Few people sight the cryptic creatures swimming in Monterey Bay, but sonorous sperm whales abound if you listen. The chatter of undersea creatures fills the soundscape, and the clicks of sperm whales pierce through the background. Once thought to be a rare occurrence, researchers at Monterey Bay Aquarium Research Institute (MBARI) detected an unprecedented number of sperm whales in Monterey Bay using underwater microphones.

Unlike their prevalent cousins the humpback whales — which are so ubiquitous in Monterey Bay that they were featured in “Star Trek IV: The Voyage Home” (along with the Monterey Bay Aquarium as the Cetacean Institute) — sperm whales often elude detection. According to Santa Cruz Whale Watching, their boats only spot sperm whales once every few years. These large, endangered predators surface briefly for air, then dive thousands of feet into the pitch-black depths to hunt. To find their prey, the whales produce loud clicks that last a fraction of a second and listen for the echo. This echolocation locates distant squids, sharks, skates and fish in the darkness of the deep ocean.

Studying deep sea creatures is challenging because the vast volume of water blocks light from reaching very far. However, MBARI says sperm whales are the loudest known animals, and water carries sound for miles. “It’s fascinating to go out into these places that really are a wilderness,” says Will Oestreich, a National Science Foundation postdoctoral scholar in the Acoustical Ocean Ecology Team at MBARI. To observe the chatter of marine life, MBARI researchers placed a microphone in outer Monterey Bay at the Deep Sea Cabled Observatory. As the name suggests, this observatory directly links to MBARI via a cable, so the instruments can continuously receive power and transmit information back to the researchers.

Since 2015, the microphone recorded the undersea soundscape continuously. Sperm whales are the loudest known animals, but their clicks last less than a second. To pick the sperm whale sounds out of seven years’ worth of recordings, Oestreich and his colleagues created an automated program that identified and analyzed sperm whale sounds.

The sheer number of sperm whales in Monterey Bay stunned the researchers. Before this study, sperm whales were thought to be quite rare. “What was shocking to me was how regularly they are here,” says Oestreich. Sightings happen infrequently, so the team was surprised to find a large number of sperm whales living in the bay throughout the year.

Since they found such large numbers of sperm whales, the team decided to track the population demographics and migration. The spacing between clicks directly relates to the animal’s body size, and female sperm whales are much smaller than males. They found that both males and females commonly swam through the bay, as well as their young. All three of these populations were present throughout the year, so it’s not just a single demographic of sperm whales coming to the bay.

Like other whales, sperm whales migrate throughout the year to different hunting and mating grounds. The total number of sperm whales in Monterey Bay cycled seasonally, and this migration is offset from the well-studied Alaska migration. Since these whales hunt in the deep ocean where sunlight doesn’t reach, this shows that the deep-sea ecosystem is still linked to the seasons. “You can know about the ecosystem as a whole just from studying one animal,” says Oestreich.

Like bats, sperm whales use their short clicks to perform echolocation to find their prey. As a click travels through the water, it bounces off of more solid objects, like the delicious squid sperm whales devour. When the whales hear this return sound, they know the location of the squid and home in on their target. “As the sperm whale starts to home in on its prey, the clicks get very, very close together and it starts to sound like a creek,” says Oestreich. As they close in, the time between clicks and the echoes gets shorter and shorter, until they suddenly stop — when the whale eats the squid. By tracking this pattern, the scientists tallied how often the whales capture their food.

While Monterey Bay contains a national marine sanctuary, sperm whales pass in and out of these protected waters. Now that MBARI scientists know that sperm whales frequent Monterey Bay, Oestreich shares that they’d like to work with conservation groups to use their data to protect this endangered species. “Knowing when these animals are here and what they’re doing … is an important part of the picture to inform their conservation.”

Go to fisheries.noaa.gov/species/sperm-whale to learn more about sperm whales and the undersea soundscape.