The RV Knorr, in Norway’s Alesund harbor in February 2014, as the crew prepares to begin its Nordic Seas Experiment. Image: Michael Collins, U.S. Naval Research Laboratory
Wide-ranging acoustic images could help researchers identify populations on the brink of collapse. 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 Godų 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.
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