Multi-scale analysis on spatial heterogeneity for abundance of Antarctica krill (Euphausia superba) in the South Orkney Islands in summer 2018

Antarctic krill (Euphausia superba) is a key species in the pelagic ecosystems of the Southern Ocean, and is also an important fishing target in China’s distant-water fisheries. A full understanding of its spatial distribution characteristics can provide theoretical support for the sustainable exploitation and conservation of E. superba resources. Based on acoustic data collected by E. superba resource survey in the South Orkney Islands in summer 2018, the survey area was divided into 7 different spatial scales and the geo-statistical method was used to research the spatial heterogeneity of abundance of E. superba at different spatial scales. The result showed that: ① as the spatial scale increased, the nugget and nugget-sill ration gradually decreased, the partial sill and sill decreased first, then increased and went down again, and 40′×40′ spatial scale had the biggest values; ② at different spatial scales, the semi-variogram was fitted by different models, spherical model had the best performance at 20′×20′ and 50′×50′ spatial scales, exponential model had the best performance at 10′×10′ spatial scale, Gaussian model had the best performance at 30′×30′, 40′×40′ and 60′×60′ spatial scales. And the fitted models showed the spatial auto-correlation was relatively strong; ③ the spatial structure of E. superba showed that, based on anisotropy, 50′×50′ spatial scale had the minimum difference of fractal dimension values, 60′×60′ spatial scale had the biggest difference of fractal dimension values. Based on isotropy, 60′×60′ spatial scale had the minimum fractal dimension value, 50′×50′ spatial scale had the biggest fractal dimension value. It is concluded that the abundance of E. superba in the South Orkney Islands had distinct spatial heterogeneity, a number of factors contributed to this phenomenon, including external factors such as ocean flow fields, the change of nutrient and thermocline caused by the exchange of water masses and internal factors such as behavior character of E. superba.