Comparison between the CCAMLR-2000 and KY 1988 surveys on environmental variability of krill in the Scotia Sea, Antarctica

We assessed the environmental variability of Antarctic krill (Euphausia superba) distribution with comparison between the CCAMLR 2000 Survey and similar scale datasets partially by the Japanese R/V Kaiyo Maru Survey in the 1987/88 austral summer season in the Scotia Sea. There were distinct differences between the 2000 and 1987/88 Surveys with regard to sea ice extent, oceanographic structure and krill distribution. The sea ice cover in 1987/88 extended northward widely during the last winter season such that sea ice remained around the South Orkney Islands until December 1987. In contrast, the sea ice cover in 1999/2000 reduced southward such that no sea ice remained around the South Orkney Islands in December 1999. The Antarctic Surface Water mass, consisting of Winter Water and Summer Surface Water, in 1987/88 extended northward and covered a large area in the Scotia Sea. In contrast, the Antarctic Surface Water in 2000 reduced southward. Geographical distribution of krill, which approximates the area of the Antarctic Surface Water, in 1987/88 extended northward with high density. In contrast, the distribution of krill in 2000 reduced southward with low density. To generally understand the above relationships between oceanographic structure and krill distribution, we introduced integrated water temperature from the surface to 200m (Q200) as an environmental index indicating the structure of the upper ocean, that is referred to as the Environmental Index (EI Q200 ). The isoline of EI Q200 =0.0 ? was located near 60S northward off the South Shetland Islands in 1987/88. In contrast, the isoline of 0.0 ? in 2000 was located in the Bransfield Strait and Weddell Sea southward off the South Shetland Islands. The Antarctic Surface Water in 1987/88 clearly developed northward compared with 2000 reduced southward. The geographical distribution of krill ranged over the area under the isolines of EI Q200 =1.0 ? in the western waters and 2.0 ? in the eastern waters of the Scotia Sea. Krill density became higher with the colder isolines of EI Q200 =0.0 ? , especially south of its steep gradient, namely, the Southern Boundary of the Antarctic Circumpolar Current. It suggested that the geographical distribution of three krill size clusters in the 2000 Survey (Siegel et al., 2002) corresponded with the distribution pattern of EI Q 200 on the whole.