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Wide-ranging acoustic images could help researchers identify populations on the brink of collapse.(Photo: news.mit.edu)

Oceanographers manage to image entire cod shoals

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Thursday, November 08, 2018, 02:50 (GMT + 9)

A team of researchers has been able to image multiple cod shoals, the largest spanning 50 kilometres (about 30 miles), which let them estimate that the average cod shoal consists of about 10 million individual fish.

These scientists, led by Nicholas Makris, professor of mechanical engineering and director of the Centre for Ocean Engineering and Olav Rune Godø from the Norwegian Institute of Marine Research, also found that when the total population of cod dropped below the average shoal size, the species remained in decline for decades.

To image cod and herring shoals, the team made use of the Ocean Acoutic Waveguide Remote Sensing (OAWRS system), an imaging technique developed at the Massachusetts Institute of Technology (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.

 

  Spatial population distributions of entire Atlantic cod spawning groups of the Northeast Arctic obtained by instantaneous imaging. The image above on the left shows, in red circles, areas where researchers imaged spawning grounds of Atlantic cod in Lofoten, Norway. The image on the right depicts a 40-kilometre-wide stable cod group containing about 40 million specimens. The image at the bottom on the left shows a 20-kilometre-scale cod group of about 1 million specimens near Andenes, Norway. (Photo: Courtesy of the researchers / mit.edu)

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.

In February and March of 2014, Makris and a team of students towed OAWRS aboard the Knorr, a US Navy research vessel that is operated by the Woods Hole Oceanographic Institution. The ship left Woods Hole and crossed the Atlantic over two weeks, during which time the crew continuously battled storms and choppy winter seas.

The RV Knorr, in Norway’s Alesund harbour in February 2014, as the crew prepare to begin their Nordic Seas Experiment. (Image: Michael Collins, U.S. Naval Research Laboratory / mit.edu)

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.

As they moved up the Norwegian coast, the researchers towed a 0.5-kilometre-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, which 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 kilometres.

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.

Back on dry land, the researchers analyzed their images and estimated that an average shoal size consists of about 10 million fish. 

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.

This research findings were published in the journal Fish and Fisheries


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