Description: The South Georgia region supports a large biomass of krill that is subject to high interannual variability. The apparent lack of a locally self-maintaining krill population at South Georgia means that understanding the mechanism underlying these observed population characteristics is essential to successful ecosystem-based management of krill fishery in the region. Krill acoustic-density data from surveys conducted in the early, middle and late period of the summers of 2001 to 2005, together with krill population size structure over the same period from predator diet data, were used with a krill population dynamics model to evaluate potential mechanisms behind the observed changes in krill biomass. Krill abundance was highest during the middle of the summer in 3 years and in the late period in 2 years; in the latter there was evidence that krill recruitment was delayed by several months. A model scenario that included empirically derived estimates of both the magnitude and timing of recruitment in each year showed the greatest correlation with the acoustic series. The results are consistent with a krill population with allochthonous recruitment entering a retained adult population; i.e. oceanic transport of adult krill does not appear to be the major factor determining the dynamics of the adult population. The results highlight the importance of the timing of recruitment, especially where this could introduce a mismatch between the peak of krill abundance and the peak demand from predators, which may exacerbate the effects of changes in krill populations arising from commercial harvesting and/or climate change.

Description: Data derived from net catches for zooplankton and micronekton during the COMICS cruise DY086 in November to December, 2017. Raw catch counts and biomass measurements have been used alongside published values to provide biomass, respiration and ingestion data between 0 and 500 metres depth (Belcher et al. 2022, Cook et al. 2023, Stowasser et al. 2020). Data values are from multiple net deployments and the number of deployments for each value are provided in the dataset. Bongo, Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) and Rectangular Midwater Trawl (RMT) nets collected small (100 μm mesh; day only), medium (330 μm mesh; day and night) and large (4000 μm mesh; day and night) samples, respectively.