Diel vertical distribution of Antarctic krill around the South Shetland Islands in February 2019 and its potential effect on biomass estimation

To address the potential effect of DVM behavior on krill biomass estimates, diel vertical distribution of krill swarms around the South Shetland Islands in Subarea 48.1 were analyzed based on data collected by the Chinese fishing vessel participating in the international 2019 Area 48 survey. Results show that more krill swarms were found in shallower depth at night than during daytime, while 16.5 % compared to 5.7 % of the corresponding total NASC value (m² n.mile^-2) integrated for layers bellow 15 m were found above 15 m at night and during daytime, respectively. The results implies that, when 15m is used for upper integration layer limit, both daytime survey and nighttime survey are likely underestimate the krill density due to the shallow distribution pattern of the animal, which may affect the nighttime estimate more than the daytime estimate.