Lowered Cameras Reveal Hidden Seasonal, Diel, and Swarm Density Shifts in Antarctic Krill Behaviors Throughout the Water Column

Mary Kathryn Kane, Plymouth Marine Laboratory, Plymouth, United Kingdom, Angus Atkinson, Plymouth Marine Lab, Plymouth, United Kingdom and Susanne Menden-Deuer, University of Rhode Island, Graduate School of Oceanography, Narragansett, United States
Antarctic krill, a key link in Southern Ocean food webs, are considered to spend most of their adult lives in coherent, highly-organized groups. Despite their importance, technological challenges have resulted in few observations of individual krill behavior in their environment. Using a stereo-camera and environmental sensor system, >125 hours of footage were collected over 67 deployments conducted throughout the water column, up to depths of 625m, along the Western Antarctic Peninsula in May-June 2013 and December 2014. Individual krill motility was quantified and compared between seasons, day and night periods, and krill densities ranging from 0.0139 krill m-3 to 52.4 krill m-3 in autumn and 8.90 krill m-3 in spring. The results suggest that: (1) contrary to traditional thought, krill were seen either alone or in unorganized swarms in both seasons, not in coherent, highly-oriented schools; (2) most of the krill observed were either at depths too deep or densities too low for traditional survey methods to detect; (3) seasonal shifts in individual krill movement behaviors reflected changes in their vertical distributions; (4) krill moved twice as quickly in late spring as in late autumn, although they were active in both seasons; (5) krill were seen feeding primarily in the water column in late spring and at the seabed in late autumn; and, (6) krill were more motile in denser swarms in both seasons. I will present video footage of these krill behaviors both in the water column and at the seabed and discuss how shifts in krill motility may allow krill to balance their feeding requirements, energetic costs, intraspecific competition, and predation risks. I will also discuss the implications of these behaviors in relation to the vertical exchange of nutrients, benthopelagic coupling, and food web dynamics.