Temporal and spatial distribution of Antarctic krill (Euphausia superba) swarms in the Bransfield Strait, Antarctic in autumn 2020

Antarctic krill (Euphausia superba) is a key species in the Antarctic marine ecosystem with important potential for commercial exploitation, however, significant heterogeneity in type and distribution of krill swarms increases the difficulty of biomass estimation and also brings the challenges for at-sea operation and working plan to fishing company. Therefore, using acoustic data collected from fishing vessels, the spatial and temporal distribution characteristics, diel differences, and swarm structures of Antarctic krill swarms in the fishing grounds of the Bransfield Strait were analysed with the aim of improving fishing efficiency. The results indicated that, in the Bransfield Strait, krill swarms were primarily located in the southern shelf waters in autumn 2020, and that the number of krill swarms increased over time in autumn. Most krill swarms can be found in water layers between the surface and the depth of 150 meters, but some swarms also occur close to the bottom. The diel distribution of krill swarms varied, with a general tendency of reversed diel vertical migration that moves up to the surface in the daytime but sinks to the deeper layer at night. The layered type of swarm with a high density is dominant in the daytime, while scattered type of swarm with low density occupies a high proportion at night. The density within swarms is correlated positively with swarm length. In general, the daytime layered type of swarms within the Bransfield Strait are suitable for fishing vessels. Understanding krill swarm distribution is beneficial to understanding their formation characteristics of krill fishing ground as well as enhancing the productivity of fishing vessels.