Krill distribution in the western Atlantic sector of the Southern Ocean during 1983/84, 1984/85 and 1987/88 based on the results of Soviet mesoscale surveys conducted using an Isaacs-Kidd midwater trawl
In this paper we consider the results of three mesoscale surveys covering a wide area of krill distribution between the Antarctic Peninsula and South Georgia Island. Surveys including from 158 to 202 stations were carried out by Soviet research vessels in the summer-autumn periods of 1983/84 and 1987/88 and in the spring-summer of 1984/85 using a grid of stations according to the standard method for Isaacs-Kidd midwater trawls operating in the layer 0 to 100 m. Overall krill biomass distribution (g/1 000 m3) and its mean values were analysed by subarea (Subareas 48.1, 48.2 and 48.3) and for the whole study area. Survey results confirm current understanding of the general pattern of krill distribution and drift in the western part of the Atlantic sector of the Antarctic (e.g. Priddle et al., 1988; Makarov, 1996). The greatest degree of spatial-temporal stability in the krill distribution field with high biomass of aggregations was observed in the western part of the area stretching from the Antarctic Peninsula to the eastern part of the South Orkneys area. The greatest variability of krill distribution on the spatial-temporal scale of the survey, which is characterised by a change in biomass of more than two orders of magnitude in different seasons, was observed in the South Georgia region and the adjacent waters of the Scotia Sea (Subarea 48.3). The possibility of krill transportation into the South Georgia region is considered. It is shown that the absence of krill in the island region during the 1983/84 season was caused exclusively by unfavourable oceanological conditions (lack of water flow from southern areas). It is also demonstrated that the absence of krill in the eastern part of the Scotia Sea may cause it to be absent from the South Georgia region regardless of oceanological conditions (spring-summer period 1984/85). The possibility of recruitment of the krill stock to the South Georgia region from the central part of the Scotia Sea is considered. Analysis of variability of mean biomass by area and season indicates that patterns of krill abundance and biomass recently observed in Subarea 48.1 (Siegel et al., 1997, 1998) are not representative of the survey area as a whole. The total biomass index in the study area during all seasons remained at a constant level (76.5-101.7 g/1 000 m3).