Abstract: We used biological data on Antarctic krill (Euphausia superba Dana), collected by fishery observers of the Antarctic krill fishery, to investigate the spatial-temporal variation in age structure. The study area was divided into small-scale units with a spatial resolution of 30 minutes (latitude) by 1 degree (longitude). The results showed that in 2009-10 the standard length (SL) of Antarctic krill ranged from 33.2 to 69.0 mm, and the mean SL and dominant SL range were 51.8 mm and 52–60 mm (52.3%), respectively. A sub-modal range of 44-48 mm was also identified. In 2010-11, the SL ranged from 22.9 to 59.3 mm, and the mean SL and dominant SL range were 44.5 mm and 42-50 mm (56.8%), respectively, and in 2011-12, the SL ranged from 18.5 to 59.4 mm, and the mean SL and dominant SL range were 41.7 mm and 36-46 mm (67.3%), respectively. Among the three years, there was a significant variation in SL (p < 0.001) and no similarity in length frequency distribution (PSI = 41.25–73.42). Clustering analysis showed that SL classes could be divided into three groups among 10 days in a month during 2009-10 to 2011-12. The SL classes could be classified into groups based on the first 10 days in February 2011 and the middle 10 days in March 2012, on the last 10 days in January to the middle 10 days in February 2010 and the middle 10 days in February 2011. The other SL classes could be classified into another group. Age 1+ and 2+ Antarctic krill occurred in waters close to the South Orkney Islands, while age 4+ Antarctic krill were found in waters further from the Islands.

Key words: Antarctic krill, South Orkney Islands, fishery-targeted population, age structure, temporal-spatial variation