We examined seasonal variation in CPUE (catch per volume of trawled water) and diurnal vertical migration of Antarctic krill (Euphausia superba) associated with the brightness categories of the day based upon the angles between the center of the sun and the celestial horizon on the Scotia Sea; South Shetland Islands (SS), South Orkney Islands (SO) and South Georgia (SG) areas, using Japanese fisheries data.
Average trawling depth usually showed a marked diurnal change during summer and winter in SS and SO, being deepest around at daytime and shallowest around at nighttime from summer to winter, but did not show such diurnal vertical migration during winter in SG, being deepest around at morning twilight and shallowest around at evening twilight. The range of trawling depth was narrower in the certain layers (usually upper layers except in winter in SG) during dusk and dawn, but was expanded during morning and afternoon in every season in every area. Diurnal changes in CPUE occurred, being largest around at daytime and smallest around at nighttime during autumn and winter in each area, but being smaller around at daytime during summer in SS and SO.
Daily average trawling depth tended to be shallower during summer and early autumn, and deepened gradually from autumn and attained the maximum depth in winter over the Scotia Sea. However, it shifted to shallower in early spring, when krill began to feed on phytoplankton. Vertical range of trawling depth was narrower during spring and early autumn, but greater during middle autumn and winter in each brightness category.