With hearts the size of a small car and arteries wide enough to swim through, blue whales are the largest animals in history. They’re also one of the noisiest. Some of their booming vocalizations, louder than jet planes, travel for hundreds of miles in the water.
Scientists use this noise to observe these marine giants and their ocean ecosystems. In two recent studies, researchers combined acoustic recordings from Monterey Bay with environmental and behavioral data to learn more about the decisions blue whales make while hunting and migrating.
“Sound travels incredibly well in water,” says Monterey Bay Aquarium Research Institute (MBARI) biological oceanographer John Ryan. “This species and others have evolved for millions of years longer than we have to use sound.”
MBARI, based in Moss Landing, has recorded underwater sounds through a deep-sea microphone—called a hydrophone—since 2015.
The hydrophone sits 3,000 feet deep on the ocean floor just outside of Monterey Bay. Anyone can tune in to the live audio through the MBARI Soundscape Listening Room, but many of the frequencies it records are too high or low for humans to hear.
Scientists—and sometimes machine learning algorithms—pick apart these recordings and isolate whale calls.
“Acoustics is a really powerful tool for studying marine mammals in general and particularly these large baleen whales like blue whales,” says Dawn Barlow, who finished her Ph.D. studying blue whale ecology at Oregon State University this month. “They are vocally active, so they produce a lot of calls. They’re low frequencies, so we can pick them up from long distances. And we can monitor them over long time periods and broad spatial scales non-invasively and without needing to get out on the water.”
Barlow was not involved in the Monterey Bay work, but uses similar methods to study blue whales in New Zealand.
The Monterey Bay studies combine sound with other types of observations like tags that suction onto the whales and remote sensing of ocean conditions.
“What’s unique about these studies is their combination with behavior,” says Barlow. “They also bring in information learned from the movement of the whales via tags. So we have the ability to listen over these long distances as well as the behavioral and environmental context of those calls.”
Both of the studies demonstrate flexibility in the behavior of blue whales—something that likely helps them survive in the ever-changing ocean environment.
Giant Gatherings
David Cade was studying the maneuverability of blue whales in 2017 when he and colleagues found huge gatherings around Monterey Bay. They saw up to 40 whales within a square kilometer—an extremely rare sight.
Cade, a former postdoctoral researcher at UCSC who now works at Stanford’s Hopkins Marine Station, began piecing together an explanation.
Alongside Ryan and collaborators, he studied whale behavior and calls, the movement of krill—tiny shrimp-like crustaceans that make up almost all of a blue whale’s diet—and oceanic conditions like currents.
“The wind comes down from the south, and then that creates a lot of upwelling, and then that creates a lot of nutrients, and then that creates a lot of plankton, and then that creates a lot of krill, and that creates a lot of whales,” Cade says. “But what’s not well understood is exactly why and when that happens. Some years in Monterey Bay, there are really high abundances of whales, and some years there are not.”
The whale supergroups that Cade saw in 2017 were feeding on enormous patches of krill. The buffet was so large that they appeared to be calling other whales to it.
“The number of krill in these patches is so big, it would have taken these 40 blue whales several days to deplete that much krill,” says Cade.
And ocean conditions change quickly, so currents or other environmental factors would likely disperse the patch before the whales could finish it.
In such cases, “it might actually be beneficial for blue whales to share the location of these resources,” Cade explains.
The whales’ behavior seemed to confirm it. The number of foraging calls increased when they found these large swarms of food.
The behavior might have to do with kin selection—an evolutionary strategy that involves helping relatives with shared genes survive even at a potential cost to the individual.
Whaling decimated the blue whale population, so “even if it’s not your brother or your cousin, everyone’s pretty closely related,” says William Oestreich, an incoming postdoctoral researcher at MBARI who contributed to the study. “And so there’s a lot of benefit at the population level to helping one another find these really short-lived but really high-quality patches of food.”
Timing it Right
Oestreich also recently used acoustic recordings of blue whales in Monterey Bay to study a different aspect of their behavior. In a recent paper, he and Ryan describe how the animals decide when to stop foraging and migrate south for the winter.
They worked with Jeremy Goldbogen’s group at Stanford University and other collaborators from around Monterey Bay to deploy bio-logging tags.
“They’re devices that have sensors like you have in your cell phone that can measure the movements of these whales underwater and also capture the vibrations produced by their calls to give us a sense of what sorts of behaviors they’re undertaking when they’re singing at different times of the day,” says Oestreich.
The team found that although the whales typically arrive at their southern destination around the same time every year, they vary their departure time by as much as four months.
When the blue whales decide to leave depends on the foraging conditions around them. In years with more abundant krill and better hunting opportunities, the whales stick around longer. They likely also use calls from other blue whales to make their decisions about when to leave.
“To me, one of the most surprising things was that they were able to match the timing of that migration flexibly with an ocean process that’s occurring over enormous spatial scales—much larger than any one individual should be able to sense,” says Oestreich.
Scientists consider blue whales to be fairly solitary. But the way sound travels in the ocean might make it possible for them to behave collectively “over spatial scales that we can’t really always wrap our heads around as terrestrial mammals,” says Oestreich.
Diving Deeper
Almost every time researchers tag whales or dive into recordings, they learn something unexpected.
“There’s just so much new information out there,” says Cade. “Every behavior is a little bit different and a little bit new.”
And studying blue whale behavior also helps scientists understand other animals.
“By looking at where blue whales are and what they’re doing, you can gain a lot of insight into the state of the ecosystem,” says Barlow. “These acoustic monitoring stations like the one at MBARI provide another way to listen in on the state of the ecosystem via the blue whales.”
Soon, the MBARI station will provide even more insight. The institute plans to establish a Blue Whale Observatory this year.
The observatory will combine several different types of technologies that will allow scientists to study the whales’ habitat, food and behavior in depth.
“Monterey Bay is one of the best places in the world to do that kind of integrative work,” says Oestreich, who will help lead the observatory alongside Ryan and MBARI researchers Kelly Benoit-Bird and Chad Walk.
Scientists estimate that after nearly going extinct from whaling, only about 10,000 blue whales exist today. The population along the West Coast is the largest in the world at around 2,000 individuals, and the data collected from the MBARI observatory will help reveal the best ways to protect these ocean giants.
“Where and when do blue whales need to be in order to gain the energy they need for this incredible life history and incredible long-distance migration,” says Ryan. “And how do those special places and times intersect with some of the threats that this endangered species still face?”
If we listen closely, the whales just might tell us.
