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Your mother probably told you there were plenty of fish in the sea. But now, thanks to a new sensor system developed by MIT associate professor Nicholas Makris and his team, it’s gotten a whole lot easier to count them.

The sonar-based technology can survey schools of tens of millions of fish over 10,000 square kilometers at once and marks a vast improvement over conventional sensing, which can survey only 100 square meters at a time. The data gleaned could provide environmentalists, scientists, and government officials with more accurate fish counts to help assess and manage overfishing.

“If you could see better what was in the ocean, and people were more aware of it, the way they are about the weather, you could better understand its health,” says Makris.

Conventional sensors use inaudible high-frequency sonar signals, which direct energy in a narrow beam. The signals are easy to direct and analyze with computers but have a range of only a few hundred meters. Makris’s real-time sensing system uses an audible range of low-frequency sonar signals, which spread through the water in all directions for thousands of kilometers.

But the signals bounce off obstacles and often return “noisy” data filled with unwanted readings. Working with scientists from Northeastern University and the U.S. Naval Research Laboratory, Makris developed a computer program that screens out sonar noise and returns data on the shape, movement, and behavior of schools once a minute. Researchers testing the technology in waters south of Long Island, NY, for example, observed tens of millions of fish congregating in changing patterns, often with a thin bridge connecting larger groups, producing an hourglass shape.

“In terms of insight into the large-scale movements of fish populations and dynamics, this is really quite revolutionary,” says fish ecology expert Tony J. Pitcher, a professor at the Fisheries Centre at the University of British Columbia. Pitcher points out, however, that the system cannot determine the species of fish. A solution, says Makris, is to track known schools of fish or to use the new technology in combination with other methods able to detect species.

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