With hearts the size of a small car and arteries wide enough to swim, blue whales are the largest animals in history. They are also among the noisiest. Some of their booming vocalizations, louder than jet planes, travel hundreds of miles through 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 during hunting and migration.
“Sound propagates incredibly well in water,” says John Ryan, biological oceanographer at the Monterey Bay Aquarium Research Institute (MBARI). “This species and others have evolved millions of years longer than we need to use sound.”
MBARI, based in Moss Landing, has been recording underwater sounds using an underwater microphone called a hydrophone since 2015.
The hydrophone sits 3,000 feet deep on the ocean floor just outside Monterey Bay. Anyone can listen to live audio through the MBARI Soundscape Listening Room, but most recorded frequencies are too high or too low for humans to hear.
Scientists – and sometimes machine learning algorithms – separate these recordings and isolate the calls of the whales.
“Acoustics is a really powerful tool for studying marine mammals in general and especially these large baleen whales like blue whales,” says Dawn Barlow, who has completed her PhD. study blue whale ecology at Oregon State University this month. “They are vocally active, so they produce a lot of calls. These are low frequencies, so we can pick them up over long distances. And we can monitor them over long periods of time and at large 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.
Monterey Bay studies combine sound with other types of observations like beacons sucking whales and remote sensing of ocean conditions.
“What’s unique about these studies is their combination with behavior,” says Barlow. “They also provide information from the movement of whales via beacons. We therefore have the ability to listen over these long distances as well as the behavioral and environmental context of these calls.
Both studies demonstrate flexibility in the behavior of blue whales, which likely helps them survive in an ever-changing ocean environment.
gatherings of giants
David Cade was studying the maneuverability of blue whales in 2017 when he and his colleagues discovered huge aggregations around Monterey Bay. They saw up to 40 whales in a square kilometer, an extremely rare sight.
Cade, a former postdoctoral researcher at UCSC who now works at Stanford’s Hopkins Marine Station, began to piece together an explanation.
Alongside Ryan and his collaborators, he studied the behavior and calls of whales, the movement of krill – tiny shrimp-like crustaceans that make up nearly all of a blue whale’s diet – and the conditions ocean like currents.
“The wind comes down from the south, and then it creates a lot of upwelling, and then it creates a lot of nutrients, and then it creates a lot of plankton, and then it creates a lot of krill, and it creates a lot of whales,” Cade says. “But what is not well understood is exactly why and when it happens. Some years in Monterey Bay there is a very high abundance of whales, and some years there are no has none.
The supergroups of whales Cade saw in 2017 were feeding on huge patches of krill. The buffet was so big they seemed to call for other whales.
“The number of krill in these areas is so great that it would have taken those 40 blue whales several days to exhaust that much krill,” Cade says.
And ocean conditions change rapidly, so currents or other environmental factors would likely scatter the area before the whales could complete it.
In such cases, “it might actually be beneficial for blue whales to share the location of these resources,” Cade says.
The behavior of the whales seemed to confirm this. The number of foraging calls increased when they found these large swarms of food.
The behavior could be linked to kin selection, an evolutionary strategy of helping relatives with shared genes to survive, even at potential cost to the individual.
Whaling has decimated the population of blue whales, so “even if it’s not your brother or cousin, everyone’s pretty close,” says William Oestreich, a new MBARI postdoctoral fellow who helped the study. “And so there’s a lot of population-level benefit to helping each other find these very short-lived but very high-quality food patches.”
The good moment
Oestreich also recently used acoustic recordings of blue whales in Monterey Bay to study a different aspect of their behavior. In a recent article, he and Ryan describe how animals decide when to stop feeding and migrate south for the winter.
They worked with Jeremy Goldbogen’s group at Stanford University and other Monterey Bay collaborators to deploy bio-registration beacons.
“These are 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 an idea of the types of behaviors they are undertaking. when they’ I sing at different times of the day,” says Oestreich.
The team found that although the whales generally arrive at their southern destination around the same time each year, they vary their departure time by up to four months.
When blue whales decide to leave depends on the foraging conditions around them. In years when krill are more abundant and there are better hunting opportunities, whales stay longer. They probably also use calls from other blue whales to decide when to leave.
“For me, one of the most surprising things was that they were able to flexibly match the timing of this migration with an ocean process that happens on huge spatial scales, much larger than what an individual should be able to feel,” says Oestreich.
Scientists consider blue whales to be quite solitary. But the way sound travels through the ocean could allow them to behave collectively “on spatial scales that we can’t always understand as terrestrial mammals,” says Oestreich.
Dive deeper
Almost every time researchers tag whales or dive into recordings, they learn something unexpected.
“There’s so much new information available,” Cade says. “Each behavior is a little different and a little new.”
And studying the behavior of blue whales also helps scientists understand other animals.
“By looking at where the blue whales are and what they’re doing, you can better understand the state of the ecosystem,” says Barlow. “These acoustic monitoring stations like MBARI’s provide another way to listen to the state of the ecosystem via blue whales.”
Soon the MBARI station will provide even more information. The institute plans to establish a blue whale observatory this year.
The observatory will combine several types of technologies that will allow scientists to study in depth the habitat, food and behavior of whales.
“Monterey Bay is one of the best places in the world to do this kind of integration 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 disappearing from whaling, only around 10,000 blue whales exist today. The population along the west coast is the largest in the world at around 2,000 individuals, and data collected by the MBARI observatory will help reveal the best ways to protect these ocean giants.
“Where and when do blue whales need to be to get the energy they need for this incredible life history and this incredible long-distance migration,” says Ryan. “And how do these special places and times intersect with some of the threats this endangered species still faces?”
If we listen carefully, the whales might just tell us.
