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 from Brightsurf:

Sound waves power new advances in drug delivery and smart materials
Sound waves have been part of science and medicine for decades, but the technologies have always relied on low frequencies.

Scientists make sound-waves from a quantum vacuum at the Black Hole laboratory
Researchers have developed a new theory for observing a quantum vacuum that could lead to new insights into the behaviour of black holes.

Remembrance of waves past: memory imprints motion on scattered waves
Now, it appears that between relativity and the classical (stationary) wave regime, there exists another regime of wave phenomena, where memory influences the scattering process.

Even if you want to, you can't ignore how people look or sound
Your perceptions of someone you just met are influenced in part by what they look like and how they sound.

Scientists achieve major breakthrough in preserving integrity of sound waves
In a breakthrough experiment, physicist and engineers at the CUNY ASRC have shown that it is possible to limit the movement of sound to a single direction without interruption even when there are deformations along the pathway.

Shaking light with sound
Combining integrated photonics and MEMS technology, scientists from EPFL and Purdue University demonstrate monolithic piezoelectric control of integrated optical frequency combs with bulk acoustic waves.

Sound waves transport droplets for rewritable lab-on-a-chip devices
Engineers at Duke University have demonstrated a versatile microfluidic lab-on-a-chip that uses sound waves to create tunnels in oil to digitally manipulate and transport droplets.

A sound treatment
University of Utah biomedical engineering assistant professor Jan Kubanek has discovered that sound waves of high frequency (ultrasound) can be emitted into a patient's brain to alter his or her state.

Light, sound, action: Extending the life of acoustic waves on microchips
Data centres and digital information processors are reaching their capacity limits and producing heat.

Cooling magnets with sound
Today, most quantum experiments are carried out with the help of light, including those in nanomechanics, where tiny objects are cooled with electromagnetic waves to such an extent that they reveal quantum properties.

Read More: Sound Waves News and Sound Waves Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.