Beneath the ocean’s surface, a symphony ripples and rolls, ricochets and hums—and whales pour their songs into the deep soundscape like streams of molten silver.
Deep within the noise, a 32-mile-long cable stretches out from the California coastline along the seafloor, tethered to the ground 3,000 feet below the surface. At its end is a two-inch-wide metal cylinder standing on three legs. This hydrophone, an underwater microphone, can record and trace the ocean’s shifting harmonies for years on end.
“Once you truly start listening to how many things make sounds in the ocean, it’s really amazing what you hear,” says Jarrod Santora, an ecosystem oceanographer and research biologist for the National Oceanic and Atmospheric Administration.
By tuning in to these underwater songs, scientists can decode the rhythms of ocean life, listening for signs of imbalance and resilience, and tracking how marine species respond to human impact. As whales navigate seas transformed by climate change, noise, and industrial activity, their voices offer a vital record of a world in flux.
“It wasn’t until I plugged in a hydrophone that I realized this world of sound can help us understand human impacts, nature, and the balance between,” says John Ryan, a biological oceanographer at the Monterey Bay Aquarium Research Institute.
In a
study published earlier this year
, which traced more than six years of acoustic monitoring in the central California Current Ecosystem, Ryan and a team of researchers found clear patterns in whale song across seasons and years. By chance, the recordings began during a massive marine heatwave unlike anything seen before in the region.
The study documented whale songs beginning in July 2015, and revealed that different species responded differently. Humpbacks have a more diverse diet and were able to adapt to harsh conditions; their songs didn’t change. But blue and fin whales feed almost exclusively on krill, and their songs were detected less often than years prior.
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The heatwave reduced the food whales rely on and triggered harmful changes in ocean chemistry, allowing toxic algae to bloom. “It caused the most widespread poisoning of marine mammals ever documented. These were hard times for whales,” says Ryan.
As prey became scarce, blue whale vocalizations dropped by nearly 40 percent alongside a collapse in krill and anchovy populations, the recent study showed.
“When you really break it down, it’s like trying to sing while you’re starving,” Ryan adds. “They were spending all their time just trying to find food.”
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An anomalous marine heatwave event
In 2013, a dense pool of unusually warm water lingered stubbornly through the winter in the Bering Sea and Gulf of Alaska. The phenomenon defied easy explanation, and its persistence confounded scientists. As this mass of warm water expanded southward along the Pacific Coast through 2014, stretching from Alaska to Mexico, it earned a nickname: “
”
In some places, ocean temperatures were more than
The Blob spread rapidly, covering a region
500 miles wide and 300 feet deep. And by 2016, it spanned over 2,000 miles of the Pacific Ocean.
Krill—tiny, shrimp-like creatures that form the foundation of the marine food web—were suddenly scarce. In years past, krill arrived in such volume that fishing nets turned pink with their abundance.
During the heatwave, they all but vanished.
“When we have these really hot years and marine heatwaves, it’s more than just temperature,” explains oceanographer Kelly Benoit-Bird, a Monterey Bay Aquarium marine biologist and co-author of the paper. “The whole system changes, and we don’t get the krill. So the animals that rely only on krill are kind of out of luck.”
Blue whales, the largest animals on Earth, were among the unlucky. Their massive jaws and pleated throats are built to engulf thousands of gallons of water at once—but only when krill are packed densely enough to make the effort worthwhile. “That swarming behavior is really critical to their survival,” Benoit-Bird says. “Each mouthful has to be worth the dive.”
And in the heat, not only did krill numbers fall, but their behavior may have also changed. With upwelling disrupted by heat, they scattered. This made it harder still for whales to find enough food.
“We don’t hear them singing,” Ryan says about the underwater sound recording from that year. “They’re spending all their energy searching. There’s just not enough time left over—and that tells us those years are incredibly stressful.”
And as increasing
trap more of the sun’s energy, the oceans—already absorbing
over 90 percent of the excess heat from climate change
—are becoming more vulnerable to extreme events like marine heatwaves and stronger El Niños. A study recently published in the Proceedings of the National Academy of Sciences found that the duration of ocean heatwaves has tripled since the 1940s. These events are now about 1°C hotter on average, with some regions experiencing spikes up to 5°C.
“There are whole ecosystem consequences of these marine heat waves,” continues Benoit-Bird. “If they can’t find food, and they can traverse the entire West Coast of North America, that is a really large-scale consequence.”
Across an ocean, blue whales repeat a trend
In the waters between New Zealand’s islands, researchers studying blue whales from 2016 to 2018 unexpectedly found eerily quiet waters during the years of The Blob, just like those heard in California.
“We were interested in understanding blue whale ecology,” says Dawn Barlow, an ecologist at the Marine Mammal Institute at Oregon State University, and lead author of the study. “And without trying, we ended up
studying the effects of marine heatwaves
—which, in this day and age, is hard to avoid when working in the ocean.”
Using underwater recorders in the South Taranaki Bight, Barlow and her team tracked two distinct vocalizations: low-frequency D calls, linked to feeding, and patterned songs, associated with mating. During years of abnormally warm water, they found fewer D calls in spring and summer—signaling a drop in foraging effort. In the following fall, blue whale song intensity also declined, suggesting reduced reproductive activity.
“When there are fewer feeding opportunities, they put less effort into reproduction,” Barlow explains.
The absence of calls has become a warning, say scientists.
“Blue whales are sentinels,” says Barlow. “They integrate many ocean processes. Where they are, and what they’re doing can tell you a lot about the health of the ecosystem.”
And the effects of a single heat wave can last long after temperatures have cooled.
“The Blob really highlighted how long-term these consequences can be,” she adds. “This isn’t just about what happens during the heatwave—it’s the lasting impacts, especially for long-lived animals like whales.”
That longevity makes them powerful sentinels. If a species capable of roaming an entire coastline begins to falter—struggling to find food, delaying reproduction—researchers say the signal is unmistakable: something deep within the ecosystem is shifting. And in places where heat waves scorch areas again and again, the transformation may be irreversible, leaving behind a sea that is profoundly—and perhaps permanently—changed.
“There’s a chance that one of these events becomes a tipping point, and that may not return to the state we had before,” says Benoit-Bird. “And that matters. For how the ocean absorbs carbon, for the fish we eat, and for the future of marine ecosystems.”
Could listening to whales help protect the ocean?
Even shallow waters, where snapping shrimp crackle like underwater firecrackers, are beginning to sound different. A study published in 2022 found that shrimp, one of the ocean’s most active noisemakers, snapped more frequently and with more force as water temperatures increased. Possibly, the scientists speculate, because they’re agitated.
One challenge for using sound to measure ocean-wide changes is establishing a baseline for what a pristine ocean sounds like. The COVID-19 pandemic offered a rare experiment. When global shipping activity came to a halt, a brief hush settled over much of the planet, including the seas, before it was catapulted to even faster production.
“Certainly the animals responded—they changed their distribution and used the habitat differently when there weren’t humans in those spaces anymore,” says Monterey Bay’s Benoit-Bird.
She recalls the way that many witnessed wildlife in empty city streets. In the ocean, the reaction was just as profound, only harder to witness. “We don’t tend to think of humans as being in the ocean in that same way,” she adds, “but we are. We’re there—we’re everywhere.”
While scientists have recently detected certain patterns, more data is needed to connect specific sounds to specific environmental changes.
“It’s so hard to get observations in the ocean,” says NOAA’s Santora. “A network like this opens the door to so many possibilities—for conservation, for management, for mitigation.”
Whales follow a slower rhythm than fish, with 80-year life spans and overlapping generations that obscure quick trends—so while populations have grown since whaling ended, only about thirty generations have passed, and scientists say we may need twice that to understand the impacts of the threats they face today.
“Science shows that climate change is impacting the oceans,” says Dawn. “We see that across trophic levels, across ecosystems. Listening and learning from these places is essential to our future. Now more than ever, it’s important to listen.”
