| Summary: | Antarctic krill form some of the highest concentrations of animal biomass observed in the world’s ocean potentially due to their prolific ability to swarm. Determining the movement of Antarctic krill within swarms is important to identify drivers of their behaviour and their biogeochemical impact on their environment. We examined vertical velocity within approximately 2000 krill swarms through the combined use of a shipborne echosounder and an acoustic Doppler current profiler (ADCP). We revealed a pronounced downward anomaly in vertical velocity within swarms of -0.6 cm.s-1 compared with vertical motion outside the swarm. The anomaly changed over the diel cycle, with smaller downward anomalies occurring at night. Swarms in regions of high phytoplankton concentrations (a proxy for food availability) also exhibited significantly smaller downward anomalies. We propose that the anomaly is the result of downward velocities generated by the action of krill beating their swimming appendages. During the night and in high phytoplankton availability, when krill are more likely to feed to the point of satiation, swimming activity is lowered and the anomaly is reduced. Our findings are consistent with laboratory work where krill ceased swimming and adopted a parachute posture when sated. Satiation sinking behaviour can substantially increase the efficiency of carbon transport to depth through depositing faecal pellets at the bottom of swarms, avoiding the reingestion and breakup of pellets by other swarm members. Antarctic krill swarms in Scotia Sea Summer 2003The dataset contains 2043 krill swarms detected using an EK60 echosounder during transects of the Scotia Sea during Jan and Feb 2003. Each of the swarms was detected using the dB difference method described in Tarling et al. 2009. Corresponding measurements of the vertical velocity within and above and below each swarm were made using an acoustic Doppler current profiler (ADCP). Velocities above and below the swarm were taken from velocities within the swarm to ...
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