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| Enlarge ImageScientists adaped a low-frequency sonar system, originally designed to survey seafloor geology, to identify fish and zooplankton. (Photo by Mike Jech, National Fisheries Science Center) |
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| Enlarge ImageThe research team towed a low-frequency broadband imaging sonar near fish schools. For comparison, they also collected sound data from a conventional high-frequency hull-mounted sonar system. Trawls collected fish samples to groundtruth sonar data. (Illustration by Jayne Doucette, Woods Hole Oceanographic Institution) |
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| Enlarge ImageTo test whether they could use low-frequency sonar to identify fish, the scientists targeted schools of Atlantic herring in deep water over Georges Bank. (Photo courtesy of Tim Stanton, Woods Hole Oceanographic Institution) |
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| Enlarge ImageThe research team aboard the NOAA ship FR/V Delaware II dockside in Woods Hole: From left, Tim Stanton (WHOI), Dezhang Chu (WHOI), Ben Reeder (former MIT/WHOI JP student, now at the Naval Postgraduate School), and Mike Jech (NOAA/NMFS). (Courtesy of Tim Stanton, Woods Hole Oceanographic Institution) |
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In the 1970s, scientists happened upon a curious phenomenon about sound
waves in the ocean and swim bladders in fish: Bony fish have gas-filled
sacs inside their abdomens called swim bladders, which help them
maintain buoyancy in water. When low-frequency sound waves (the kind
used by the Navy to track submarines, or by industry to search for oil
and gas beneath the seafloor) come in contact with swim bladders, they
resonate much like a tuning fork and return a strong echo.
For decades, Tim Stanton, a scientist at Woods Hole Oceanographic
Institution (WHOI), made a mental note of this phenomenon, thinking
about how it could best be exploited someday to help detect fish. That
is something the Navy (to use sonar more effectively) and the National
Marine Fisheries Service (to assess fish stocks) are both keenly
interested in. For the latter, low-frequency sonar systems were too
large and expensive for practical use.
Stanton kept the idea in the back of his mind during the 1980s, when he
was a scientist at the University of Wisconsin, Madison, where Dezhang
Chu, now Stanton’s colleague in the WHOI Department of Applied Ocean
Physics and Engineering, was a graduate student, along with Mike Jech,
now a scientist at the Northeast Fisheries Science Center in Woods
Hole, Mass. Stanton watched the development of smaller, cheaper commercial
low-frequency sonar systems made to survey the seafloor, both for
marine geology research and for the oil and gas industry to site
offshore rigs and pipelines. By the end of the 1990s, Stanton, Chu, and
Jech thought the time was ripe to adapt the technology to count fish.
A broad range of sound
Funded by the Office of Naval Research, Stanton bought a low-frequency
sonar system—a bright yellow, snazzy-finned device that looks like a
cross between a 1950s Cadillac and something out of Star Wars. But he
didn’t get it because of its looks. Towed on a line by boat, the sonar
transmits a nearly continuous broad range of low-frequency sound waves,
from 1 to about 100 kilohertz (kHz). In contrast, conventional “fish
finders” transmit and detect sound at only a single high frequency—120
kHz, for example.
The strength of the echoes from objects in the ocean strongly depends
on sound frequency transmitted. Low frequencies provide essential information,
because those are the ones that detect gas pockets in the seafloor and
in the fish bladders. Transmitting a broad range of low frequencies is
also critical, Jech said. Just as light spans a spectrum of wavelengths
representing colors, sound waves can be transmitted over a wide range
of frequencies; the greater the range of frequencies you transmit, the
greater the variety and detail of sound signals—and potential
information—you get back.
“It’s like seeing in just black and white, versus in color,” Jech said,
pointing to a red buoy and a green buoy bobbing in the waves outside
his window. “If they were in black and white, you couldn’t distinguish
between them.”
Testing a new idea
The low-frequency sonar had never before been used to look for fish.
Enlisting the expertise of WHOI Research Specialist Jim Irish, Stanton
and Chu field-tested it to see what signals it could collect—in tanks
at the University of New Hampshire; in a Florida lake, and on two
one-day cruises aboard the WHOI coastal research vessel Tioga. Then in
2005, it was time to test the system on the real thing: Atlantic
herring in deep water over Georges Bank off Cape Cod.
“We had a three-day window of opportunity to test,” Stanton said. “We slept very little for those three days.”
They towed the sonar system near the surface and deep in the water,
over and through fish schools. For comparison, they also collected
sound data from a conventional, high-frequency 120 kHz, hull-mounted
sonar system.
“To minimize or eliminate impact on marine mammals,” Jech noted, “we
are very careful when we do our experiments to keep the transmitted
sound level low, and we use directional beams, much like a flashlight
directs light in one direction.”
Deciphering the data
Once the sound data were collected, the work truly began. Much the way
astronomers analyze radio or gamma waves gathered by space probes to
infer and discern what they can't physically see in the depths of space, acousticians such as Stanton,
Chu, and Jech have conducted years of laboratory experiments so that
they can interpret sound signals that can illuminate the depths of the oceans.
Instead of detecting blurry patches of fish, as they would with
traditional sonar, the scientists used data from the new system to
detect individual fish and determine that fish swimming in adjacent
schools were, in fact, the same size. The team was also able to discern
that herring swim bladders resonated at a specific frequency—3
kHz—giving them the ability to detect disinct species.
That not only demonstrates the value of using the sonar system to
detect fish via their bladders, it opens new possibilities. With
funding from the National Oceanic and Atmospheric Administration, the
team will now explore the ocean using this broadband sonar sysem to distinguish
fish from shrimp and other zooplankton, which have their own particular
acoustic “fingerprints.” They may be able to learn how to distinguish
herring from cod, which may resonate at 1 kHz, or from sharks and tuna,
which don’t have swim bladders, Jech said. And they may even be able to
decipher frequency variations that could distinguish the smaller swim
bladders of juvenile herring from larger ones of adults, and thus get a
handle on the demographics of fish populations.
Such information is “vital from a fish management standpoint,” Stanton
said, “but it would also be valuable for biologists studying life in
the oceans. We’re trying to take an old fundamental discovery and, with
new technology, bring it to life.”
—Lonny Lippsett
Gone Fish AssessingIf you think U.S. Census Bureau officials have their hands full locating and counting everyone who lives in the United States, consider the men and women at the National Oceanic and Atmospheric Administration’s National Marine Fisheries Service who are charged with counting fish in U.S. waters.
Their mission is to prevent the decline of fish stocks and the loss of habitats that fish and shellfish need to breed, spawn, feed, or grow. But a whole lot of water gets in the way of their ability to locate those essential habitats and to estimate fish stocks.
In recent years, scientists at Woods Hole Oceanographic Institution have been lending a hand, applying new technologies and methods to see into the sea and help assess and maintain fisheries.
Find out more:
New ‘Eyes’ Size Up Scallop Populations
Undersea Robot Swims Where Trawls Dare Not Go
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Posted: September 27, 2006 [top] |
