Sounds of a feeding frenzy
Each autumn night below the surface of the Gulf of Maine, a feeding frenzy takes place. It’s occurred for eons, but until just recently, human observers had little idea what really took went on down in the cold, dark Atlantic waters. Now new research out of Northeastern University brings the scene into focus.
In a study published this month in Nature, lead author Purnima Ratilal and her colleagues describe the voracious habits of eight species of marine mammals, mostly whales, that gather to feast on tremendous shoals of spawning herring on Georges Bank. The animals’ behavior has been mapped for the first time on a large scale thanks to an innovative approach to underwater listening. “We’re the first to simultaneously map vocalizations of so many species,” says Ratilal.
Previously, marine scientists had two main methods for monitoring animal behavior: direct observations from boats, with range limited to a couple kilometers, and tagging and tracking individual animals, which yields a narrow view of an ecosystem.
In this study, the researchers found a way to gain a much more comprehensive view. Back in 2006, Ratilal and her colleagues deployed a line of underwater microphones known as hydrophones, provided by the Office of Naval Research Ocean Acoustics Program. The array was roughly 200 meters long, with 160 hydrophones positioned at varying intervals along its length. When towed behind a research boat, it allowed the researchers to instantaneously monitor acoustic signals across an area 100,000 square kilometers in size (about four times the size of Massachusetts).
Initially, Ratilal and her colleagues intended to use the array to map the behavior of Atlantic herring, a keystone species in the Gulf of Maine ecosystem. In particular, they wanted to chart the patterns that culminate in the formation of massive spawning shoals, one of the great events in the ocean. That research led to the publication of groundbreaking work. Later, Ratilal realized that in addition to detecting herring, the array of hydrophones had picked up some other important noises.
“We were out there mapping fish, not marine mammals. But even though we weren’t surveying marine mammals, we noticed at sea that we were hearing a lot of calls,” she says.
The calls came from a variety of whale species, but figuring out which species was making them, and where they were coming from, took a long time. The hydrophone array had collected an immense, almost overwhelming, amount of acoustic data and between 2009 and 2014, Ratilal and her colleagues developed signal processing techniques that allowed them to decipher it.
By searching in different frequency bands, they teased out the noises of different species. Below 100 hertz, they found large baleen whales like fin whales and blue whales. In the 300-800 hertz range they found signals from humpbacks. In the 800-1000 hertz range, they detected the characteristic whistles of smaller toothed whales like orcas and pilot whales.
The signal data allowed the researchers to acoustically track mammals’ movements through the Gulf of Maine and across time.
“It’s a technical tour de force to sort all this out in four dimensions,” says Jesse Ausubel, a program director at The Rockefeller University and adjunct faculty member at the Woods Hole Oceanographic Institution.
As Ratilal and her colleagues mapped the different marine mammal species, patterns started to emerged. They noticed that the intensity of calls picked up at night, when the herring shoals form and the different species of whales move in to feed. They also observed that whale species tended to cluster together, forming species-specific feeding areas that only slightly overlapped.
While the exact reason for this partitioning is unknown, it fits with other evidence that whales hunt socially, either using calls to let their brethren know where the food is or, in some cases, actually working together to corral fish. As a result, it’s natural that members of the same species would end up eating together.
“[The partitioning] comes about, I think, because of natural foraging behaviors of individuals species and how they might cooperate with each other. That might exclude individuals from other species,” says David Sims an expert on marine predator behavior at the University of Southampton in the United Kingdom.
Ratilal hopes future work will use the hydrophone approach she’s helped develop alongside other established methods for observing marine animal behavior. The effort would provide researchers with complimentary perspectives on what’s now looking like the large-scale, coordinated movements of underwater species.