In September 2006, the team ventured out into the Gulf of Maine with two ships: one that sent out sound waves and another that detected the rebounding echoes with a string of 160 hydrophones. Together, they visualized the movements of a quarter of a billion herring. During the day, these fish stick to the ocean floor and largely keep their distance. But come sunset, they gather to spawn, rising to the surface and aggregating into a kilometers-wide mega-orgy—a shoal of 250 million fish all busy creating millions more baby fish.

While working on the herring, the team kept on hearing whales in their recordings. They initially focused on humpbacks, reputedly among the most vocal of the whales. “We were amazed at the quality of the data we got,” says Ratilal. “We found 2,000 calls from humpbacks each day.” But even though the herring were spawning throughout the gulf, the herring-eating humpbacks were clustered in two separate locations. Why weren’t they going after the fish in the middle? “We thought there might be other whale species occupying the regions in between. And sure enough, we found them.”

Each whale species calls within a certain frequency range and makes its own distinctive repertoire of sounds. Using this information, the team could look at their recordings and extract the locations of five huge filter-feeding species (the blue, fin, humpback, sei, and minke) and three toothed ones (sperm, pilot, and killer).

The whales seemed to divide the herring between them, with each species sticking to its own particular part of the Gulf. The blue whales stayed away from the humpbacks, which swam apart from the minkes, which lived separately from the seis. “You find the same species in these same areas day after day,” says Ratilal. “It’s quite stable.”

It’s possible that the larger whales like blues stay away from shallower regions, leaving those to the smaller minkes and pilots. But in truth, no one knows why or how the whales carve up the oceans between them. It’s not surprising that they do—you can see similar partitioning among, say, plant-eaters on the African grasslands—but it’s rare to see such stark visual evidence of these divisions. Check out Ratilal’s map: that’s a huge body of water. See those rings of color? Those are the territories of animals that are the size of ships.

Wang et al, 2016. Nature

“[Ours] is the only technique that can instantaneously monitor marine mammal and fish populations over very large areas,” she says. She calls her technique Passive Ocean Acoustic Waveguide Remote Sensing (POAWRS), and the “Passive” bit is important. When the team studied the herring, they found the fish by sending out sound waves and capturing the echoes. But whales are so vocal that the first bit is unnecessary. “We’re just listening in,” says Ratilal.