(CN) — A new behavioral study shows that feeding at the ocean’s surface plays a big role in New Zealand blue whales' foraging strategy by allowing them to optimize their energy use.
Details of this study are published Wednesday in the journal PeerJ, in which Oregon State University researchers discuss their findings about the feeding habits of these ocean giants.
Blue whales currently hold the title of being the largest mammals on Earth, reaching 70 to 90 feet long and weighing in at 200,000 to 300,000 pounds. It’s difficult to fathom just how large these creatures are: roughly the length of three school buses, with hearts the size of a small car.
Because of their enormous size, the whales must balance the energy gained through their food intake with the energy costs of feeding, which involves diving, holding their breath or opening their mouths, which slows their movement in the water. They have an added challenge: Their prey of choice are tiny krill, and the whales have to find and eat large volumes of them to make any energetic headway.
"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 OSU's Marine Mammal Institute.
Co-authors of the study include Dawn Barlow, a doctoral student in Torres' lab, Todd Chandler, who was responsible for capturing drone footage used in the study, and Jonathan Burnett of OSU's Aerial Information Systems Laboratory.
"In this population of whales in New Zealand, they foraged more in areas where their prey was dense and shallow,” Torres continued. "Their dives were relatively short, and they were feeding more at the surface, which requires less energy."
A great deal of what researchers know about blue whale foraging consists of data collected from tags placed on the whales, which record travel and diving patterns, including acceleration, or lunging, toward patches of food. Surface feeding on the other hand is not as well understood, partly because it is harder to analyze tag data and quantify the size of prey patches at the water's surface, Barlow said.
During a 2017 field research trip off the coast of New Zealand to study blue whales, Torres and her team observed surface feeding from their boat on multiple occasions. They noted the density of the krill patches was greater the closer they were to the surface.
The data researchers collected from this trip showed that blue whales had relatively short dive times overall, spanning about 2.5 minutes. This was interesting when compared to other blue whale populations, such as those off the coast of California which average dives of about 10 minutes. When they observed the surface foraging, the dive time of New Zealand blue whales dropped even more, down to 1.75 minutes.
The researchers used a drone to capture stunning footage of a blue whale surface feeding on a patch of krill, which illustrated the feeding process in never-before-seen detail including decision-making about whether or not to eat patches of krill near the ocean's surface. This video was first shared first publicly after the research trip and it went viral, giving researchers another source of data to describe surface feeding behavior.
"The drone footage fills a gap in our understanding of surface feeding," Barlow said.
This valuable footage allowed the researchers to see how the whale used its right eye to target the prey. They were able to quantify the recognition distance from the whale to the prey and could measure how widely the whale opened its mouth to feed. They also saw from the footage showed the whale's decision to rotate from one side to the other to better capture the krill.
"The video allows us to describe a lot of really cool kinematics and body movement coordination by the whale that we haven't been able to see before," Torres said.
"The footage also allowed us to see the prey response in new way. We can see when the krill begin to flee as the whale approaches, which is really amazing. At the whale's fastest speed and acceleration, the krill begin to jump away just eight tenths of a second before the whale strikes at the krill patch," Torres continued.
Torres added that though they had surface-feeding footage from just one whale, the footage included four encounters between that whale and surface prey patches, providing insight into decision-making processes by the whale in response to the size and orientation of the prey patches.
"This footage highlights the value of using drones for study and observation of whales," she said. "Drone footage could be a good complement to data collected from tags for studying surface behaviors of whales."
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