Oceanographers produce the very first images of cod shoals

For the most part, mature Atlantic cod is a solitary creature that spends most of its time far below the surface of the ocean, grazing on bony fish, squids, crabs, shrimp and lobsters – unless the spawning season occurs, others by the millions, forming huge shoals resembling frenetic islands teeming with the sea.

These high schools of spawning can provide clues to the health of the entire cod population – a key indicator for tracking the recovery of the species, particularly in areas such as New England and Canada, where cod has been severely depleted by decades of overfishing.

But the ocean is a cloudy place and the fish are very mobile by nature, making them difficult to map and count. Today, a team of MIT oceanographers has traveled to Norway – one of the last areas of the world where cod still thrives – and used a synoptic acoustic system to illuminate, for the first time, entire schools of cod almost instantly, in 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 for Marine Research, was able to image several shoals of cod, the larger covering 50 kilometers, about 30 km. miles. The researchers estimated, from the images they produced, that the shoal of cod consists of about 10 million fish.

They also found that when the total cod population fell below the average size of shoals, the species has been declining for decades.

"This average size of shoals almost looks like a lower limit," Makris said. "And what's sad is that it seems to have been crossed almost everywhere for cod."

Makris and his colleagues published their findings in the journal Fish and Fishing.

Echoes in the depths

For years, researchers have attempted to image schools of cod and herring using high-frequency sonar instruments mounted on the hull, which direct narrow beams under research vessels. movement. These vessels cross a portion of the sea in a pattern similar to that of a lawn mower, performing imaging of shoal slices by emitting high frequency sound waves and measuring the time required for signals returned by a fish. to return to the ship. But this method requires that a ship moves slowly in the waters to get accounts; a study can take several weeks and generally only allows sampling of a small portion of a large, often benign bench, which is completely missing between survey lines and never captures the dynamics of the shoals.

The team used Ocean Acoutic Ocean Waveguide Remote Sensing (OAWRS), an imaging technique developed at MIT by Makris and co-author Purnima Ratilal, which emits low-level sound waves. frequency that can cover a much wider range than that at high frequency. sonar. The sound waves are basically tuned to bounce off the fish, especially on their swim bladder – a gas-filled organ that reflects sound waves – in the manner of echoes from a tiny drum. As these echoes return to the ship, researchers can group them together to get a snapshot of millions of fish across vast expanses.

Make pbadage

In February and March 2014, Mr. Makris and a team of students and researchers traveled to Norway to count cod, herring and capelin at the peak of their spawning season. They towed OAWRS aboard the Knorr, a US Navy research vessel operated by the Woods Hole Oceanographic Institution and best known as the ship on which the researchers discovered the remains of the Titanic.

The ship left Woods Hole and crossed the Atlantic for two weeks, during which the crew continuously battled storms and rough seas in the winter. When they finally arrived on the south coast of Norway, they spent the next three weeks observing herring, cod and capelin all along the Norwegian coast, from the town of Alesund north of Norway. the Russian border.

"The underwater terrain was as treacherous as the land, with seamounts, ridges and submerged fjord cbads," recalls Makris. "Billions of herring would be hiding in one of these submerged fjords near Alesund during the day, about 300 meters deep, and would go up at night on shelves some 100 meters deep. by instantly photographing entire benches, stretching kilometers, and adjusting their behavior. "

A window through a hurricane

While traveling up the Norwegian coast, the researchers towed a set of pbadive pbadive microphones 0.5 km long and a device emitting low-frequency sound waves. After photographing herring schools in southern Norway, the team then headed north to Lofoten, a dramatic archipelago of steep cliffs and mountains, illustrated in the most well-known way. in "Descent into the Maelstrom" by Edgar Allen Poe, in which the poet emphasized the abundance of cod. .

To date, the Lofoten remain the main cod spawning area and the Makris team was able to produce the very first images of a whole cod bank, covering about fifty kilometers.

Towards the end of their journey, the researchers had planned to image a last cod region, just as it was expected that a hurricane strike. The team realized that there would only be two windows of relatively quiet wind to operate its imaging equipment.

"So we went there, got good data and fled to a nearby fjord when the wall of the eyes hit," recalls Makris. "We ended with a 15-meter sea at dawn and the Norwegian Coast Guard, in a young and strangely soothing voice, urging us to evacuate the region." The team was able to photograph a slightly smaller shoal, about ten kilometers, before finishing the expedition.

On the edge

Back on the mainland, the researchers badyzed their images and estimated that an average size of shoals was about 10 million fish. They also examined historical cod data from Norway, New England, the North Sea, and Canada, and found an interesting trend: regions – such as New England – that experienced a steady decline in cod stocks did when the total cod population fell below about 10 million, the same number as a medium shoal. When cod fell below this threshold, it took decades for the population to recover, if ever.

In Norway, the cod population has always remained above 10 million inhabitants and has been able to recover, returning to its preindustrial level over the years, even after significant declines in the middle of the 20th century. The team also photographed schools of herring and found a similar trend throughout history: when the total population fell below the average level of a herring spawning bank, fish recovery took decades.

Makris and Godø hope that the team's results will serve as a measure 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 and you can not see what's going on," says Makris. "It's a game for all, until you start enlightening and seeing what's going on, and you'll be able to appreciate, understand and manage it properly." He adds, "Even though fieldwork is difficult, time consuming and expensive, it is essential to confirm and inspire theories, models and simulations."

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