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A commentary by Hill et al. (2019) raised some questions concerning the analyses of trends in krill density presented in Cox et al. (2018). We have re-examined our analyses and conclude that our approach does not "contain multiple errors" as asserted by Hill et al. (2019). Specifically, we reiterate that our statistical analysis of the publicly available version of KRILLBASE does not support the notion of a precipitous decline in krill density between 1976 and 2016. We respond to the technical criticisms made by Hill et al. (2019) of our work. We include a section where we suggest some approaches to help build consensus on detecting ecological trends in the Southern Ocean in the future. These approaches include: integrated assessments incorporating all relevant data, utilising data that will become available in the future from emerging technologies, and suggestions of other potential population monitoring methods that may be effective for krill. [ABSTRACT FROM AUTHOR]

The Antarctic marine environment is changing, and changes in the Southwest Atlantic sector have included decreases in sea ice and increases in water temperature. Associated with these changes is a reported 38% and 81% per decade decline in the numerical density (hereafter density) of Antarctic krill Euphausia superba Dana, 1850, between 1976 and 2003. Few changes in other components of the ecosystem that could be attributed to such a change, such as a mass decline in krill-dependent predators, have been detected. In an ecosystem so dependent on this keystone species, a massive population decline in krill ought to have had an obvious effect. In the absence of such an effect, it is timely to revisit the issue of the purported decline in krill density. The original analysis that indicated a decline in krill density was based on the 2004 version of KRILLBASE, a database of net samples. We analysed the publicly available and updated version (version 1, accessed 30 November 2017) and our analyses did not suggest a significant decline in krill density. Rather, after accounting for sampling heterogeneity and habitat variables, average krill density appears to have been stable but with considerable inter-annual variability. Since our results were unable to find any evidence for a decline in krill density we recommend a re-appraisal of many of the paradigms that underlie much of the recent thinking about ecosystem change Antarctic waters. Such a revision is necessary to provide a firmer foundation for predictions of the effects of climate change and resource extraction on the Southern Ocean ecosystem. [ABSTRACT FROM AUTHOR]