Characteristics of interannual variation in aggregation and diurnal vertical migration of Antarctic krill at South Georgia during winter

We elucidated characteristics of interannual variability in density index, recruitment index and diurnal vertical distribution for Antarctic krill in krill fishing grounds (hotspots) at South Georgia during winter based on Japanese krill fishery data during the period 1990-2012. High density and predictable fishing grounds occurred in the restricted areas throughout the study period. Main results are as follows:

1. Median CPUEs did not show particularly low values in winter even when krill abundance and predator performance were poor in the previous summer.
2. Krill recruitment index at South Georgia suggested the strong influence in recruitment from the Antarctic Peninsula in the 1990s, whereas the influence was speculated to be from the Weddell sea during 2000-2006.
3. Median winter trawling depth (a proxy for krill vertical migration) for each daytime and night-time was significantly positively correlated with average krill body length in winter. This could be the optimal behavior of krill to balance food intake against predation risk by Antarctic fur seals, the most abundance krill-eating predator there.