Mesoscale Dynamics of Blue and Fin Whales and Antarctic Krill off East Antarctica

Blue and fin whales occur seasonally in the Southern Ocean to forage, primarily on Antarctic krill. These species are typically found in different pelagic environments, with blue whales often associated with sea ice and fin whales occupying areas away from sea ice. When they co-occur, however, it is not known if they compete over the same type and size of krill swarms, or if they have distinct swarm size, density, or krill life stage preferences. We investigated co-occurrence of blue and fin whales and krill in an open water area off East Antarctica using a bottom-moored passive acoustic recorder and an echosounder over one week in early February 2019. Using only calls with high signal-to-noise ratios, we measured whale occurrence in the vicinity of the moored recorders. Fin whale 20 Hz and 40 Hz calls were commonly recorded during the first four days, but almost entirely absent during the last three days. Blue whale song and D calls, on the other hand, were persistent during the first six days, but substantially decreased on the seventh day. Both 40 Hz and D calls were more common than songs for both species, possibly indicative of social and foraging behaviors. The distribution and abundance of acoustically-detected krill varied throughout the study period. Dense backscatter layers at 20-40 m depths were common during the first three days, but they were largely absent in subsequent days, replaced by more dispersed layers. On the sixth day, a dense, shallow backscatter layer appeared and coincided with an increase in blue whale D calls and fin whale 40 Hz calls. These data shed light on foraging preferences of these two large krill consumers in the Antarctic ecosystem.