Report of On-line Krill Ageing Workshop (August and November 2021)

An international workshop to investigate a potential use of ageing method for the Antarctic krill (Euphausia superba, Dana) based on growth band counts observed in krill eyestalks was held in August and November 2021. The workshop was attended by 14 participants from 7 nations and investigated the technique using laboratory bred known age krill that had been reared under varying light conditions. The index of average percentage of error (an estimate of the precision of band counts vs. known absolute age) using these 53 known age krill was 17.5 and a Coefficient of Variation of 24.8%, suggesting poor accuracy against absolute age. Correlation in band counts between workshop participants were also reviewed. The band counts by the participants did not have any correlation with absolute age of krill, but showed very weak (not significant) increase with the size of krill. The uncertainty in locating the appropriate counting axis on the section and the difficulty in distinguishing the endocuticle structure where growth bands form within the eyestalk was perceived to be the major issue in using this method. The workshop concluded that caution should be exercised, and this method should not be more widely applied for estimating the age of krill unless it can be validated in the future.