Doppler radar for fish could revolutionize ocean monitoring
On September 29, 2006, on board Oceanus, a 177-foot research vessel out of Woods Hole, Massachusetts, acoustic engineer Nicholas Makris captured an unusual reflection on his sonar screens. At first, a dark speck appeared off the northern flank of Georges Bank. The speck grew and grew, transforming into a teeming mass that stretched for 25 miles. On the screen, it appeared as a huge, undulating swarm that moved in sync—much like a boisterous crowd doing “the wave” around a football stadium. Underwater, hundreds of millions of shimmering Atlantic herring schooled together and then, just as quickly, dispersed into shallower waters for a night of “synchronized reproductive activities.”
Back in a spacious, high-ceilinged office at MIT, Makris spent the following years refining the novel sonar technology that could soon revolutionize fisheries science. By monitoring entire shoals over vast swaths of the ocean in real time, the new method represents a millionfold increase in the rate at which an area can be surveyed. Think of it as a kind of Doppler radar for fish.
Oceans cover 70 percent of the earth’s surface, but because light atten-uates in water, scientists have for decades known more about the surface of the moon than about the dim depths of the ocean. Despite a federal mandate to take accurate counts of wild-fish populations that, in turn, help establish limits on what fishermen can catch, the existing data remain an educated guess extrapolated from trawl nets towed along given transects. Even combined with high-frequency sonar, these surveys usually illuminate a narrow column of water. Moreover, high-frequency, high-resolution sonar comes with a trade-off in range: surveying a section of the Atlantic Ocean the size of Massachusetts takes at least two weeks and offers a fleeting glimpse of an entire population, almost like trying to watch an entire film by viewing a couple of pixels in a single frame.
Makris, a towering, broad-shouldered man with thick eyebrows and an upright bass of a voice, is the head of MIT’s Laboratory for Undersea Remote Sensing. He has used sound to predict the intensity of hurricanes and measure the density of Arctic ice. His latest project, Ocean Acoustic Waveguide Remote Sensing (OAWRS), evolved out of a 1993 mapping project in which Makris and his colleagues bounced sound waves off the seafloor to chart previously uncharted peaks running down the middle of the Atlantic Ocean. In 2001, the Office of Naval Research asked him to look into apparitions off the coast of New Jersey. When Makris showed that these phantoms were fish, his new project took off. Makris began intentionally propagating and bouncing sound off Atlantic herring. Unlike conventional sonar, his repurposed Cold War technology employs low-frequency sound beams which could, in a perfectly quiet ocean, travel from one pole to the other. “We’re talking about audible sound,” Makris says. “In fact, it’s a little higher than my range. It’s more like Robert Plant singing ‘Black Dog.’”
When these sound waves hit a fish, its swim bladder resonates like a tuning fork; capturing these reflected echoes with a large underwater array of hydrophones, he creates images of the passing clouds of fish—some of which had never been seen in their entirety. “We were about a hundred miles southeast of Manhattan, in one of the busiest shipping lanes in the world, dodging all these big tankers, and we found these monstrous shoals of fish,” he says. “National Marine Fisheries wasn’t aware it was there, and we just watched it evolve.”
With support from the National Oceanographic Partnership Program (a consortium of the National Science Foundation, the Alfred P. Sloan Found-ation, and the Office of Naval Research), Makris and his colleagues plan to hit the water by 2013 with lighter, more versatile equipment that may eventually be capable of telling one fish species’ echo from another by estimating the size of a swim bladder.
In the long term, Makris’s images could reveal how entire ecosystems respond to management closures. But before the orphaned military and geophysical technology becomes a reality for environmental researchers, the project needs to overcome some formidable odds: a slow-moving federal bureaucracy, increasing underwater noise from offshore oil and gas exploration, and the unintended consequences of putting a giant fish-finder into the wrong hands. During his 2006 expedition, commercial fishermen got wind of the massive school’s coordinates and netted record hauls.
If knowledge is power, then the conservation success of this new fish sonar will clearly be a delicate balancing act. ❧
—Peter Andrey Smith
Art ©Kitty Kilian/ImageZoo