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.

We provide estimates of krill biomass density in three survey strata defined by the U.S. AMLR Program: Elephant Island (EI), the West Shelf (WA), and the Bransfield Strait (or South Area (SA)). We include density estimates from U.S. AMLR research vessel surveys conducted from 1996 to 2011 and from gliders deployed in 2018-19. We also include density estimates from fishery-independent surveys conducted by fishing vessels from 2013 to 2019. The combined set of estimates demonstrates that krill biomass density varies over time, including within a season, and among survey strata (Fig. 1). We do not currently know how much of this variation is attributable to measurement error. While SC-CAMLR and its working groups have devoted considerable time to ensuring that techniques for collecting and analyzing acoustic data are comparable across platforms, differences in the nets used during these surveys (e.g., mesh sizes) likely add uncertainty to the time series of biomass estimates. We show that using length-frequency distributions (LFDs) from biased samplers (e.g., krill in penguin diets) can subsequently bias estimates of krill biomass up when compared to estimates based on LFDs from the IKMT nets historically used by the U.S. AMLR Program (Fig. 5). The LFDs of krill collected from commercial trawls are, on average, more similar to LFDs from penguin diets than from IKMTs, likely because commercial trawls tend to have larger mesh sizes than scientific trawls. Therefore, the time series of biomass estimates from fishing vessels (starting in 2013) and gliders (2018/19) that rely on commercial trawls or predators should be considered the maximum potential biomass density in the region and used to document temporal changes in biomass time series, rather than as absolute estimates. Going forward, we propose that acoustic surveys conducted by fishing vessels use a standard gear, or at the least a standard cod-end liner of <5 mm, when conducting fishery-independent surveys. The temporal and spatial variability observed in biomass density estimates raises two concerns related to using individual biomass estimates for computing catch limits. First, if surveys are conducted infrequently, biomass estimates used to calculate catch limits might be either too high, increasing risks to krill-dependent predators, or too low, forgoing valuable catch. Second, if surveys are conducted frequently, catch limits will themselves be highly variable and survey costs will increase. At present, we do not know the optimal survey frequency. We propose WG-ASAM recommend that an average density, computed over time and multiple survey strata, be used to determine catch limits.