A HUMPBACK whale surfaces, its mouth distended with krill and thousands of litres of water. It is the final stage of bubble-net hunting, a sophisticated technique employed by these huge mammals. A whale and its partner, visible just below the water’s surface, have together created a trap for the krill – their main food source – by swimming around exhaling columns of bubbles through their blowholes. The spiral of columns surrounds the crustaceans, creating a barrier they are unwilling to swim through. They move close together, and that’s when the whales dive, turn and swim upwards into the krill, mouths gaping.
It is an effective strategy, but not well understood. A drone took this photo as part of a project led by David Johnston of Duke University in Durham, North Carolina, to learn more about the whales’ behaviour. A laser altimeter fitted to the drone allows his team to calculate its altitude and thus the sizes of the whales and their bubble nets.
The picture was taken about 200 kilometres off the western Antarctic Peninsula. The whales feed here all summer, building up supplies of fat. They need to, because they then migrate to their breeding grounds in the Gulf of Panama and will not eat again until they return to the Antarctic, six months later.
Duke Marine Robotics and Remote Sensing Lab NOAA Permit 14809-03, ACA Permit 2017-034
This article appeared in print under the headline “Forever blowing bubbles”
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