In 2023, SC-CAMLR endorsed the krill assessment and proposed revisions to the precautionary catch limits for Divisions 58.4.1 and 58.4.2-East (para. 2.98). These assessments were based on recent biomass estimates from a 2019 survey conducted by Japan in Division 58.4.1 and a 2021 survey conducted by Australia in Division 58.4.2-East, and stock assessments using the Grym (SC-CAMLR-2023 para. 2.91). These biomass estimates had been endorsed by WG-ASAM-2023 and published in peer-reviewed literature.
However, the Scientific Committee noted that sea ice coverage had prevented the surveys in Division 58.4.2-East from surveying shelf-break areas where high krill densities had been recorded in the BROKE-W survey in 2006 (Jarvis et al., 2010), and recognised that variable sea ice coverage is an important challenge for acoustic biomass surveys in polar regions (para. 2.93).
Here, we investigate how sea ice may have influenced the krill biomass estimates in the BROKE-W (2006) and TEMPO (2021) surveys. We carried out this work by comparing krill biomass density data between 2006 and 2021 surveys, and inside and outside of the median sea ice boundary. This approach allowed us to control for inter-survey krill biomass density changes. We also compare krill biomass densities to historic krill fishing areas.
In Division 58.4.2-East there was a statistical difference (KS-test, D = 0.54, p < 2e-16) in mean areal biomass density observations between the 2006 survey (9.5 gm-2) and TEMPO surveys (6.2 gm-2). While we found a statistical difference between the distribution of krill biomass density inside and outside the median sea ice boundary in 2006, there was no difference in 2021, suggesting a density dependence in krill - sea ice interaction. We also found no intra-survey difference in krill densities within and outside historic krill fishing areas.
We suggest the differences in krill biomass estimates between 2006 and 2021 were not caused by sampling differences, but simply because there were more krill in the eastern sector of Division 58.4.2 (55°E–80°E) in 2006 than 2021. We recommend metrics of sea ice coverage be developed for krill surveys and reported for each stratum.