Preliminary biomass estimation of Antarctic krill based on the swarm-based method for CCAMLR Division 58.4.1 in 2018/19 using data obtained by Japanese survey vessel Kaiyo-maru

Biomass (B₀) of Antarctic krill for CCAMLR Division 58.4.1 in 2018/19 was preliminarily estimated as 4.349 million ton (CV=54.8 %) based on the swam based method using data obtained by Kaiyo-maru. The point estimate was comparable with the estimate in 1996 by BROKE (4.83 million ton with CV=17 %). However, they can not be compared directly because (1) biomass estimation methods were different, (2) timing of the surveys were different (Kaiyo-maru survey commenced about 40 days earlier than BROKE) and (3) areal coverages were different primary because of difference of positions of sea ice edge especially in the western part of the Division. The CV of biomass estimate of Kaiyo-maru survey was high primary because the spatial distribution of biomass densities was highly heterogeneous. The swarm based method potentially includes non-krill echoes but also excluding aggregations of krill which are smaller than size of swarm set in the Echoview template used in this analysis. Comparison with B₀ estimated by the dB difference method is necessary before producing finial B0 in Division 58.4.1 in 2018/19.