Widening the net: spatio-temporal variability in the krill population structure across the Scotia Sea

Abstract

Resolving the spatial variability in the population structure of Antarctic krill (Euphausia superba) requires a synoptic sample, as in the design of the CCAMLR 2000 Survey. However, this approach is not appropriate for assessing temporal variability. The size of krill in the diet of Antarctic fur seals (Arctocephalus gazella) has been shown to provide a good representation of the temporal changes in the krill population structure from within their foraging area. At Cape Shirreff, South Shetland Islands, krill in nets had modal size classes of 46–48 mm and 52–54 mm in length and appeared to reflect the presence of larger krill offshore and smaller krill inshore; krill in the diet of fur seals contained both modes, indicating that the foraging area of fur seals integrated the spatial variability in the krill population. At Signy Island, South Orkney Islands, krill in nets and fur seal diets had a modal size class of 52 mm when sampled simultaneously; however, krill in the diet of seals showed a decline in size later in the season with an overall mode of 48 mm. At Bird Island, South Georgia, there was little overlap between net samples, with a modal size class of 30–40 mm, and fur seal diets, with distinct modes of 44 and 54 mm; and there was also much greater spatial variability in the size of krill in these net samples than in those from the other two locations. Extending the comparison of krill size in the diet of seals, to include spatially congruent net samples collected immediately prior to the CCAMLR 2000 Survey, showed almost complete overlap and indicated an even greater spatial variability in the krill population structure at South Georgia. Interactions between the oceanographic transport and enhanced growth rates of krill at South Georgia may combine to produce a higher degree of spatial variability in the krill population compared to other locations and this may limit the use of differences in krill length as an indicator of their provenance. This study underlines the importance of using data from multiple sources when considering large-scale krill population dynamics; information that is crucial to the effective management of the commercial exploitation of krill.