Public KRILLBASE data support a southward shift of SW Atlantic krill since 1926-1939, but the reported 1400 km south-west shift depends on cells with one or two cruise-seasons

Atkinson et al. (2021) reported that the centre of gravity (CoG) of post-larval Antarctic krill in the SW Atlantic moved about 1400 km south-west from 1926-1939 to 1996-2016, mostly after 1976-1995. They gave no uncertainty interval. We applied their comparable-cell method to the public 2016 KRILLBASE-abundance release (standardised net-haul densities, 2 x 5 degree cells sampled in all three eras). The result agreed with the published value: 43 cells and 1398 km. We then resampled whole cruise-seasons, removed single cruise-seasons and cells, used robust cell summaries, and kept only well-sampled cells. With all cells, the southward component was resolved: 631 km (95% cruise-season bootstrap interval 383-783 km). The total distance was very uncertain (507-1729 km). In 23 of the 43 cells, one era has data from only one or two cruise-seasons. With the 20 cells that have at least 3 cruise-seasons in every era, the shift was 654 km (249-1250). The westward component fell from 22.4 to 8.1 degrees of longitude, and its interval included zero. The southward component stayed resolved (456 km, 201-738). One 1976 cruise-season is the only 1976-1995 source in eight cells. If we remove it, the westward shift after 1976-1995 decreases from 25.1 to 11.0 degrees. In well-sampled cells, the southward shift after 1976-1995 was not resolved (315 km, -8 to 586). A Tweedie GLM with cell fixed effects gave a 68% decrease in density after 1976-1995 (95% CI 37-84%). Its north-south difference had the sign of a contraction but was not resolved. Limitations: one method, public data that differ slightly from the authors' screened set, no larval data, and no propagation of the standardisation uncertainty. The data support a southward shift of several hundred kilometres, but not a resolved 1000-1400 km south-west jump after the 1990s.

Antarctic krill · Euphausia superba · KRILLBASE · range shift · centre of gravity · cluster bootstrap · Southern Ocean · Scotia Sea · sampling bias · reanalysis