Aggregation characteristics of Antarctic krill (Euphausia superba) in the South Orkney Islands in summer 2018

Aggregation is a typical behavior of Antarctic krill (Euphausia superba). The distribution and type of aggregation is also the focus of E. superba behavior study. Based on acoustic data collected by E. superba resource survey, the aggregation of E. superba around the South Orkney Islands from January to February 2018 was investigated using the Simrad EK60 (38 and 120 kHz) echosounder. The results indicated that a total of 3 224 E. superba aggregations were detected, and the depth, length, thickness, area and density of E. superba aggregation were (96.00±64.33) m, (218.52±455.66) m, (11.19±13.98) m, (1 894.40±9 345.72) m², and (114.11±159.60) ind/m³, respectively. In different periods of a day, significant differences were found between depth, length, thickness and area of E. superba aggregations, but density of E. superba aggregation had no significant difference. At different depths of E. superba aggregation, there were significant differences between depth, length, area of E. superba aggregation, but no significant differences were found between thickness and area of E. superba aggregation, respectively. Three types of E. superba aggregation were classified clearly. Type I had the highest density (325.90±221.30) ind/m³, smallest area (379.64±433.73) m² and deepest depth (158.06±67.54) m; type II had the greatest length (1 089.60±1 189.56) m and the area (15 601.25±30 243.33) m² was much larger than other two types; type Ⅲ had the smallest depth (78.91±52.88) m and smallest density (48.87±50.33) ind/m³, but it had the greatest number of E. superba in the aggregation.