Nav: Home

Oceanographers produce first-ever images of entire cod shoals

November 06, 2018

For the most part, the mature Atlantic cod is a solitary creature that spends most of its time far below the ocean's surface, grazing on bony fish, squid, crab, shrimp, and lobster -- unless it's spawning season, when the fish flock to each other by the millions, forming enormous shoals that resemble frenzied, teeming islands in the sea.

These massive spawning shoals may give clues to the health of the entire cod population -- an essential indicator for tracking the species' recovery, particularly in regions such as New England and Canada, where cod has been severely depleted by decades of overfishing.

But the ocean is a murky place, and fish are highly mobile by nature, making them difficult to map and count. Now a team of oceanographers at MIT has journeyed to Norway -- one of the last remaining regions of the world where cod still thrive -- and used a synoptic acoustic system to, for the first time, illuminate entire shoals of cod almost instantaneously, during the height of the spawning season.

The team, led by Nicholas Makris, professor of mechanical engineering and director of the Center for Ocean Engineering, and Olav Rune Godø of the Norwegian Institute of Marine Research, was able to image multiple cod shoals, the largest spanning 50 kilometers, or about 30 miles. From the images they produced, the researchers estimate that the average cod shoal consists of about 10 million individual fish.

They also found that when the total population of cod dropped below the average shoal size, the species remained in decline for decades.

"This average shoal size is almost like a lower bound," Makris says. "And the sad thing is, it seems to have been crossed almost everywhere for cod."

Makris and his colleagues have published their results in the journal Fish and Fisheries.

Echoes in the deep

For years, researchers have attempted to image cod and herring shoals using high-frequency, hull-mounted sonar instruments, which direct narrow beams below moving research vessels. These ships traverse a patch of the sea in a lawnmower-like pattern, imaging slices of a shoal by emitting high-frequency sound waves, and measuring the time it takes for the signals to bounce off a fish and back to the ship. But this method requires a vessel to move slowly through the waters to get counts; one survey can take many weeks to complete and typically samples only a small portion of any particular expansive shoal, often completely missing shoals between survey tracks and never capturing shoal dynamics

The team made use of the Ocean Acoutic Waveguide Remote Sensing, or OAWRS system, an imaging technique developed at MIT by Makris and co-author Purnima Ratilal, which emits low-frequency sound waves that can travel over a much wider range than high-frequency sonar. The sound waves are essentially tuned to bounce off fish, in particular, off their swim bladder -- a gas-filled organ that reflects sound waves -- like echoes off a tiny drum. As these echoes return to the ship, researchers can aggregate them to produce an instant picture of millions of fish over vast areas.

Making passage

In February and March of 2014, Makris and a team of students and researchers headed to Norway to count cod, herring, and capelin during the height of their spawning seasons. They towed OAWRS aboard the Knorr, a U.S. Navy research vessel that is operated by the Woods Hole Oceanographic Institution and is best known as the ship aboard which researchers discovered the remnants of the Titanic.

The ship left Woods Hole and crossed the Atlantic over two weeks, during which time the crew continuously battled storms and choppy winter seas. When they finally arrived at the southern coast of Norway, they spent the next three weeks imaging herring, cod, and capelin along the entire Norwegian coast, from the town of Alesund, north to the Russian border.

"The underwater terrain was as treacherous as the land, with submerged seamounts, ridges, and fjord channels," Makris recalls. "Billions of herring actually would hide in one of these submerged fjords near Alesund during the daytime, about 300 meters down, and come up at night to shelves about 100 meters deep. Our mission there was to instantaneously image entire shoals of them, stretching for kilometers, and sort out their behavior."

A window through a hurricane

As they moved up the Norwegian coast, the researchers towed a 0.5-kilometer-long array of passive underwater microphones and a device that emitted low-frequency sound waves. After imaging herring shoals in southern Norway, the team moved north to Lofoten, a dramatic archipelago of sheer cliffs and mountains, depicted most famously in Edgar Allen Poe's "Descent into the Maelstrom," in which the poet made note of the region's abundance of cod.

To this day, Lofoten remains a primary spawning ground for cod, and there, Makris' team was able to produce the first-ever images of an entire cod shoal, spanning 50 kilometers.

Toward the end of their journey, the researchers planned to image one last cod region, just as a hurricane was projected to hit. The team realized there would be only two windows of relatively calm winds in which to operate their imaging equipment.

"So we went, got good data, and fled to a nearby fjord as the eye wall struck," Makris recalls. "We ended with 30-foot seas at dawn and the Norwegian coast guard, in a strangely soothing young voice, urging us to evacuate the area." The team was able to image a slightly smaller shoal there, spanning about 10 kilometers, before completing the expedition.

On the brink

Back on dry land, the researchers analyzed their images and estimated that an average shoal size consists of about 10 million fish. They also looked at historical tallies of cod, in Norway, New England, the North Sea and Canada, and discovered an interesting trend: Those regions -- like New England -- that experienced long-lasting declines in cod stocks did so when the total cod population dropped below roughly 10 million -- the same number as an average shoal. When cod dropped below this threshold, the population took decades to recover, if it did at all.

In Norway, the cod population always stayed above 10 million and was able to recover, climbing back to preindustrial levels over the years, even after significant declines in the mid-20th century. The team also imaged shoals of herring and found a similar trend through history: When the total population dropped below the level of an average herring spawning shoal, it took decades for the fish to recover.

Makris and Godø hope that the team's results will serve as a measuring stick of sorts, to help researchers keep track of fish stocks and recognize when a species is on the brink.

"The ocean is a dark place, you look out there and can't see what's going on," Makris says. "It's a free-for-all out there, until you start shining a light on it and seeing what's happening. Then you can properly appreciate and understand and manage." He adds "Even if field work is difficult, time consuming, and expensive, it is essential to confirm and inspire theories, models, and simulations."
-end-
This research was supported, in part, by the Norwegian Institute of Marine Research, the Office of Naval Research, and the National Science Foundation.

Related links

ARCHIVE: Whales dine with their own kind http://news.mit.edu/2016/whales-feed-species-specific-hotspots-0302

ARCHIVE: Team observes genesis of fish shoals http://news.mit.edu/2009/animal-behavior-0326

ARCHIVE: One fish, two fish: New sensor improves fish counts http://news.mit.edu/2006/fish

Massachusetts Institute of Technology

Related Sound Waves Articles:

Sound waves direct particles to self-assemble, self-heal
Berkeley Lab scientists have demonstrated how floating particles will assemble and synchronize in response to acoustic waves.
Laser, sound waves provide live views of organs in action
Biomedical engineers are now able to take a live, holistic look at the inner workings of a small animal with enough resolution to see active organs, flowing blood, circulating melanoma cells and firing neural networks.
Sound waves boost older adult' memory, deep sleep
Gentle sound stimulation -- such as the rush of a waterfall -- synchronized to the rhythm of brain waves significantly enhanced deep sleep in older adults and tripled their ability to recall words, reports a new study.
Towards mastering terahertz waves?
Terahertz waves allow for the detection of materials that are undetectable at other frequencies.
Sound waves create whirlpools to round up tiny signs of disease
Mechanical engineers at Duke University have demonstrated a tiny whirlpool that can concentrate nanoparticles using nothing but sound.
The sound of quantum vacuum
Quantum mechanics dictates sensitivity limits in the measurements of displacement, velocity and acceleration.
The sound of a healthy reef
A new study from the Woods Hole Oceanographic Institution (WHOI) will help researchers understand the ways that marine animal larvae use sound as a cue to settle on coral reefs.
Knots in chaotic waves
New research, using computer models of wave chaos, has shown that three-dimensional tangled vortex filaments can in fact be knotted in many highly complex ways.
How to sound the alarm
A group of risk experts is proposing a new framework and research agenda that they believe will support the most effective public warnings when a hurricane, wildfire, toxic chemical spill or any other environmental hazard threatens safety.
Sound waves may hold potential to treat twin pregnancy complications
The early-stage feasibility study involving sheep suggests High Intensity Focused Ultrasound -- a technique already used for treating some cancers -- could help a condition called Twin-Twin Transfusion Syndrome.

Related Sound Waves Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Climate Crisis
There's no greater threat to humanity than climate change. What can we do to stop the worst consequences? This hour, TED speakers explore how we can save our planet and whether we can do it in time. Guests include climate activist Greta Thunberg, chemical engineer Jennifer Wilcox, research scientist Sean Davis, food innovator Bruce Friedrich, and psychologist Per Espen Stoknes.
Now Playing: Science for the People

#527 Honey I CRISPR'd the Kids
This week we're coming to you from Awesome Con in Washington, D.C. There, host Bethany Brookshire led a panel of three amazing guests to talk about the promise and perils of CRISPR, and what happens now that CRISPR babies have (maybe?) been born. Featuring science writer Tina Saey, molecular biologist Anne Simon, and bioethicist Alan Regenberg. A Nobel Prize winner argues banning CRISPR babies won’t work Geneticists push for a 5-year global ban on gene-edited babies A CRISPR spin-off causes unintended typos in DNA News of the first gene-edited babies ignited a firestorm The researcher who created CRISPR twins defends...