Distribution, biomass and characteristics of the Euphausia superba fishery around South Georgia (Subarea 48.3)
Krill distribution in the South Georgia area and many aspects of its variability have been extensively described in scientific literature. A summary of recent findings is discussed in this paper. There has been less focus on data concerning krill biomass assessments. The existing uncertainty in krill biomass assessment can be largely attributed to high variability and the wide range of biomass values obtained which in turn leads to uncertainty in interpreting krill biomass for the area. It was therefore only possible to begin to explain the reasons for the variability in the scale of krill swarms and to attempt to determine more reliable values of its biomass after a number of census surveys had been conducted. Results of the nine most representative surveys conducted by Soviet scientists in the area from 1974 to 1988 are summarized in this paper. The most important survey data and biomass assessments have been tabulated and illustrated by various survey maps. It was found that the distribution of krill swarms corresponds largely with areas where currents turn into eddies. Among these areas, aggregations of krill were observed most often to the east of the island and around 37°W. When the water circulation system is unfavourable, which means the absence of eddy formations, krill do not usually form aggregations and its density is low. When water dynamics are favourable for krill aggregations its biomass around South Georgia is approximately 400-800 thousand tonnes. Unfavourable conditions correspond to a biomass of an order of magnitude less. Short-term fluctuations of krill biomass in the South Georgia area are extremely significant. Biomass may differ by as much as eight times in a month. It also usually decreases in spring, regardless of the type of water circulation. Annual differences in krill biomass are undoubtedly associated with the volume of krill brought here by waters of the Scotia Sea Secondary Front. More extensive studies incorporating trawl and acoustic surveys of different temporal and spatial scales are recommended in order to make reliable assessments of krill biomass.