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.

Leave a Comment

Real Time Web Analytics

Pin It on Pinterest

Get more stuff like this
in your inbox

Subscribe to our mailing list and get interesting stuff and updates to your email inbox.