The blue whales in New Zealand waters have been feeding at the ocean surface as a way to optimise their energy use.
When blue whales feed on krill, a lot of energy gets used up in the process of diving, holding their breath and opening their mouth as they plunge into the swarm of krill. And, considering how the massive size of these whales compares to that of krill, these large whales definitely need to consume a lot of krill.
“People think about whales having to dive deep to get to the densest prey patches, but if they can find their prey in shallow waters, it's actually more energetically profitable to feed near the surface," said Leigh Torres, an assistant professor and director of the Geospatial Ecology of Marine Megafauna Laboratory at Oregon State University's Marine Mammal Institute.
And that’s precisely what the blue whales in New Zealand waters have been doing.
These blue whales were found to forage more in areas where krill was dense and shallow. Torres elaborated, “Their dives were relatively short, and they were feeding more at the surface, which requires less energy.”
Torres is one of the authors of a paper published in the journal PeerJ, which described the surface feeding behaviour of blue whales in New Zealand.
Although tags attached to whales can give a glimpse into whale behaviour and diving patterns, surface feeding has not been well understood. This is partly because it is difficult to analyse tag data and quantify the size of the prey patches at the ocean surface.
In February 2017, Torres and her team observed blue whales off the coast of New Zealand, and used a drone to film surface feeding on the krill patches.
They noticed that the density of krill patches was greater near the ocean surface. In addition, the overall dive times of the whales were shorter than that of other blue whale populations (like that off the Californian coast).
On average, the New Zealand population clocked about 2.5 minutes, while other populations clocked about 10 minutes. In fact, when the New Zealand whales foraged, their time dropped to 1.75 minutes.
The drone footage captured four encounters between a blue whale and surface prey patches, providing an insight into the decision-making processes of the whale in response to the size and orientation of the prey patches. For instance, it showed how the whale used its right eye to target the prey and showed the whale's decision to rotate from one side to the other to better capture its prey.
According to Torres, the drone footage was invaluable in revealing “a lot of really cool kinematics and body movement coordination by the whale that we haven't been able to see before.”