Multibeam echosounder observations reveal interactions between Antarctic krill and air-breathing predators

A multibeam echosounder (MBE) was deployed on an inflatable boat (length = 5.5 m) to observe swarms of Antarctic krill Euphausia superba in the nearshore environment off Livingston Island, South Shetland Islands, Antarctica. Visual observations of air-breathing predators, including penguins and fur seals, were made from the boat at the same time. MBEs extend the 2-dimensional acoustic observations that can be made with conventional vertical echosounders to 3 dimensions, enabling direct observation of the surface areas and volumes of entire krill swarms. Krill swarms exhibited a wide range of various size metrics (e.g. height, length and width) but only a narrow range of surf ace-area-to-volume ratios or 'roughnesses', suggesting that krill adopt a consistent group behavior to maintain swarm shape. The variation in R was investigated using generalized additive models (GAMs). GAMs indicated that the presence of air-breathing predators influenced swarm shape (R decreased as the range to predators decreased, and the swarms became more spherical), as did swarm nearest-neighbor distance (R decreased with increasing distance) and swarm position in the water column (R decreased in the upper 70% of the water column). Therefore, swarm shape appears to be influenced by a combination of behavioral responses to predator presence and environmental variables. MBEs have the potential to contribute much to studies of krill, and can provide data to improve our understanding of the behavior of krill in situ.