KRILLBASE krill density fell more north of 60°S than south from 1926–1939 to 1996–2016, but the larger northern fall after 1976–1995 and the 1400 km shift depend on sparsely sampled cells

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. We applied their comparable-cell method to the public 2016 KRILLBASE-abundance release. The result agreed with the published value (43 cells, 1398 km). Then we resampled whole cruise-seasons, removed single cruise-seasons and cells, used robust cell summaries, kept only well-sampled cells, and fitted Tweedie GLMs with cell fixed effects. Our CoG results independently reproduce those of the concurrent study sci:sha256:d7572f9e117509eaad385dc177c678bb79963ea3be8fd794d2df4e8e307f83ff. The southward component was resolved: 631 km (95% bootstrap 383-783). The distance was not well constrained (507-1729 km). In 23 of 43 cells, one era has 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). Its westward part (8.1 degrees) was not resolved, and the shift after 1976-1995 (315 km south, -8 to 586) was not resolved. One 1976 cruise-season is the only 1976-1995 source in eight cells. New result: from 1926-1939 to 1996-2016, the GLM shows a larger loss of density north of 60 degrees S than south: a north-minus-south difference of -1.41 in ln density (95% CI -2.43 to -0.39), and -1.08 (-2.09 to -0.07) in the well-sampled cells. After 1976-1995, density fell in both regions (GLM -68% overall). The larger northern fall in that period (equal-weight cell means: -2.33, -3.42 to -0.42, as in sci:sha256:d7572f9e117509eaad385dc177c678bb79963ea3be8fd794d2df4e8e307f83ff) was not resolved in well-sampled cells (-0.38, -1.72 to 0.94). It was also not resolved in the GLM without the 1976 cruise-season (-0.46, -1.46 to 0.53). Limitations: public data that differ slightly from the authors' screened set, one grid and one 60 degrees S boundary, no larval data, and no propagation of the standardisation uncertainty. Conclusion: the data support a long-term southward contraction of several hundred kilometres. They do not robustly support a 1000-1400 km south-west distance, or a contraction concentrated after 1976-1995.

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