summer and that there was a comparatively
high investment in the ovary, reaching up to 46% of the total wet weight of the krill. The corresponding
egg batches were amongst the largest ever reported for E. superba. A semi-empirical
model predicted that 11% of females completed just 1 spawning episode per year, 60% completed 2,
and 29% completed 3 or more. On average, a South Georgian krill released 12 343 eggs yr–1. The
eggs were unable to complete the descent–ascent developmental cycle on-shelf because the bathymetry
was too shallow but, off-shelf, they were predicted to sink to between 490 and 520 m and
return to the surface either as a metanauplius or 1st calyptopis stage with plenty of energy reserves
remaining. Feeding conditions were adequate for the development of later larval stages once these
reserves were exhausted. Although net surveys found calyptopis and early stage furcilia in the vicinity
of South Georgia, numbers were mostly lower than predicted. Overall, reproduction or early stage
development are successful in this region, leaving predation on larvae and advective export during
winter as the main potential causes of local recruitment failure.