Time series of annual biomass estimates of Antarctic krill around the South Shetland Islands during the period of 2013 to 2019 is provided to aid the discussion on biomass estimation in Subarea 48.1. The estimates were based on data described in the working document ASAM-2019-04r1, which presented the time series of krill density estimates resulting from the annual acoustic surveys conducted by the Chinese krill fishing vessel FU RONG HAI from 2013 to 2019. A wrong data column (biomass density instead of acoustic density) for the year 2019 in Table 3 of ASAM-2019-04r1 was also corrected for future reference.

With the recent development of techniques and protocols for the acquisition and processing of acoustic data collected aboard krill fishing vessels, a large volume of acoustic data accumulated by fishing vessels become usable. The acoustic data collected in a main fishing ground (or hotspot) in the Bransfield strait by one of the Chinese krill fishing vessel, namely F/V Fu Rong Hai, is used to demonstrate the seasonal dynamics of the krill stock in the hotspot. It shows that the monthly differences in krill biomass is so large that it cannot be explained solely by individual growth, and krill flux must have played an important role that need to be addressed in the future.

Length frequency distribution (or length PDF) of Antarctic krill is an important input for converting the acoustic density to biomass estimate. For the 2019 Area 48 multi-national krill survey, various sources of length data were combined to provide the krill PDF for biomass estimation, and its potential effect was also discussed. To address this issue further, the potential effect of krill length PDF on krill biomass estimate was presented for a series of different mean and standard deviation (SD) of the length distribution assuming a normal PDF distribution. The actual krill length PDF sampled by two different gears (IKMT vs RMT8) during the Chinese R/V Xuelong2 survey in January 2021 in Area 58 was also compared to provide some insight on the matter.