Where are the whales?

When it comes to mon­i­toring the abun­dance and behav­iors of whales, most research and con­ser­va­tion efforts rely on visual obser­va­tions. People go out on a boat and sys­tem­at­i­cally scan the ocean, clip­board in hand. “But the ocean is very vast, and it takes time to do this kind of survey,” said Purnima Ratilal, asso­ciate pro­fessor in the Depart­ment of Elec­trical and Com­puter Engi­neering at Northeastern.

Fur­ther­more she said in a press release issued by Northwestern, sperm whales are known to dive for up to an hour at a time, so you can never be sure you’ve seen all the whales in a given radius because they may just be too deep to observe.

That’s why Ratilal is using acoustic methods to com­pli­ment cur­rent whale mon­i­toring efforts. In a paper recently pub­lished in the Journal of the Acoustic Society of America, Ratilal and her team present the first high-​​resolution acoustic data local­izing sperm whales in shallow con­ti­nental shelf waters.

Ratilal’s exper­tise is in under­water acoustic sensing methods, par­tic­u­larly long-​​range sensing with densely-​​sampled towed hydrophone arrays. This is basi­cally a long hose with dozens of under­water micro­phones affixed to it every half meter or so. The hydrophones pick up sound from all direc­tions, and then algo­rithms also devel­oped in Ratilal’s lab parse the data to iden­tify where a sound of interest is coming from, in both direc­tion and range, then delete the noise coming from other direc­tions and areas.

“We get a cacophony of nat­ural sounds and then have to dis­tin­guish indi­vidual whale calls from one another,” she explained.

Last year, she and her team were out testing their new towed array device, co-​​developed by BAE Sys­tems and Ein­horn Engi­neering with funding from the National Sci­ence Foun­da­tion, in the coastal waters between southern Cape Cod and Maine when they came across a lone sperm whale swim­ming on the con­ti­nental shelf on the Gulf of Maine. Nor­mally sperm whales like to stick to the con­ti­nental slope, which is deeper and far­ther out to sea. This is where the squid live—“their favorite food,” Ratilal said.

This lone ranger may have been scav­enging for food along the shal­lower shelf, but it’s unclear exactly why he’d strayed from his mates. Nev­er­the­less, he was sounding the char­ac­ter­istic short clicks that sperm whales use to echolo­cate their prey, which in turn Ratilal’s group used to locate the whale.

Pre­vious acoustic work used a handful of omni­di­rec­tional hydrophones to home in on a whale’s exact loca­tion. But in that case, the method could only localize whale sounds in the vicinity of the hydrophones. Ratilal’s method allows her to probe fur­ther out into the sea and also to localize deep diving whales not vis­ible on the surface.

The team also used the method to find a larger group of whales swim­ming on the deeper con­ti­nental slope south of Cape Cod. In that case, they were able to use their data to sep­a­rate the indi­vidual sounds of about six or seven dif­ferent whales. Ratilal said. “We’re char­ac­ter­izing and dis­tin­guishing the dif­ferent whales’ clicks,” she explained.

The towed arrays aren’t strangers to the ocean. Both the Navy and the oil industry use towed arrays for their var­ious geo­phys­ical sur­veys, which Ratilal believes could be used to simul­ta­ne­ously pro­tect whales from large marine ves­sels as well as pro­vide useful mon­i­toring data using her approach. “If they’re already out there with their arrays, they could be get­ting seismic prospecting data and at the same time they could sense the whales in the vicinity and make sure they’re at a safe dis­tance,” she said.