Critical Population Density Brings Order to Chaos
Northeastern University and MIT researchers have observed—for the first time—the origin of a mass gathering and the subsequent migration of hundreds of millions of animals. Utilizing a new imaging technology invented by the researchers, they were able to instantaneously image and continuously monitor entire shoals of fish containing hundreds of millions of individuals stretching for tens of kilometers off Georges Bank near Boston.
They found that once large shoals of Atlantic herring reach a critical population density, a “chain reaction” triggers the synchronized movement of millions of individual fish over a large area. The phenomenon is akin to a human “wave” moving in a sports stadium. They also observed that the fish “commute” to the shallower waters of the bank, where they spawn in the darkness, then return to deeper water and disband the following morning.
The findings, published in the latest issue of Science, confirm general theories about the behavior of large groups of animals that, until now, had not been verified in nature. Previously, these theories for diverse animal groups, ranging from flocks of birds to swarms of locusts, had only been tested with computer simulations and laboratory experiments.
“As far as we know, this is the first time we’ve quantified this behavior in nature and over such a huge ecosystem,” said Nicholas C. Makris, professor of mechanical and ocean engineering at MIT, who co-led this project with Northeastern professor Purnima Ratilal.
As part of the project, two research vessels were equipped with Ocean Acoustic Waveguide Remote Sensing (OAWRS) technology, developed by professors Makris and Ratilal. Both OAWRS and conventional sensing methods depend on acoustics to locate objects by bouncing sound waves off of them. OAWRS, however, captures images of a 100 kilometer diameter area every 75 seconds, providing far more complete coverage of fish population and behavior than conventional methods. In addition, OAWRS does so at a lower frequency than conventional methods, which allows the sound to travel much greater distances with lower intensity and still provide useful information.
"After analyzing the data carefully during the initial days at sea, I noticed what seemed to be a daily pattern of fish shoal formation,” said Ratilal, assistant professor of electrical and computer engineering at Northeastern. “When I predicted what would happen the following day, and it turned out to be right, we knew we had discovered something really important."
Makris and Ratilal see potential in using OAWRS to better monitor—and conserve—fish populations. Large oceanic fish shoals provide vital links in the ocean and human food chain, they explained, but their sheer size makes it difficult to collect information using conventional methods.
Northeastern PhD. students Mark Andrews and Zheng Gong contributed to this research. Additional collaborators include J. Michael Jech of the Northeast Fisheries Science Center, Olav Rune Godoe of the Institute of Marine Research in Norway, as well as others from MIT, Northeastern and the Southeast Fisheries Science Center. The project was sponsored by the National Oceanographic Partnership Program, the Office of Naval Research and the Alfred P. Sloan Foundation, and is a contribution to the Census of Marine Life.