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<\/p>\n[By\u00a0Nancy Bazilchuk]<\/p>\n
A 100-year-old equation and a fibre-optic cable off the coast of Svalbard led researchers to discover they could detect swimming whales \u2014 even if they were completely silent. The discovery broadens the tools that biologists could someday use to detect and monitor these marine giants.<\/p>\n
Fibre-optic cables lace the planet\u2019s oceans, carrying everything from telephone conversations to banking information. In 2020, NTNU researchers figured out that those cables can also serve as a passive listening device, allowing them to detect the deep rumbles of whale sounds in the frigid waters off Svalbard.<\/p>\n
Now researchers have discovered how to use fibre optic cables to \u201clisten\u201d to whales, even when they don\u2019t make any sound. \u00a0The secret is that whales, like ships, push water ahead of them as they swim \u2013 which creates a very low frequency pressure wave that the fibre-optic cables can detect.<\/p>\n
\u201cIf the whales are silent, their body movement causes disruptions in the water and the sediments, so that we can detect them, even if they aren\u2019t making any noise,\u201d said Martin Landr\u00f8, head of NTNU\u2019s Centre for Geophysical Forecasting and the senior author of a new paper just published in PNAS, the Proceedings of the National Academies of Science.<\/p>\n
But no one, including the researchers, had thought to look at these very low frequency waves to detect whales \u2013 until now.<\/p>\n
\u201cThe main challenge in detecting these low frequency signals is that they decay very fast with distance. A big ship is easy to detect as it moves a lot of water, but a whale is much smaller, so that it moves a lot less water. This means that they need to dive into the water column to be detected,\u201d said Robin Andre R\u00f8rstadbotnen, the first author of the paper, and a postdoc at the Centre for Geophysical Forecasting.<\/p>\n
One of the many advantages to the fibre-optic cable network in the waters around Svalbard is that there are a number of ships, both small and large, that cruise these same waters.<\/p>\n
And if you think about, the shape of a ship\u2019s hull isn\u2019t that different from the shape of a whale\u2019s body.<\/p>\n
But unlike whales, all ships are required to have what\u2019s called an Automatic Identification System, or AIS. That identifies both the ship itself and where it is travelling, allowing researchers to track the ships as they sail along or across the fibre-optic cables.<\/p>\n
The researchers could detect the actual sound \u2013 the acoustic signal \u2013 of the different ships in the data from the fibre-optic cables, of course. They could also detect how fast the ships were travelling.<\/p>\n
That\u2019s the same approach the researchers had previously used to hear whale vocalizations.<\/p>\n
This time, however, they realized that they could use the fibre-optic cable to detect the pressure wave generated as the ship moved through the water.<\/p>\n
From boiling water to pressure waves<\/p>\n
A pressure wave from water movement is much lower frequency than a whale vocalization. It\u2019s also information that the researchers hadn\u2019t looked at before.<\/p>\n
\u201cThe big surprise was that this fiber could detect this,\u201d Landr\u00f8 said.<\/p>\n
Suddenly they had a way to interpret the low-frequency data because they already had so much information about the different ships.<\/p>\n
Bubbles in boiling water may not seem to have much in common with pressure waves from swimming whales, but researchers were able to use the equations to interpret their information on swimming whales. Screenshot.<\/p>\n
\u201cThere are a lot of cruise ships during the summer months in Svalbard, sailing along the fibres we interrogated. By looking at one ship, Le Commandant Charcot, over three cruise seasons, we obtained a lot of very valuable information that could help us better understand these low-frequency signals and also identify aspects that need further research,\u201d R\u00f8rstadbotnen said.<\/p>\n
They also used a well-known equation published in 1917 by a famous physicist named Lord Rayleigh that described how bubbles in boiling water collapse. \u00a0That may sound very different than moving ships and whales, but the basic physics applies.<\/p>\n
\u201cThis is the way we calibrated it,\u201d Landr\u00f8 said. \u201cWe know how fast the ships are moving, we know exactly where they are, because we have the AIS information. So that gives you a way to fine tune the understanding of these low-frequency signals. And that was our major observation, which is new.\u201d<\/p>\n
When you think about it, a whale, moving through the water, isn\u2019t that different than a ship, pushing water out of the way as the motor propels the ship forward.<\/p>\n
\u201cThat opened the possibility that maybe this can be used to detect silent whales,\u201d Landr\u00f8 said. \u00a0\u201cWhen we saw this, we knew the next step was to look for whales.\u201d<\/p>\n
A blue whale and a bit of luck<\/p>\n
Here\u2019s where chance \u2013 call it a little bit of luck \u2013 stepped in.<\/p>\n
\u201cWe were lucky to find a whale, a blue whale, that was vocalizing close to the surface, and thenwhen it stopped vocalizing, it dove down,\u201d Landr\u00f8 said.<\/p>\n
A blue whale seen from below. Blue whales are the largest animal on Earth, but their population was decimated by commercial whaling during the last century. Researchers don\u2019t know the current global population size, but they believe it is between 5,000-15,000 mature individuals, which is just 3-11% of the population size in the early 1920s. Photo: NTB<\/p>\n
So first, the researchers could see the blue whale\u2019s vocalizations using the acoustic data from the fibre-optic cables. This was by now a routine way for them to detect whales that made sound.<\/p>\n
But now they knew that they should also look at the low-frequency data from the fibre-optic cables to see what the blue whale looked like in those data. And they could interpret what they saw with the help of Rayleigh\u2019s equation.<\/p>\n
Suddenly they could follow the track of the whale even as it stopped making sounds, and dove deeper into the ocean.<\/p>\n
Another tool for conservation work<\/p>\n
Landr\u00f8 and his colleagues have previously suggested that researchers explore the use of fibre-optic cables for a range of remote sensing applications, from detecting earthquakes and monitoring undersea pipelines for sabotage to linking information from satellites to create an Earth-Ocean-Atmosphere-Space Observatory.<\/p>\n
A blue whale seen from below. Blue whales are the largest animal on Earth, but their population was decimated by commercial whaling during the last century. Researchers don\u2019t know the current global population size, but they believe it is between 5,000-15,000 mature individuals, which is just 3-11% of the population size in the early 1920s. Photo: NTB<\/p>\n
\u201dThis could be a game-changing global observatory for Ocean-Earth sciences,\u201d Landr\u00f8 said after a paper about how such a system could work was published in\u00a0Nature Scientific Reports.<\/p>\n
The ability to detect silent whales is another step in using the existing fibre-optic network to better understand the planet and its far-flung inhabitants, such as whales.<\/p>\n
While some few whale populations are in reasonable shape,\u00a0most whales were heavily exploited over time. But the very nature of whales \u2013 big marine animals that roam the oceans \u2013 makes population estimates challenging.\u00a0 Being able to track silent whales could change all that.<\/p>\n
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