CCAMLR

Metadata for krill biomass time series obtained by Chinese krill fishing vessel in Subarea 48.1 is submitted with accompanying description of the acoustic survey and data analysis. Distribution of Antarctic krill for the 2013/14 to 2015/16 and the 2017/18 to 2018/19 fishing seasons were mapped by the Chinese F/V Fu Rong Hai to understand the interannual variations of Antarctic krill around the South Shetland Islands, one of the major fishing ground for Antarctic krill in Area 48. Acoustic data were collected with ship-mounted Simrad EK60 scientific echosounder system. A swarm-based Echoview EV template was used for echo-integration, and the areal krill density with coefficient of variance (CV) was calculated following Jolly and Hampton (1990). The results suggest that krill distribution displays an obvious interannual variations. Mean krill density of the whole survey area reveals an increasing trend during the last 6 years, increased from 20.22 g m^-2 in December 2013 to 70.13 g m^-2 in February 2019.

The effect of differences in the methodology used for the CCAMLR 2000 krill acoustic survey and the 2019 synoptic survey were considered. These were 1) the effect of acoustic surveying at night, 2) differing trawl selectivity between the vessels that participated in the survey, and 3) the effect of krill discrimination method. Surveying during the day only produced a 6% increase in the standing stock estimate and the trawl selectivity between vessels had minimal effect on the krill length distributions. The choice of krill discrimination method had a marked effect on the per-stratum biomass estimates. Areas with most of the krill backscatter in well-defined schools were least sensitive to the choice of discrimination method.

As a contribution to the joint effort on the large-scale international synoptic krill survey of Area 48 (2019 Area 48 survey), the Chinese krill fishing vessel F/V Fu Rong Hai conducted the acoustic survey around the South Shetland Islands and the Elephant Islands in the Subarea 48.1 from 5^th to 10^th February 2019. Implementation and preliminary results on the estimates of krill biomass density were presented to the WG-EMM 2019 meeting. After the meeting, the acoustic data were processed further using the swarm-based method to identify krill backscatter as recommended by CCAMLR. Updated analysis shown that mean krill biomass density was 80.7 g m^-2 with a CV of 13.6% in the survey area. However, the krill biomass estimates might be updated pending on the outcome of krill length clustering analysis and discussion on the methodology during this SG-ASAM meeting.

To address the potential effect of DVM behavior on krill biomass estimates, diel vertical distribution of krill swarms around the South Shetland Islands in Subarea 48.1 were analyzed based on data collected by the Chinese fishing vessel participating in the international 2019 Area 48 survey. Results show that more krill swarms were found in shallower depth at night than during daytime, while 16.5 % compared to 5.7 % of the corresponding total NASC value (m² n.mile^-2) integrated for layers bellow 15 m were found above 15 m at night and during daytime, respectively. The results implies that, when 15m is used for upper integration layer limit, both daytime survey and nighttime survey are likely underestimate the krill density due to the shallow distribution pattern of the animal, which may affect the nighttime estimate more than the daytime estimate.