CCAMLR

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.

Acoustic data on Antarctic krill in Subarea 48.1 are available in several spatial scales that are relevant to the work of WG-ASAM-2021. The various spatial scales (or areas with different size and shape) available from CCAMLR that could be considered by WG-ASAM-2021 to provide biomass estimates of Antarctic krill in Subarea 48.1, together with the US AMLR survey strata and the Chinese fishing vessel survey transects, are presented in a single chart for easy reference. The distribution of krill catches in Subarea 48.1 is also shown to aid consideration in choosing a suitable/practical spatial scale to be used for providing an interim management advice for the Antarctic krill fishery in 2021.

In February and March 2021 an acoustic-trawl survey was carried out to estimate the biomass of Antarctic krill (Euphausia superba) in the eastern sector of the CCAMLR Division 58.4.2 (area = 775,732 km².) The survey was run from the Australian Research Vessel RV Investigator operating a calibrated EK80 scientific echosounder and an RMT-8 + 1 net. Krill were identified using the swarms-based technique and estimated mean areal biomass density was 6.4 gm^-2. Differences were found between day and night mean areal biomass densities (t-test, p = 1.2e-07) and the statistical distribution of biomass densities (KS-test, p <2e-16). Day-night biomass density differences were suggestive of krill migrating to shallow water at night and so being missed by hull mounted echosounders. We therefore used the day time only observations which resulted in a 35% reduction in total transect length from 1,327 nautical miles reduced to 861 nautical miles. Day time mean areal biomass density was 8.3 gm^-2 with a total biomass 6.477 million tonnes, with CV = 28.9%.

The authors have analyzed the krill size composition in the catches taken by fishing vessels and the RV Atlantida in the local fishing ground within several day to exclude  any influence of krill drift on results obtained. The fishing ground was located in Subarea 48.2 (SSMU SOW). This comparative analysis  clearly demonstrate both the differences in the krill length composition from catches taken by research and commercial trawls, as well as the differences in the krill length composition from catches taken by commercial trawls with different constructions and when using traditional and continuous fishing technology. The most vulnerable to the gear construction and fishing technology is the keeping of recruitment group and adult krill in the catches.